Organic counterions TFA (trifluoroacetate) C1 (HCOO -, formate) C2 (CH 3 COO -, acetate) C3 (CH 3 CH 2 COO -, propionate) C4 (CH 3 (CH 2 ) 2 COO -, butyrate) C5 (CH 3 (CH 2 ) 3 COO -, pentanoate) C6 (CH 3 (CH 2 ) 4 COO -, hexanoate) C8 (CH 3 (CH 2 ) 6 COO -, octanoate) C10 (CH 3 (CH 2 ) 8 COO -, decanoate)

-Acknowledgements -

These three years as a PhD student in Bordeaux were the greatest experience in my life filled with a variety of many interesting meetings and events. For the given opportunity to live unforgettable a life and scientific experience in a wonderful group I would like to thank my supervisor Reiko ODA. Thank you very much for being the driving force in my study, for leading me and even pushing when it was necessary during these three years of scientific research, for all your advises and bright ideas that made my work more interesting and versatile, for your great personality which made our international group a family. I would like to thank my co-supervisor Sylvain NLATE, who helped me to master organic synthesis and was ready to help with any questions.

This PhD thesis was created in Chimie et Biologie des Membranes et des Nanoobjets (CBMN). I would like to thank Erick DUFOURC, who is the director of the institute. I am very grateful to all the members of the jury. I would like to thank Muriel BLANZAT and Christophe TRIBET who kindly agreed to be reviewers for this manuscript. I am also grateful to Dario BASSANI who took over the duties of the president of the jury; Larry ROMSTED who kindly agreed to be the member of jury for my thesis defense despite all the technical troubles which we had to meet and, of course, Michel LAGUERRE who was an invited member of the jury.

I would like express gratitude to all collaborators of this project who made a great contribution to this PhD helping me to combine different techniques and approaches. Especially, to Dario BASSANI who in addition to his great talent to explain and teach made each of our meetings cosy and friendly by his humor; Larry ROMSTED and his PhD student Changyao LIU who were patient in teaching me chemical trapping technique and always hospitable on the other side of the Atlantic; Michel LAGUERRE and Massimiliano PORRINI who helped me to get insight into the molecular dynamic simulations. Thank you Max for being not just a collaborator with who was nice to work, but also a good friend. I also would like to thank Stéphane MASSIP and Brice KAUFFMANN for deepening my knowledge and improving my skills in the crystallography experiments and Axelle GRELARD for being always ready to discuss and help with any questions concerning the NMR experiments.

I am happy to meet in our lab people who became my close friends and mentors. I would like separately to express my gratitude to Dima who to the end was following my work, giving me suggestions concerning the experiments and sharing his experience in writing. Also to Guillaume -Acknowledgements -4 who was always ready to help me in my research as well as my English; very often he was more excited about my results than me, it encouraged me not to give up even in hard times. Moreover, I am thankful for the opportunity to play ping pong and relax by having very intense sets; for the all happy hours that we spent together in IECB with great food, drinks and nice company, alcohol always makes people closer. All that made my PhD life funnier, brighter and lovely.

At last but not at least I would like to thank all those with whom I was working with during these three years: Emilie POUGET, Dmitriy DEDOVETS, Jiaji CHENG, Hélène CARRIE, Guillaume LE SAUX, Alexandre CUNHA, Rajat Kumar DAS, Marie-Christine DURRIEU, Gregor KEMPER, Yutaka OKAZAKI, Laurent PLAWINSKI. Guys, each of you is a part of the great collective that made me happy to come to the lab every day. Thank you for all our talks and discussions, your advice, smiles, laugh and of course light, wonderful atmosphere at the working place.

I really appreciate the help and support of my friends and family during all my way. Thank you all. Благодарю тебя, любовь моя.

-Résumé -La présente étude est une approche holistique axée sur l'étude des effets spécifiques d'ions sur les propriétés d'auto-assemblage d'agents tensioactifs cationiques. Nous avons étudié l'effet de divers contre-ions sur les caractéristiques d'auto-organisation de tensioactifs cationiques en solution aqueuse. Afin d'obtenir une meilleure compréhension de l'effet des interactions ioniques et moléculaires à l'interface sur les propriétés globales, nous avons utilisé des approches différentes.

Nous avons combiné une étude expérimentale portée sur les propriétés en solution (concentration micellaire critique ou CMC, degré d'ionisation, nombre d'agrégation), avec des approches expérimentale (piégeage chimique) et informatique (simulations de dynamique moléculaire) centrées sur l'étude des propriétés à l'interface micelle -eau, en déterminant la concentration interfaciale des contre-ions et de l'eau. De plus, nous avons étudié l'impact de la nature du contreion sur la croissance des micelles par rhéologie. Enfin, outre l'examen des propriétés des tensioactifs en solution, les effets spécifiques d'ions sur les structures cristallines des agents tensioactifs gemini ont été étudiés.

Dans le premier Chapitre, nous présentons une étude bibliographique sur l'auto-assemblage de tensioactifs. En premier lieu, nous décrivons les molécules amphiphiles, leur classification et les forces motrices d'auto-assemblage. La description thermodynamique des processus de micellisation pour amphiphiles à une seule chaîne et amphiphiles gemini, avec un accent particulier sur les tensioactifs ioniques, est donnée. Afin d'élucider la façon dont les contre-ions de tensioactifs ioniques peuvent affecter leur micellisation, nous introduisons des effets ioniques spécifiques aussi appelés effets Hofmeister. Historiquement, il a pu être observé que les tentatives précédentes pour décrire les interactions impliquées dans ces phénomènes par des théories simplifiées ont leurs limites. Bien que de nouvelles forces impliquées dans l'effet ionique soient régulièrement décrites, il n'y a toujours pas de consensus quant aux les forces dominantes dans les effets ioniques spécifiques.

Pour résumer les connaissances acquises à ce jour, il peut être dit qu'en plus des forces électrostatiques et de dispersion, les forces d'hydratation jouent un rôle important dans l'effet Dans le troisième Chapitre, les propriétés physiques de la solution micellaire sont étudiées.

On observe que la CMC, le degré d'ionisation et l'énergie libre de formation de micelles dépendent fortement de la nature du contre-ion et suivent la série de Hofmeister. Inversement, le nombre d'agrégation des micelles ne montre pas une dépendance importante par rapport au contre-ion. Pour les halogénures monoatomiques, une corrélation monotone entre les propriétés d'agrégation et les propriétés physiques des ions est observées. Ces propriétés sont leur taille, polarisabilité, énergie libre d'hydratation, le numéro de l'hydratation, pKa de l'acide conjugué et le nombre lyotrope. Des agents tensioactifs gemini associés à des contre-ions carboxylate d'alkyle montrent une diminution essentiellement linéaire de la CMC avec l'augmentation de la longueur de la chaîne. Ceci indique une formation d'agrégats plus favorable quand l'hydrophobicité de l'anion augment. A contrario, les agents tensio-actifs avec des ions polyatomiques inorganiques ont un comportement plus complexe.

Bien que toutes les propriétés ioniques influencent la CMC, l'énergie libre d'hydratation des ions a l'impact le plus clair sur la micellisation. De plus, considérant les propriétés hydrophile/hydrophobes intrinsèques des contre-ions, les surfactants associés aux ions plus hydrophobes, ayant donc un degré d'ionisation et une énergie libre d'hydratation plus faibles, favorisent la micellisation. L'inverse est constaté quand le surfactant est associé à un contre-ion plus hydrophile.

Nous avons également démontré qu'en changeant les propriétés de la tête cationique, il est possible d'influencer significativement l'ordre de la série de Hofmeister et donc les interactions responsables des effets ioniques spécifiques. Nos résultats préliminaires ont montré que, pour un gemini où le méthyle du groupe de tête est substitué par un proton, l'effet Hofmeister peut être inversé.

Afin obtenir une meilleure compréhension des propriétés des agrégats, les propriétés interfaciales des micelles formées ont été caractérisées en couplant une approche expérimentale avec des simulations numériques, permettant ainsi d'étudier la nature de l'interaction des contre-ions avec les macromolécules.

Le Chapitre IV décrit l'étude expérimentale des propriétés interfaciales des micelles effectuées par une technique appelée piégeage chimique. Cette méthode permet d'estimer les concentrations du contre-ion et de l'eau à l'interface micellaire. Les résultats obtenus ont démontré -Résumé -7 que contre-ions hydrophobes et donc moins solvatés sont fortement associés avec le groupe de tête et sont situés principalement dans la région interfaciale de la micelle. D'autre part, la concentration interfaciale d'ions plus solvatés est faible et indique un plus faible degré d'interaction entre le groupe de tête le contre-ion. Ces résultats sont en bon accord avec les propriétés physiques micellaires en solution et corrèlent avec la série de Hofmeister. Dans le cinquième Chapitre, nous introduisons une approche numérique basée sur des simulations de dynamique moléculaire (DM) afin de caractériser l'effet de la nature de l'anion sur les propriétés structurelles des micelles à l'échelle atomique. Tout d'abord, les simulations de DM ont été utilisées pour enquêter sur les propriétés interfaciales des micelles en parallèle avec les expériences de piégeage chimique citées précédemment. Les résultats ont montré une bonne concordance entre les simulations et les résultats expérimentaux, les ions hydrophobes ayant tendance à s'associer fortement avec la micelle et étant principalement situés à proximité des groupes de tête, formant ainsi des paires ioniques de contact. Contre-ions hydrophiles, ayant une association plus forte avec l'eau, interagissent moins avec des groupes de tête de la micelle. En outre, il a été constaté que le contre-ion affecte les propriétés structurelles des micelles, telles que la compacité, la rugosité et la sphéricité. Les simulations ont démontré que les anions hydrophobes favorisent la compacité et la sphéricité de la micelle, la stabilisant et diminuant sa rugosité. Ceci contraste avec les anions hydrophiles qui induisent une plus faible compacité micellaire et moins de stabilité. De plus, nous avons clairement montré un début de pénétration du coeur hydrophobe de la micelle par les contre-ions carboxylate d'alkyle de chaine hydrophobe plus longue, augmentant ainsi le volume de la micelle. Il est intéressant de noter que même si la polarisabilité des ions n'a pas été considérée dans les simulations de DM, les résultats obtenus sont en bon accord avec les observations expérimentales.

Dans le Chapitre VI, les effets spécifiques d'ions dans des systèmes plus complexes ont été étudiés. Jusqu'à présent le système étudié était des micelles sphériques. Cependant avec l'augmentation de la concentration en agent tensio-actif, des micelles filandreuses enchevêtrées peuvent être formées. La viscosité augmente drastiquement à cette concentration, et rend donc possible l'étude par rhéologie des propriétés mécaniques des solutions contenant ces micelles filandreuses. Il a été montré que la nature des contre-ions affecte fortement la croissance micellaire et présente une tendance semblable à ce qui a été montré pour des micelles sphériques, suivant ainsi la série de Hofmeister. En raison de leur association avec les groupes de tête, les contre--Résumé -8 hydrophobes écrantent la charge micellaire et favorisent leur croissance, alors que les ions hydrophiles écrantent la charge micellaire dans une moindre mesure, induisant une croissance filandreuse à des concentrations beaucoup plus élevées. Identiquement à la CMC, l'augmentation observée de la viscosité du système qui est imputée à la croissance des micelles filandreuses, concorde bien avec l'énergie libre d'hydratation de l'anion.

Les effets spécifiques d'ions sur les propriétés d'auto-assemblage de tensioactifs gemini examinés jusqu'ici ont été faits en solution. Il était donc intéressant d'aller plus loin et d'étudier l'effet des contre-ions sur le processus la cristallisation des systèmes gemini/contre-ion.

Le Chapitre final décrit l'impact de la nature du contre-ion sur les structures cristallines de tensioactifs gemini. Il a été constaté que contrairement ce qui a été observé en solution aqueuse où l'hydratation de l'anion joue un rôle primordial, les anions ayant un rayon plus petit s'associent plus fortement avec les groupes de tête cationiques. Les résultats diamétralement opposés pour un même système, qu'il soit en solution ou à l'état solide démontrent l'importance des interactions ion-eau sur les effets spécifiques des ions.

En résumé, utilisant différentes approches, il a pu être montré les effets ionique spécifiques d'ions qui influencent le comportement des agrégats micellaires dans des solutions aqueuses, dépendent fortement de l'énergie libre d'hydratation des contre-ions. En d'autres termes, sur leurs propriétés hydrophiles/hydrophobes. Il est notable que la polarisabilité des ions, qui fournit des informations sur les forces de dispersion, et donc sur la spécificité ionique, semble moins bien concorder avec les propriétés d'agrégation. En revanche, nous observons une très bonne corrélation entre l'énergie libre d'hydratation de l'ion et les propriétés des agrégats de tensioactifs. Ces résultats suggèrent fortement que l'énergie libre d'hydratation d'ions peut fournir des informations sur les effets spécifiques d'ions en solution aqueuse. Cependant, il faut noter que les propriétés du substrat (gemini dans notre cas) doivent être prises en compte non moins rigoureusement afin de prédire entièrement les effets Hofmeister.

Mots-clés: effets spécifiques d'ions, d'auto-assemblage, tensioactifs gemini, micellisation, propriétés interfaciales, piégeage chimique, simulations de dynamique moléculaire, rhéologie, micelles, structure cristalline.

-Abstract -

The present study is a holistic approach focused on the investigation of ion specific effects on the self-assembly properties of cationic surfactants. We studied the effect of various counterions on the self-organization features of cationic surfactants in aqueous solution. In order to obtain a more comprehensive understanding of the effect of interfacial ionic and molecular interactions on aggregate properties we used different approaches. We combined an experimental study focused on the bulk solution properties (critical micelle concentration or CMC, ionization degree, aggregation number, etc.), experimental (chemical trapping) and computational (molecular dynamic simulations) approaches focused on investigating the interfacial micellar properties by analyzing the interfacial counterion and water concentrations. Moreover, the impact of counterion nature was investigated studying the growth of wormlike micelles using rheology. Besides the examination of the surfactants properties in solution, the ion specific effects on the crystalline structures of gemini surfactants were studied.

In the first Chapter we present a bibliographic study of the self-assembly of surfactants. At the beginning we introduce amphiphilic molecules, their classification and forces driving their selfassembly. The thermodynamic description of micellization process for single chain and gemini amphiphiles with particular emphasis on ionic surfactants is given. In order to elucidate how counterions of the ionic surfactants can affect their micellization, we introduce ion specific effects also known as Hofmeister effects. Following the history we can see how all previous attempts to describe the interactions involved in these phenomena by simplified theories have their limitations.

Introducing new forces that might be responsible for the ion specificity was done at almost every decade, however there is still no general agreement as to which forces and ion properties drive the ion specific effects. Summarizing all knowledge obtained so far, we can say that in addition to electrostatic and dispersion forces, one should also take into account the hydration forces that play a significant role in ion specificity.

In the second Chapter we introduce the cationic gemini surfactants C 2 H 4 -1,2-((CH 3 ) 2 N + C 10 H 21 ) 2 denoted as 10-2-10, and a variety of counterions chosen for this study.

To investigate the ion specific effects we focused on the monoatomic halides, polyatomic inorganic and alkyl carboxylate anions. Here we also give the synthetic procedures used to obtain 10-2-10 gemini with different of counterions.

-Abstract - [START_REF] Debye | Light Scattering in Soap Solutions[END_REF] In the third Chapter, the physical properties of the micellar solution are studied. It was observed that the CMC, ionization degree and free energy of micellization of the surfactants strongly depend on the counterion nature and follow the Hofmeister series. Conversely, the aggregation number of the micelles did not show a significant dependence on the counterion. We found that for monoatomic halides, the obtained properties of aggregation correlate monotonously with the physical properties of the ions such as size, polarizability, hydration free energy, hydration number, pKa of conjugated acid and lyotropic number. Gemini surfactants associated with alkyl carboxylate counterions primarily show a linear decrease in CMC with increasing chain length, indicating that with increasing hydrophobicity of the anion, aggregate formation is more favorable.

On the contrary, the surfactants with polyatomic inorganic ions have a more complex behavior. We found that all properties of the ion influence the CMC values; however, the hydration free energy of the ions has the most obvious impact on the micellization behavior. Moreover, classifying anions according to their hydrophobic/hydrophilic properties, we see that: surfactants associated with more hydrophobic counterions, thus having lower ionization degree and free energy of micellization, favor surfactant micellization. Whereas surfactants with hydrophilic counterions, characterized by a high ionization degree and free energy of micellization, disfavor the formation of aggregates.

We also demonstrated that by changing the properties of the headgroup, we can significantly affect the order of Hofmeister series and hence the interactions responsible for ion specific effects.

Our preliminary results showed that increasing the hardness of the cationic headgroup for gemini by replacement of one methyl by a proton, we could reverse the Hofmeister effects.

To obtain a better understanding of the properties of aggregates, the interfacial properties of the formed micelles were characterized using experimental and computation approaches. This gave us access to the information about direct interaction of ions with macromolecules.

Chapter IV describes the experimental study of the interfacial properties of the micelles done by a probing technique called chemical trapping. This method allows to estimate the ion and water concentrations within the micellar interface. The obtained results demonstrated that poorly hydrated counterions are strongly associated with the headgroup and are located primary in the interfacial region of the micelle. On the other hand, interfacial concentration of highly hydrated ions is low and indicates a low degree of counterion-headgroup binding. These results are in a good agreement with investigated micellar physical properties in bulk solution and all correlate with Hofmeister series.

-Abstract -

In the fifth Chapter we introduce a computational approach based on molecular dynamic (MD) simulations applied to characterize the effect of anion nature on the structural properties of the micelles at the atomic level. Foremost, the MD simulations were used as computational approach to investigate interfacial properties of the micelles in parallel with experimental chemical trapping. We found that the obtained simulations are coherent with the experimental results: the hydrophobic ions tend to associate strongly with the micelle and are primarily located in the vicinity of the headgroups forming contact ion pairs. Hydrophilic counterions, having a stronger association with water, interact less with headgroups of the micelle. Furthermore, it was found that counterion nature affects the structural properties of the micelles, such as compactness, roughness and sphericity. MD simulations demonstrated that hydrophobic anions promote the compactness and sphericity of the micelle stabilizing it and decreasing its roughness. At the same time, micelles with hydrophilic anions are characterized by lower compactness and stability. Moreover, we clearly showed that alkyl carboxylate counterions with increasing chain length start to penetrate micellar hydrophobic core thereby increasing the volume of the micelle. It is interesting to note that although the polarizability of the ions was not considered in the MD simulations, obtained results are in a good agreement with experimental observations. In Chapter VI, the ion specific effects in a system of higher complexity were studied. Until now our study was focused on spherical micelles. However with increasing surfactant concentration, large entangled wormlike micelles can be formed. Because there is a significant change in viscosity at this concentration, rheological study can be used to examine the mechanical properties of solutions containing wormlike micelles. It was shown that the nature of the counterions strongly affects the micellar growth and exhibits a similar tendency to what was shown for spherical micelles, thus following Hofmeister series. Due to their association with the headgroups, the hydrophobic counterions screen the micellar charge and favor their growth, whereas hydrophilic ions screen micellar charge to a lower extent and exhibit micellar growth at much higher concentrations. Similarly to the CMC, the observed increase in viscosity of the system, attributed to micellar growth, correlated with the hydration free energy of the anion.

The ion specific effects on the self-assembly properties of gemini surfactants discussed so far was examined in aqueous solutions. However our interest went further and we investigated the effect of counterions on the assembly process in crystals.

-Abstract -

In the final Chapter we demonstrate the impact of the counterion nature on the crystalline structures of gemini surfactants. It was found that contrarily to the aqueous solution, where hydration of anion plays a primordial role, in the crystal structure the anions with smaller radius display a stronger association with the cationic headgroups. Comparison of the results obtained for the same system in aqueous solution and in solid state showed the importance of ion-water interactions in ion specific effects.

Summarizing the results obtained by different approaches, we can conclude that ion specific effects which determine the behavior of micellar aggregates of cationic quaternary ammonium gemini in aqueous solutions strongly depend on the free energy of hydration of the counterions, in others words, on its hydrophilic/hydrophobic properties. It is interesting to note that the polarizability of the ions, which provides information on the dispersion forces and thus on ion specificity, seems to correlate less with the aggregation properties. In contrast, the free energy of hydration of the ion, correlates very well with properties of surfactant aggregates. These results strongly suggest that ion hydration free energy can provide information about the ion specific effects in aqueous solution. However, one should note that the properties of substrate (the gemini in our case) should be taken into account not less carefully in order to fully predict Hofmeister effects.

Keywords: ion specific effects, self-assembly, gemini surfactants, micellization, interfacial properties, chemical trapping, molecular dynamic simulations, rheology, wormlike micelles, crystal structure.

-Glossary -

-General introduction -

It has been a long time now that surfactant molecules have played an important role in many fields of scientific research such as biomaterials, colloid chemistry, polymer chemistry, matter science and nanomaterials. This is due to their particular capacity to self-assemble in solution forming a variety of aggregates with different properties based on their amphiphilic nature.

Surfactants contain both hydrophilic and hydrophobic moieties in one molecule. The selfassembling features, such as stability or critical micelle concentration as well as morphology of the aggregates can be controlled by the molecular structure of surfactants or the physical-chemical conditions such as solvent, temperature, added salts, etc. For the ionic surfactants, one of the crucial parameters that determine their self-assembling behavior is the nature of counterions.

Understanding the influence of ion specific effects on the properties and structure of aggregates still remains one of the chalenging problems in the chemistry of proteins, biomembranes and associated colloids such as micelles, emulsions and vesicles. Franz Hofmeister was the first who reported in 1888 that different salts have different ability to precipitate proteins from aqueous solution. Based on these observations, Hofmeister proposed the order of cations and anions that reflects their effect on the proteins. Later on, it was shown that many properties of proteins and colloids in aqueous solution correlate with Hofmeister series. Many studies have been done in order to explain these phenomena theoretically and experimentally. However, there is still no consensus on how different forces play the roles in cooperative manner. Thus, the prediction of morphologies and properties of aggregates for a particular ion molecule in a given condition remains a major barrier for routine applications.

In this work, we combine experimental and computational approaches in order to rationalize ion specific effects of ionic surfactant based on the balance of forces which controls their selforganization process and aggregate morphology.

In the first Chapter, we present an overview of the literature dealing with the self-assembly of surfactants. Structure of the main classes of amphiphiles and their self-organization features are discussed. The thermodynamic description of micellization process for single chain and gemini amphiphiles with particular emphasis on ionic surfactants is given. This is followed by the introduction of the Hofmeister effects and main classical theories that were created in order to describe these phenomena based on all the forces that contribute to ion specific effects. At the end, we give some examples of very recent studies of Hofmeister effects.

The main goal of the present work is to compare the effects of different anions on the selfassembly of cationic surfactants. We mainly focused on the gemini surfactants with the same headgroups, however, some comparison was performed in order to investigate the impact of the headgroups nature on the ion specific effects. The Chapter II contains the introduction of the systems chosen for this study, as well as the synthetic procedure for the surfactant molecules.

The micellization of gemini surfactants with different counterions was first characterized in the bulk solution as discussed in the Chapter III. To obtain better understanding of the properties of aggregates, the interfacial properties of the formed micelles were characterized using experimental and computational approaches. This gave us access to the information about direct interactions of ions with macromolecule. Chapter IV describes the experimental study using chemical trapping technique that allowed us to estimate the ion and water concentrations within the micellar interface.

To complete and support these results computer modeling discussed in Chapter V was used. There we show how applying the molecular dynamic simulations to our micellar systems allows us to characterize the effect of anion nature on the structural properties of the micelle including its morphology and counterion density within the interface.

In the Chapter VI the ion specific effects in the system of higher complexity was studied.

The impact of the counterions nature on the large worm like micelles was investigated by rheology.

In the last Chapter, we focused on the effect of counter ions on the packing of gemini amphipile molecules in the crystals. Comparison of the results obtained for the same system in aqueous solution and in solid state showed the importance of ion-water interactions in the ion specific effects.

This comprehensive study allowed us to shed light on the relations between ion type and the hydration, hydrogen bonding, electrostatic, ion pairing, and polarization interactions that balance the hydrophobic effect and control the physical properties of solutions of ionic amphiphiles.

Chapter I.

Amphiphiles. Bibliographic study

1. Amphiphilic molecules. Types and properties.

Amphiphilic molecules are largely studied and used for different fields of science and technology due to their unique self-assembling behavior.

Amphiphiles are called surfactants when they exhibit an interfacial activity by lowering the surface tension of the liquid or interfacial tension between two liquids assembling at the interface.

Numerous variations are possible in the types of head groups and tail groups of surfactants. For example, the head group can be anionic, cationic, zwitterionic, or nonionic [1]. Its size can also vary changing the properties of the surfactant molecule. The hydrophobic moiety can have different lengths as well as various substitutes [2]. Besides, among others the linkages between the hydrophilic and hydrophobic parts provides a large variety of amphiphiles (Figure I-1). Hydrophilic head group can be covalently linked to single, double or triple hydrophobic alkyl chains. Two hydrophilic head groups covalently linked by one hydrophobic alkyl tail result in so-called bolaform amphiphiles.

Two surfactant molecules covalently linked at their charged head groups are termed a gemini (dimeric) amphiphile [2,3]. Due to the presence of two ambivalent moieties in the surfactant molecules, they associate spontaneously in solution, forming the supramolecular structures. Tanford proposed two main antagonistic forces that drive the self-assemble process [4,[START_REF] Tanford | Theory of micelle formation in aqueous solutions[END_REF]. One is hydrophobic interactions between the aliphatic chains. This energy represents a favorable process of bringing the surfactant's tail from contact with water to contact with other tails, thus providing the stimulus for selforganization. At the same time, repulsive forces between polar headgroups such as electrostatic interactions, hydration and steric effects, prevent the formation of large aggregates.

A variety of factors affect the self-assembly process of amphiphiles. Apart from experimental conditions (concentration, temperature, pH, ionic strength, etc) aggregate formation to a large extend depends on the structural properties of the molecules resulting in different aggregate morphologies. Based on the free energy model proposed by Tansford, Israelachvili et al. [START_REF] Israelachvili | Intermolecular and Surface Forces[END_REF][START_REF] Israelachvili | Theory of self-assembly of hydrocarbon amphiphiles into micelles and bilayers[END_REF] developed a geometry-based approach to predict the shape of formed aggregates. The dependence between the surfactant molecules and associated aggregates was described by critical packing parameter (cpp), p = v/a 0 l, where v and l are volume and extended length of the hydrophobic part respectively, and a 0 is a surface area occupied by the surfactant headgroup. Critical packing shape is determined by the balance between the hydrophobic effects for assembly of the amphiphile tails (which causes the organization of amphiphilic molecules and, hence, reduces a 0 ) and the tendency of hydrophilic headgroups to maximize their contact with water (and increases a 0 ). The balance between these two main forces leads to an optimal area per amphiphilic headgroup, for which the interaction energy is minimum [START_REF] Ramanathan | Amphiphile nanoarchitectonics: from basic physical chemistry to advanced applications[END_REF]. Thus, due to difference in p, a variety of structures such as spherical micelles, rodlike micelles, vesicles, bilayers can be formed, for clarity see the Figure I-2.

Although this description remains largely conceptual and not to be taken literary, or "visually", the global behavior of the amphiphilic molecules can be well described, i.e. big head amphiphiles tend to form spherical micelles whereas upon decreasing the size of the headgroup (either sterically or by decreasing the repulsion [screening the apparent charge]) they tend to form less curved aggregates, cylindrical micelles, towards bilayers, then inversed micelles, etc.

Micellization

Ionic surfactant self-assembly

The behavior of ionic surfactants at low concentrations is very similar to the one of classical electrolyte (salt). However, with increasing surfactant concentration, a heterogeneous system occurs due to the formation of self-assembled aggregates called micelles. This concentration is called forces between hydrophobic tails [START_REF] Debye | Note on light scattering in soap solutions[END_REF][START_REF] Debye | Light Scattering in Soap Solutions[END_REF]. After that, numerous theoretical studies [START_REF] Tanford | Theory of micelle formation in aqueous solutions[END_REF][START_REF] Ooshika | A theory of critical micelle concentration of colloidal electrolyte solutions[END_REF][START_REF] Reich | Factors Responsible for the Stability of Detergent Micelles[END_REF][START_REF] Halsey | On the Structure of Micelles[END_REF][START_REF] Hoeve | On the Statistical Mechanical Theory of Micelle Formation in Detergent Solutions[END_REF][START_REF] Poland | Hydrophobic bonding and micelle stability; the influence of ionic head groups[END_REF] were developed in order to improve the model proposed by Debye. The overview of all these studies was done by L. Romsted [2]. Another model was proposed by Nagarajan and Ruckenstein [START_REF] Nagarajan | Theory of surfactant self-assembly: a predictive molecular thermodynamic approach[END_REF] in order to give a thermodynamic description of the process of micelle formation. According to this theory there are five main contributions to the aggregation free energy per surfactant molecule of size N and shape S well described by Zana [START_REF] Zana | Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications[END_REF]:

( ) = + ( ) + ( ) + ( ) + ( , ) (I-1)
where a is the area per molecule on the aggregate surface and R is the aggregate size that is related to N and a by aggregate geometry (for spherical micelles, = 4

). These five main thermodynamic terms are as follows:

(1)

The first force that provides the micellization is the hydrophobic effect. It can be characterized as the free energy per surfactant molecule g hc gained by isolating hydrocarbon chains from water. This free energy is negative and is approximately independent of aggregate geometry S, its contribution favors the increase in aggregate size and has linear dependence in N with a reduction of roughly k B T per hydrocarbon chain [START_REF] Zana | Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications[END_REF].

(2)

The formation of surfactant aggregates generates an interface between the hydrophobic core region consisting of the surfactant tails and the surrounding water medium. This contribution g int is unfavorable for the aggregate and can be described as follows:

( ) = ( - ) (I-2)
where 1 is the interfacial tension of the core-water interface (~ hydrocarbon-water interfacial tension) and a min is the minimum area per molecule (i.e., the interfacial area occupied by a headgroup).

(3)

The third contribution of the free energy is attributed to the packing of the non-polar chains into the hydrophobic core g el , namely the difference in the chain length regarding relaxed length l 0 (typical for liquid hydrocarbons).

( ) = ′( -) (I-3)
The elastic constant kʹ depends on the chain statistics, as well as the packing parameter.

(4)

The contribution to the ( ) from the presence of polar headgroups includes steric repulsion between the headgroups. From the entropy of mixing per molecule for steric free energy g st we obtain:

( ) = + - - (I-4) (5)
For the ionic surfactants there is one more term that makes a significant impact on the free energy of aggregates formation: electrostatic repulsion between the headgroups g es .

Oppositely to g int , it acts to increase the area per molecule. The free energy of the electrostatic repulsion based on Poisson-Boltzmann theory is given by:

= + ( + ) ⁄ -( + ) ⁄ (I-5) + - + ( + ) ⁄
where = 4 ⁄ is a dimensionless charging parameter depending on two length: For the surfactant of the shape S and chemical potential μ, the aggregation number N and area per molecule a (as well as aggregate size R) can be determined as follows:

( ) ( ) = , ( ) ( ) = (I-6)
By increasing the chemical potential (μ > k B TlnФ cmc , where k B is Boltzmann constant and Ф cmc is volume fraction of the surfactant at the cmc), micellization takes place:

Ф = [ * /( )], = * Ф (I-7)
where T m is temperature at the micellization point and average aggregate size at the cmc, N*, can be calculated now from the expression of ( ) at its minimum.

Ionic gemini surfactant self-assembly

In the present work our interest is focused on the cationic gemini surfactants: two alkyl chains bound with two monovalent cationic headgroups that are linked by a spacer. The selfassembly properties for such kind of amphiphiles significantly differ from their single chain length analogs. The CMC for gemini surfactants is typically several orders of magnitude lower than that of corresponding monochain analogs. It strongly depends on the chain length of the hydrophobic part, nature of the headgroup, spacer length and type of counterions [START_REF] Zana | Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications[END_REF][START_REF] Menger | Gemini Surfactants[END_REF].

The presence of two tails, polar headgroups and spacer should be considered for the estimation of the thermodynamics of aggregate formation, Equation (I-1). Here we would like to give a short overview of how the dimeric nature of the surfactant contributes to each thermodynamic term described above.

(1)

The hydrophobic term per g hc surfactant molecule now should include the contribution of the two tails plus spacer. However, only the spacer moiety, that integrates the micellar hydrophobic core (s core ) should be taken into account. This penetrating moiety is taken as difference between the total spacer length (s) and the mean head-head distance, = -⁄ ⁄ , where b is the segment length of the spacer model proposed by Gaussian [START_REF] Zana | Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications[END_REF].

The co-localization of the hydrophobic chains and spacer in one molecule from the beginning favor the micellization, hence the gain in hydrophobic energy per aliphatic group is smaller than for monomeric surfactants.

(2)

The interfacial contribution g int in the case of gemini surfactants should be modified regarding the presence of the spacer. The spacer chain length that occupies the core-water interface and participates in shielding is proportional to a 1/2 . Thus, this contribution can be given by:

≃ ( -) ⁄ (I-8)
where 2 is the interfacial tension of the spacer-water interface and w is a spacer width.

However, this correction takes place just when spacer does not consist of hydrocarbons, otherwise 2 = 1 and g int term is the same as for the single chain analogs.

(3) As we already mentioned, the presence of short spacer would keep the two hydrophobic chains at closer distance than they would stay in single chain surfactants. This feature of gemini structure reduces the entropy of the tail chain. In comparison with a single chain surfactant, where the area of the tail that interacts with water equals approximately a tail ~ v/R, for gemini the presence of the spacer reduces the accessible area per tail to a sp ~ (sb) 2 , where s is the number of segments of a Gaussian model for the spacer. Taking this into account thermodynamic contribution can be presented as:

≃ ≃ (I-9) (4)
The last contribution that has to be modified is electrostatic repulsion of the headgroups.

Similar to the impact of the spacer on the chains, short linker decrease the distance between the headgroups in respect to single chain analogues. This results in a nonuniform charge distribution of ion pairs over the micellar interface. The solution to this challenge is well described by Camesano and Nagarajan in their work [START_REF] Camesano | Micelle formation and CMC of gemini surfactants: a thermodynamic model[END_REF]. They recommend a semi-empirical correction factor to g es contribution, which disappears when the spacer is longer than the mean interhead distance and the charges are uniformly distributed on the micellar surface.

In addition, besides all the thermodynamic terms that were discussed above, for the ionic surfactants, the impact of counterions on the aggregate formation is very strong.

Ion specific effects

Hofmeister series

The most important contribution to the discovery of ion specific effects was done by Franz Hofmeister in 1888. He published a series of papers [START_REF] Hofmeister | Zur Lehre von der Wirkung der Salze[END_REF] where he described the different ability of a variety of salts to precipitate proteins. This work was later translated in English [START_REF] Kunz | Specific ion effects in colloidal and biological systems[END_REF]. Based on his observations, Hofmeister organized cations and anions according to their ability to precipitate the proteins. Now this ordering is known as Hofmeister series, Figure I-4. However it would be fair to note that the first investigations of salt effects on the viscosity of aqueous solutions were done by Poiseuille in 1847 [START_REF] Poiseuille | Sur le mouvement des liquides de nature très différente dans les tubes de très petits diamètres[END_REF], who observed that some salts increase the viscosity of water, whereas others decrease it. It was not easy to understand observed phenomena in the 19 th century due to the limited available analysis techniques. However, this did not stop Hofmeister from performing seminal experiments and he was able to draw non trivial conclusions from this work about specific ion effects that allowed him to order salts according to their "water withdrawing capability": "… the colloid precipitating effect of a salt is dependent on its water absorbing capability, one can expect that this activity against the various colloid substances should be constant in relation to other salts. It also can be expected, that the precipitating capability of salts is parallel to other physical and chemical properties, if it has been proved or it is most likely, that these properties are dependent on the water absorbing capability of the salts" [START_REF] Kunz | Specific Ion Effects[END_REF]. This was on his opinion the most important effect of salts on the colloid system. Nevertheless, his observations also seemed to be related with what we would call specific ion adsorption, hydration-dehydration, ion exchange effects that occur at colloidal surfaces.

There are countless phenomena related to ionic effects in physical chemistry and biochemistry. A number of reports can be found on the ion dependence of the surface tension of aqueous solutions [START_REF] Pegram | Hofmeister Salt Effects on Surface Tension Arise from Partitioning of Anions and Cations between Bulk Water and the Air-Water Interface[END_REF][START_REF] Weissenborn | Surface Tension and Bubble Coalescence Phenomena of Aqueous Solutions of Electrolytes[END_REF]. Bubble interactions and coalescence are also known to be influenced by salt effects; colloidal particles in solution are strongly influenced by background salt. A lot of biological systems are linked with ionic effects. They participate in the osmotic regulation of the cells, enzymes activites, bacteria growth and other vital processes.

Nonetheless, a unifying theory is still missing and the Hofmeister series yet represents an appealing challenge in colloid and interface science. Still today there is a debate about the relative importance of direct ion-ion interactions and of ion-water interactions to explain or even to predict these effects. Some scientists are convinced that the proper description of the dispersion forces will finally solve the problem, others think that the proper geometry of ions or charged headgroups and of water is decisive.

First attempts to explain ion specificity

After Hofmeister published his works on ion specificity in 1880s, a number of studies were done to elucidate the discovered phenomena. For example, Robertson in 1911 [START_REF] Robertson | Contributions to the Theory of the Mode of Action of Inorganic Salts upon Proteins in Solution[END_REF] found that the influence of added salts on the precipitation of proteins depends on the concentration of the salt employed. At low and at high concentrations the salts can act as precipitants and as coagulants respectively. In 1920 Loeb [28] showed that pH of the solution of proteins has an important influence on the salt effects.

First attempts to explain ion specificity were essentially related to the specific interactions between ions and water. Such kind of interactions would make a strong influence on the physicochemical properties of salt solutions, particularly on conductivity and viscosity.

Kohlraush has performed a large number of experiments to study the conductivity properties of the electrolyte solutions, establishing that they obey Ohm's law [START_REF] Adamson | A textbook of physical chemistry[END_REF]. He described that the mechanism for conductivity was based on the "independent migration" of ions through the aqueous solution due to the presence of an electric field. The current which goes through the solution is given by the product of the velocity of the ion and its charge. The electrochemical mobility, μ i (cm 2 V -1 s -1 ), is given by the ratio between the velocity and the applied electric field.

The conductivity measurements showed that the mobility of the ions in water has a nonlinear dependence on the ion size and follows the order: Cs + > Rb + >K + > Na + > Li + for alkali metal cations, and Br -> I -> Cl ->F -for halide anions (Figure I-5). The observations do not correlate with expectations: the more mobile anions are the ones with smaller radii. This could be explained by the fact that the actual effective size of ions in water is very different from that in a crystal. Small ions (e.g. Li + , F -) are strongly hydrated and move bearing several water molecules with them and so they move slowly compared with bigger, poorly hydrated ions (e.g. Cs + , Br -). As we already mentioned the first study on electrolyte viscosities was done by Poiseuille in 1847. Jones and Dole in 1929 [START_REF] Jones | THE VISCOSITY OF AQUEOUS SOLUTIONS OF STRONG ELECTROLYTES WITH SPECIAL REFERENCE TO BARIUM CHLORIDE[END_REF], Cox and Wolfenden in 1934 [START_REF] Cox | The Viscosity of Strong Electrolytes Measured by a Differential Method[END_REF], and several other groups further refined the specific ion effect on water viscosity. It was noted that some aqueous electrolytes enhance the viscosity relative to pure water, whereas, some others decrease it.

As was discussed by Marcus [START_REF] Marcus | Effect of Ions on the Structure of Water: Structure Making and Breaking[END_REF][START_REF] Jenkins | Viscosity B-Coefficients of Ions in Solution[END_REF], the effect of ions on the structure of water can be described by B coefficient of the dynamic viscosity of an electrolyte solution, η, which derives from the Jones-Dole [START_REF] Jones | THE VISCOSITY OF AQUEOUS SOLUTIONS OF STRONG ELECTROLYTES WITH SPECIAL REFERENCE TO BARIUM CHLORIDE[END_REF] expression (originally expressed in terms of the fluidity 1/η):

[( * ⁄ ) -] = ⁄ + + ⋯ (I-10)
where η* is the viscosity of the solvent (water), A is a coefficient that can be calculated from the conductivity of the electrolyte and B is an "ion specific parameter" known as Jones-Dole viscosity B coefficient. Usually Equation (I-10) holds well for c < 0.1 M, and for more concentrated salt solutions further terms are needed (e. g. an extra term Dc 2 is added) [START_REF] Jenkins | Viscosity B-Coefficients of Ions in Solution[END_REF].

According to Gurney [START_REF] Gurney | Ionic processes in solution[END_REF], ions having positive value of viscosity coefficient, B > 0, increased the viscosity of aqueous solutions, were deemed to be "structure making". Ions having negative value of viscosity coefficient, B < 0, decreasing the viscosity of water, were called as "structure breaking". It was supposed that a "water structure" (which can be thought as a dynamic fluctuating hydrogen bond network) existed, and that this was specifically affected by electrolytes.

Ions dedicated to be "water-structure makers" were called "kosmotropes", whereas, "water-structure breakers" were called "chaotrops".

The term structure-making and structure-breaking was used for many years. However, it is somewhat misleading. According to recent studies, it was shown that at least monovalent ions do not influence the structure of water beyond the first hydration shell [START_REF] Collins | Ions in water: Characterizing the forces that control chemical processes and biological structure[END_REF][START_REF] Smith | The Effects of Dissolved Halide Anions on Hydrogen Bonding in Liquid Water[END_REF][START_REF] Zhang | Interactions between macromolecules and ions: the Hofmeister series[END_REF][START_REF] Sala | Specific ion effects in aqueous eletrolyte solutions confined within graphene sheets at the nanometric scale[END_REF]. Therefore, there is no significant long-range structuring of water due to the presence of ions.

Overview of the classical theories

Extensive work devoted to the study of electrolyte bulk solutions, colloidal systems within bulk or on the interface has allowed the development of theories that try to explain the behavior of the solutions containing anions. Here we will discuss the different theories which describe the intermolecular interactions present in salt solutions (from electrolyte to colloid systems). These simplified models or "classical theories", consider the solvent as a continuum characterized by dielectric constant [START_REF] Barthel | Physical Chemistry of Electrolyte Solutions: Modern Aspects[END_REF].

First we will briefly overview the electrolyte (salt) solutions. The electrostatic Coulomb forces are considered to be the main interactions between ions in water. In this context electrostatic self energy will be introduced.

Next we will discuss theories developed for the more complex colloid solutions. First one is called electric double layer (EDL) theory. It describes the electrostatic forces between two interacting colloidal particles separated by the electrolyte medium. Later, the modified version of this model, DLVO theory, was proposed. In this theory, the new term, attractive van der Waals forces are introduced on top of the repulsive electrostatic contribution. We will then introduce the Lifshitz theory, which also takes into account other non-electrostatic forces such as Keesom and Debye potentials.

All these well-known theories and concepts can be easily traced in physical chemistry textbooks, as, for example, ref. [START_REF] Berry | Physical Chemistry[END_REF][START_REF] Ninham | Molecular Forces and Self Assembly[END_REF].

Electrostatic forces in the electrolytes

3.3.1.a Coulomb forces

In classical theory of electrolyte solutions the first assumption is that ionic interactions between the colloid particles could be described by Coulomb electrostatic interactions only:

( ) = ± (I-11)
where z 1 e and z 2 e are the charges of the ions, r is the interionic distance, ε 0 is the permittivity in vacuum and ε r is the static dielectric constant of solvent. The ions are considered as point charges and r is assumed to be far beyond larger than their actual physical size.

In the electrolyte solution, the Coulomb forces are screened by cooperative effects of neighboring ions. Thus, the potential of mean force between two ions in an electrolyte solution can be approximated as:

( ) ∝ ( ) (I-12)
where D is inverse of the Debye screening length:

= ⁄ (I-13)
T, k B , e, and are the absolute temperature, Boltzmann constant, unit charge, and number density of ions, while ν 1 and ν 2 are the stoichiometric coefficients for the specific electrolyte respectively.

3.3.1.b Self energy

The self energy is the energy required to bring in an ion from "infinity" (no interaction) to a spatial location r in the surrounded medium (i.e. dielectric mideum) [START_REF] Wang | Fluctuation in electrolyte solutions: The self energy[END_REF][START_REF] Wang | Effects of image charges on double layer structure and forces[END_REF]. The self-energy consists of two terms electrostatic and nonelectrostatic contributions.

Here an electrostatic term is introduced, known also as Born self energy, in order to present the electrostatic forces in the solution of electrolytes. Assuming that the charge e ν located on the surface of a sphere of radius a, the electrostatic self energy can be described as [START_REF] Ninham | Molecular Forces and Self Assembly[END_REF]:

= ( - ) (I-14)
For the dilute electrolyte ( D a << 1) this equation can be simplified as follows:

= (I-15)
The Equation (I-15) gives Born self energy of an ion immersed in a dielectric medium.

However, one should keep in mind that in addition to the electrostatic Born self free energy, there is the nonelectrostatic contribution, the dispersion self free energy, which has a significant contribution to the self energy of an ion [START_REF] Boström | Dispersion Self-Free Energies and Interaction Free Energies of Finite-Sized Ions in Salt Solutions[END_REF][START_REF] Boström | Contributions from Dispersion and Born Self-Free Energies to the Solvation Energies of Salt Solutions[END_REF].

Self energy can be used to estimate solubility by comparing the free energies of a solid ionic crystal with the free energies of transfer of ions to the solute [START_REF] Boström | Contributions from Dispersion and Born Self-Free Energies to the Solvation Energies of Salt Solutions[END_REF][START_REF] Rashin | Reevaluation of the Born model of ion hydration[END_REF].

The balance of forces in associated colloids

3.3.2.a The electrostatic double layer (EDL)

The force between two colloidal particles can be considered using the double-layer model. It is the basis for the later developed DLVO theory (see subchapter 3.3.2.b), which still remains the core of colloid science.

The spontaneous charging of the particle surface in a liquid medium results in the formation of a double layer. The EDL can be defined as follows: "An electrical double layer is a nonhomogeneous region of finite thickness containing significant variations in charge density across its thickness which consequently produces a potential drop across this dimension; the non-homogeneity invariably arises as a consequence of the competition between entropy and energy effects in the system's attempt to minimize its free energy" [START_REF] Macdonald | Comparison and discussion of some theories of the equilibrium electrical double layer in liquid electrolytes[END_REF]. There are several theories that tried to describe the EDL of colloids. First, Helmholtz proposed that two layers of opposite charge formed at the electrode/electrolyte interface and were separated by a small distance, H, where the electrical potential (Ψ s ) decreases linearly away from the surface of the bulk, see the Solving the Poisson-Boltzmann equation that is used to describe the interaction energy between the ions within the electric field, and accounting the induced osmotic pressure due to different ionic densities within the EDL and bulk, the force per unit area ( D l > 1) can be described as follows [START_REF] Ninham | Molecular Forces and Self Assembly[END_REF]:

( ) = ƒ( ) (I-16)
The prefactor depends on the assumption of the constant charge or potential on the surface.

More details about EDL theories can be found in following references [START_REF] Grahame | The Electrical Double Layer and the Theory of Electrocapillarity[END_REF][START_REF] Ninham | Electrostatic potential between surfaces bearing ionizable groups in ionic equilibrium with physiologic saline solution[END_REF][START_REF] Parsons | The electrical double layer: recent experimental and theoretical developments[END_REF]

3.3.2.b The DLVO theory

In the light of what has been discussed above, it is clear that, electrostatic potential does not reflect any specific electronic property of the molecules/ions but only their net charges and stoichiometric coefficients (i.e., monovalent salts such as KBr and LiAc should behave identically).

Therefore, the theory that would include the contribution which accounts for the stability of colloid suspensions via van der Waals attractive interactions was introduced in 1941 by Derjaguin and Landau [START_REF] Derjaguin | Theory of the stability of strongly charged lyophobic sols and of the adhesion of strongly charged particles in solutions of electrolytes[END_REF]. Independently, Verwey and Overbeek obtained the same results in 1948 [START_REF] Verwey | Theory of the stability of lyophobic colloids[END_REF]. In the DLVO theory two types of forces were considered to act between two like-charged colloidal particles (Figure I-7):

(i) repulsive: electrostatic (long-range interactions),

(ii) attractive: van der Waals (short-range interactions).

Attractive interactions between two individual molecules in DLVO theory are attributed to the dispersion potential (also called London or van der Waals forces). They have a quantum mechanical nature and describe the interaction between two instantaneous dipoles and depend on the polarizability and ionization potential of the molecule. Van der Waals interactions are inherent to the ions, neutral molecules and solvents. Hamaker obtained attractive forces by pairwise summation of London forces between atoms. For the two particles modeled as planar interfaces at the distance l, potential of interaction per unit area is [START_REF] Ninham | Molecular Forces and Self Assembly[END_REF]:

( ) = - (I- 17
)
where A is the Hamaker constant. However DLVO theory still could not account for ion specificity. Measured forces between cationic bilayers [START_REF] Pashley | Direct measurements of surface forces between bilayers of doublechained quaternary ammonium acetate and bromide surfactants[END_REF] or osmotic forces in lamellar phases [START_REF] Tsao | Osmotic stress measurements of dihexadecyldimethylammonium acetate bilayers as a function of temperature and added salt[END_REF] for bromide and acetate containing systems, showed that forces at the same salt concentration differ by a factor of 50. This can be due to [START_REF] Shubin | Electrical Double Layer Structure Revisited via a Surface Force Apparatus: Mica Interfaces in Lithium Nitrate Solutions[END_REF][START_REF] Israelachvili | Measurement of the hydrophobic interaction between two hydrophobic surfaces in aqueous electrolyte solutions[END_REF][START_REF] Ninham | The Rideal Lecture. Vesicles and molecular forces[END_REF]:

(1) The DLVO theory does not take into account the other forces of nonelectrostatic nature (many body forces) such as permanent dipole-dipole (Keesom) and induced dipolepermanent dipole (Debye)

(2) Attractive forces were treated assuming that an intervening liquid behaves the same way as bulk liquid, exhibiting the same properties up to the surface

(3) Surfaces considered to be molecularly smooth (4) The number of fitting parameters attributed to the EDL theory increases the number of assumptions necessary to treat the data [START_REF] Tanford | Theory of micelle formation in aqueous solutions[END_REF] The electrostatic and quantum mechanical forces can be treated separately [START_REF] Israelachvili | Intermolecular and Surface Forces[END_REF] The medium is consider as continuum These are the reasons why the DLVO failed at the concentration higher than 0.1 M.

3.3.2.c Lifshitz theory

In order to fix it, the next model called Lifshitz theory took into account all many body forces. This theory, developed in 1955, is based on DLVO theory and retains its assumptions:

electrical and quantum mechanical forces can be treated independently, surfaces of the interacting particles are molecularly smooth, an intervening liquid has bulk liquid properties up to the surface.

However, in this theory, the calculation of the attractive forces is done using quantum field theory.

This theory accounts for all nonelectrostatic forces. They are many body dipole-dipole, dipoleinduced dipole and induced dipole-induced dipole forces. They are temperature dependent and cannot be derived from summation of two-body molecular potentials.

The extension of Lifshitz theory, developed by Dzyaloshinski, Lifshitz and Pitaevski [START_REF] Dzyaloshinskii | The general theory of van der Waals forces[END_REF] including the interactions of the particles in the intervening liquid medium, invokes the complex mathematical analysis of quantum field theory. The attempts to simplify this model resulted in the collapse of the whole theory to a semiclassical theory [START_REF] Ninham | Molecular Forces and Self Assembly[END_REF][START_REF] Kunz | The present state of affairs with Hofmeister effects[END_REF].

Even though this theory failed to comprehensively describe the contribution of all forces involved in stabilizing colloidal systems (note that it is still based on main assumptions of DLVO theory), it provides the first insight into the understanding of the long-range forces responsible for the molecular recognition, where many body potentials seem to play the main role.

3.4.What is missing in classical theories? Important interactions

We recall that standard classical theories of colloidal and physical chemistry do not involve ion specificity and fail to predict the behavior of the system for salt concentrations higher than 0.1 M. The first attempts to include specific ion effects by accounting dispersion many body forces were done introducing the Lifshitz theory. Nevertheless, the calculated difference in forces for various salts was still very weak. Hereupon modifications of Lifshitz theory [START_REF] Ninham | Ion Binding and Ion Specificity: The Hofmeister Effect and Onsager and Lifshitz Theories[END_REF] as well as the later studies [START_REF] Boström | Surface Tension of Electrolytes: Specific Ion Effects Explained by Dispersion Forces[END_REF][START_REF] Boström | Ion Specificity of Micelles Explained by Ionic Dispersion Forces[END_REF][START_REF] Boström | The Influence of Ionic Dispersion Potentials on Counterion Condensation on Polyelectrolytes[END_REF] have been focused on describing the Hofmeister effects with refined calculations on many body forces.

Hydration of the ion also plays an important role suggesting that hydration forces are important. Moreover, as it was shown later [START_REF] Collins | Why continuum electrostatics theories cannot explain biological structure, polyelectrolytes or ionic strength effects in ion-protein interactions[END_REF], the water structure around the ion should be considered in order to give better estimation of the ion specificity.

Another approach widely used for a last decades to study the molecular systems is computational simulations. The most famous is molecular dynamic (MD) simulations. It gives the opportunity to follow the mechanisms and dynamics of the molecular assembly systems at an atomistic level. The first MD simulations also emphasized the importance of ion polarizability [START_REF] Jungwirth | Specific Ion Effects at the Air/Water Interface[END_REF].

Nevertheless, some resent studies [START_REF] Lund | Specific Ion Binding to Macromolecules: Effects of Hydrophobicity and Ion Pairing[END_REF][START_REF] Mason | Specificity of Ion-Protein Interactions: Complementary and Competitive Effects of Tetrapropylammonium, Guanidinium, Sulfate, and Chloride Ions[END_REF][START_REF] Horinek | Rational design of ion force fields based on thermodynamic solvation properties[END_REF] showed that MDs can provide ion specificity without inclusion of polarizability, using nonpolarizable force field. However, in these cases Lennard-Jones parameters that describe system accounting repulsive and attractive forces should be chosen carefully.

The theory that would describe ion specific effects has to account the following features:

(1) The charge density of the ion, defined as the electric charge per unit volume, i.e. ion size, is the most important parameter that affects its properties [START_REF] Collins | Ions in water: Characterizing the forces that control chemical processes and biological structure[END_REF]. The main challenge in this case is to define the proper ion radius. It can be treated as radius delivered from the crystal structure or effective radius, which includes the first hydration shell. This is not a trivial question and there is no final consensus on this issue. Besides, when the ion is polyatomic the charge density is no longer homogenously distributed, this introduces a higher level of system complexity.

(2) Considering the water-solid interface, an intriguing result was reported by Lund et al. [START_REF] Lund | Specific Ion Binding to Macromolecules: Effects of Hydrophobicity and Ion Pairing[END_REF][START_REF] Lund | Specific Ion Binding to Nonpolar Surface Patches of Proteins[END_REF] where they showed that hydrophobic anions interact to the higher extent with hydrophobic surface of the macromolecule through hydrophobic attraction, whereas hydrophilic anions interact more favorably via electrostatic forces with cationic surfaces.

Hence electrostatic effects may not always be dominant, the chemical structure and geometry of the whole system can even overcome electrostatic repulsions.

(3) Ion specific effects cannot be characterized just by well-defined ion specific parameters that would describe properties in solution. The properties of the ions is strongly influenced by their environment, i.e. headgroups [START_REF] Vlachy | Hofmeister series and specific interactions of charged headgroups with aqueous ions[END_REF] or counterions [START_REF] Mason | Specificity of Ion-Protein Interactions: Complementary and Competitive Effects of Tetrapropylammonium, Guanidinium, Sulfate, and Chloride Ions[END_REF] in their vicinity.

This fact can also explain why the order in the Hofmeister series slightly differs between various sources. For example, Yang [START_REF] Yang | Hofmeister effects: an explanation for the impact of ionic liquids on biocatalysis[END_REF] in his review noticed that depending on the enzymes, their activity in salt solution can change following or not following Hofmeister series, depending on the nature of enzymes. The order of the Hofmeister series can be reversed by slight variation of peptide properties by capping one of the functional group [START_REF] Paterová | Reversal of the Hofmeister Series: Specific Ion Effects on Peptides[END_REF].

(4) It follows from the foregoing that the structure and chemical composition of the macromolecules should be known in order to be able to estimate any Hofmeister effects.

A number of recent works [START_REF] Lund | Specific Ion Binding to Nonpolar Surface Patches of Proteins[END_REF][START_REF] Paterová | Reversal of the Hofmeister Series: Specific Ion Effects on Peptides[END_REF][START_REF] Vrbka | Specific Ion Effects at Protein Surfaces: A Molecular Dynamics Study of Bovine Pancreatic Trypsin Inhibitor and Horseradish Peroxidase in Selected Salt Solutions[END_REF], especially in MD simulations, emphasize that ionprotein interactions cannot be described by simplified models of a uniformly charged protein "surface". The surface of the protein accessible for the ions and water should be estimated in order to predict the specific interactions and therefore analyze the Hofmeister effects.

(5) Finally, ion specific effects are salt concentrations and pH dependent (especially for biomolecules such as proteins and enzymes) [START_REF] Kunz | Specific Ion Effects[END_REF]. As we discussed previously at a salt concentration lower than 0.1 M, electrostatic forces dominate, whereas for higher concentrations due to the screening of the electrostatic interactions, ion specificity is much more pronounced.

Accounting all these features that should be taken into account to characterize the ion specific effects, it seems that all simplified theories would be rather helpless. However the opposite was proven by Collins and his "concept of matching affinities" [START_REF] Collins | Ions in water: Characterizing the forces that control chemical processes and biological structure[END_REF][START_REF] Collins | Ions from the Hofmeister series and osmolytes: effects on proteins in solution and in the crystallization process[END_REF].

The latest attempts to shed light on Hofmeister effects

Relatively recently, Collins proposed a simple theory that provides a better understanding of a multitude of experimental results in biology and colloidal science. For the ion classification, Collins uses terms kosmotropes and chaotropes depending on the sign of Jones-Dole viscosity B coefficient (see subchapter 3.2). Note that here, the used terms describe the affinity of water for the ion and not the water-structuring around it. Rather kosmotropes and chaotropes are understood as a characterization of the "degree of hydration". Small ions of high charge density (kosmotropes, shown above the line) bind nearby water molecules tightly, thus immobilizing them, whereas large monovalent ions of low charge density (chaotropes, shown below the line) actually "free up" nearby water molecules, allowing more rapid motion than in bulk solution, see The law of matching affinities is based on the observation that, for single valence ions, the oppositely charged ions with alike size tend to come together in solution to form contact ion pairs whereas oppositely charged ions with differing size tend to stay apart [START_REF] Collins | Ions in water: Characterizing the forces that control chemical processes and biological structure[END_REF][START_REF] Collins | Ions from the Hofmeister series and osmolytes: effects on proteins in solution and in the crystallization process[END_REF]. The concept of matching water affinities explains the different types of interactions that:

"Cations and anions form direct stable ion pairs if their hydration enthalpies (considered to be a measure of "water affinities") match" [START_REF] Salis | Models and mechanisms of Hofmeister effects in electrolyte solutions, and colloid and protein systems revisited[END_REF]. Two hard ions of high charge density being strongly hydrated of opposite charge have a strong reciprocal attraction. Coming close, they form direct ion pairs liberating the hydration spheres between them in the bulk solution. For the soft ions of low charge density the association of opposite charged ions is favorable due to the decrease of ion-water contact. Hence, the weakly hydrated chaotropic ions can also form direct ion pairs expelling also the hydration water between them. The interaction of one hard and one oppositely charged soft ion is then straightforward: here, the attraction by the soft ion is not strong enough so that the hard ion loses its hydration shell. As a consequence, a soft/hard ion pair is always separated by water and cannot form strong ion pairs, see This concept can be applicable to explain many phenomena in Hofmeister effects [START_REF] Kunz | Specific Ion Effects[END_REF], i.e.

why hard anions are usually considered as salting-out and soft anions as salting-in, whereas for cations it is just the other way around. Soft cations are salting-out and "hard cations" salting-in.

Besides, MD similations, done in the group of Jungwirth, confirm Collins' law of matching water affinity [START_REF] Vrbka | Quantification and rationalization of the higher affinity of sodium over potassium to protein surfaces[END_REF][START_REF] Jungwirth | Ions at Aqueous Interfaces: From Water Surface to Hydrated Proteins[END_REF]. Encouraging results were obtained by Vlachy et al. [START_REF] Kunz | Specific Ion Effects[END_REF][START_REF] Vlachy | Hofmeister series and specific interactions of charged headgroups with aqueous ions[END_REF]. Using a computational approach they succeeded in classifying the headgroups in a Hofmeister-like series in order to estimate the contribution of the nature of the headgroups on the ion specific effects. It was shown that according to the law of matching water affinity, hard headgroups tend to form contact ion pairs with hard counterions, whereas for the soft headgroups ion pairing with poorly hydrated soft counterions is preferable, Figure I-10. However, one should remember that this concept of matching water affinities is a very simplified one that should only be taken as a rule of thumb. In real systems the additional effects, implementing more complex interactions need to be considered [START_REF] Leontidis | Effects of Monovalent Anions of the Hofmeister Series on DPPC Lipid Bilayers Part II: Modeling the Perpendicular and Lateral Equation-of-State[END_REF].

The latest studies

In this last part of our introduction we would like to discuss some of the latest studies on Hofmeister effects including theoretical, computational and experimental approaches.

For example, Ruckenstein and Huang [START_REF] Ruckenstein | Specific ion effects on double layer forces through ion hydration[END_REF] in their study discovered how does the nature of an electrolyte influence to the strength of repulsion between two charged plates. Conversely to the classical DLVO theory, their theory considers the hydration of the ions and reflects the specificity for the ions of opposite signs to that of the surface charge. As was reported, the strength of the repulsive force between two charged plates for different anions present in the solution follows the order OH -< Ac -< F -∼ Cl -< Br -< H 2 PO 4 -< I -< NO 3 -< ClO 4 -. And for the cations follows the order Another work by Song and co-workers [START_REF] Song | Ion specific effects: decoupling ion-ion and ion-water interactions[END_REF] which demonstrates how complex ion-ion interactions, originating from electrostatic interactions, and ion-water interactions manifest themselves in the Hofmeister effects. It was shown that analyzed water diffusion constant changing (at least partially) due to electrostatic interactions between the ion and water. It also was found that energy attributed to the perturbation of the hydrogen bonds of water is important for the charge density at water-air interface. It will be directly influenced by ion nature.

In addition to the theoretical studies, Hofmeister effects also were investigated by experimental and computational approaches.

Willott et al. [START_REF] Willott | Anion-Specific Effects on the Behavior of pH-Sensitive Polybasic Brushes[END_REF] investigated the ion specific effects on the polymer brushes 1 with different hydrophobicity. First, it was shown that the behavior of these polymer brushes in the aqueous solution is strongly dependent on the salts concentration. Second, these systems as well are ion dependent. The strongly kosmotropic acetate anions display low affinity for the hydrophobic polymers, and largely unscreened electrosteric repulsions allow the brushes to remain highly solvated at higher acetate concentrations. The mildly chaotropic nitrate and strongly chaotropic thiocyanate anions exhibit a polymer hydrophobicity-dependent affinity for the brushes. Recently the group of Chaimovich [START_REF] Lima | Effect of Counterions on the Shape, Hydration, and Degree of Order at the Interface of Cationic Micelles: The Triflate Case[END_REF] published a work where they discussed the impact of counterions on the dodecyltrimethylammonium micelles focusing on the properties of triflate (Tf -).

Combining experimental and computational results they showed that the in comparison to other micelle types, the DTATf micelles have a higher average number of monomers per aggregate, an uncommon disk-like shape, smaller interfacial hydration, and restricted monomer chain mobility.

These results demonstrate that the -CF 3 group in Tf -was directly responsible for the observed shape changes by decreasing interfacial hydration and increasing the degree of order of the surfactant chains in the DTATf micelles.

Concerning the systems of higher level of complexity such as rod-like micelles, the effect of different anions on the rheological behavior of cetyltrimethylammonium-salicylate wormlike micelles was investigated by Alkschbirs et al. [START_REF] Alkschbirs | Effects of some anions of the Hofmeister series on the rheology of cetyltrimethylammonium-salicylate wormlike micelles[END_REF]. Such wormlike micelles, being a dynamic system, exhibit interesting viscoelastic properties due to their ability to break and reform within a characteristic lifetime. Hence the each system can be described in terms of time required for breaking or reptation. This time for each process calls relaxation time. In the work of Alkschbirs et al. it was shown, that the systems in the presence of salts present shorter relaxation times due to the charge neutralization associated with the adsorption of the anions on the micelles surface. The magnitude of the effect depends on the nature of the anions, which follows the Hofmeister series.

In our work we elucidate the interactions contributing to ion specific effects at molecular and bulk solution levels by carrying out complementary experiments and simulations to determine the effect of systematic changes of amphiphile structure, with a particular focus on headgroup structure and counterion type, on their aggregate properties. The goal of this comprehensive analysis is to get insight into the effect of interfacial ionic and molecular interactions on aggregate properties (via physical characterization) based on determination of interfacial water, headgroup, and counterion concentrations by experiment (chemical trapping) and by simulation (molecular dynamics and density functional theory, MD/DFT). The analysis of the relationship between the properties of the ions such as hydration, hydrogen bonding, electrostatic, ion pairing, and polarization interactions and the behavior of amphiphile aggregates in the solution gave us better understanding of the interactions contributing to the balance of forces that control the physical properties of solutions of ionic amphiphiles.

As was discussed above, such ion property as ion size and polarizability are directly related to the dispersion forces that seem to play an important role in the stabilization of the colloids.

Moreover, the size and morphology of the ion can have some specific steric effect on the structure of aggregates. Hydration free energy, estimated from Born self energy, gives an information about interactions between ions and water molecules. Ions with higher negative values of free hydration energy (hydrophilic) interact with water more favorably forming the hydrated ion spheres, whereas ions having lower negative hydration energy are weakly hydrated and are called hydrophobic. The hydration number is another measure of ion hydration. The electron density of the ion, that is one of the most important characteristic of the ion specificity, can be correlated with pKa values of the conjugated acid. Finally, as would be discussed later, lyotropic number is directly related to the Hofmeister series. Correlation between these anions properties and the properties of self-assembly of the cationic gemini surfactants would be discussed in this work.

Chapter II. Synthesis and characterization of cationic gemini surfactants Introduction

As was described in Chapter I, the influence of ion specific effects on the self-assembly process of amphiphile molecules has remained unsolved challenge for a long time. Even though many experimental and computational studies were performed in order to understand the contribution of ion properties to the Hofmeister effects, the unifying theory is still missing and thus the Hofmeister series yet represent an appealing challenge in colloid and interface science.

Simplifying longer, they behave like catanionic amphiphile [START_REF] Edlund | Phase Behavior and Phase Structure for Catanionic Surfactant Mixtures: Dodecyltrimethylammonium Chloride-Sodium Nonanoate-Water System[END_REF][START_REF] Koehler | Microstructure and Dynamics of Wormlike Micellar Solutions Formed by Mixing Cationic and Anionic Surfactants[END_REF] or mixed micelle [START_REF] Oelschlaeger | Synergistic effects in mixed wormlike micelles of dimeric and singlechain cationic surfactants at high ionic strength[END_REF]; in the later case, counterion tail contributes to the hydrophobic effect.

Another type of cationic surfactant that was investigated is tertiary 10-2-10 X gemini (X = I -, Br -, Cl -). For these gemini, one of the two methyl groups from a quaternary gemini surfactant headgroups have been replaced by a proton as shown in Figure II-1. These gemini were compared with the quaternary one since the variation of the hardness/softness of the headgroups could change the balance of forces that control the self-assembly. Weakly hydrated ions should form ion pairs with quaternary ammonium headgroups more easily than strongly hydrated ions and the interactions should follow a Hofmeister series, as is generally observed with ionic surfactants. However, the headgroups with tertiary ammonium may be more hydrated and may interact with the counterions by hydrogen bonds, which would be strongest for the small anion, e.g., F -, and the Hofmeister series might be reversed.

Synthesis and characterization of 10-2-10 gemini

In this paragraph we focus on the synthesis and characterization of cationic 10-2-10 gemini surfactants with various counterions. The details of the synthetic procedures will be described in the experimental section.

General synthetic procedure for quaternary 10-2-10 gemini compounds with different counterions

Quaternary ammonium 10-2-10 gemini surfactant with different counterions were synthesized according to the procedure reported for 14-2-14 X salts by Manet et al. [START_REF] Manet | Counteranion Effect on Micellization of Cationic Gemini Surfactants 14-2-14: Hofmeister and Other Counterions[END_REF] with modification, due to the high solubility of 10-2-10 amphiphile molecules compared to their 14-2-14 analogue.

First of all 10-2-10 Br was synthesized by a dialkylation reaction of N,N,Nʹ,Nʹtetramethylethylenediamine (TMEDA) with decylbromide. This compound was then used for the counterion exchange reactions. The procedure of salt metathesis reactions is closely related to the affinity of the anion to the headgroup, as well as the acidity of the acid. Thus, we have used three ) surfactant were formed before the complete ion exchange. In the case of acetate and fluoride anions their silver salts are commercially available, thus the ion exchange can take place directly leading to the formation of the corresponding 10-2-10 X (Figure II-2, synthetic route B). Thus, for route A and B, ion exchange takes place, and a precipitate of AgBr is observed. After AgBr was removed, amphiphiles having X counterions were isolated and purified. the deconvolution of the peak and the sensitivity of the machine, these results show that the experimental stoichiometry of Br -is in good agreement with the theoretical one. The excess of carbon typically comes from atmospheric contamination. Furthermore, the position of the N1s peak (around 402 eV) strongly suggests that all nitrogen atoms in this compound are in their ammonium form [START_REF] Mittal | Acid-Base Interactions: Relevance to Adhesion Science and Technology[END_REF] (Figure II-4).

Synthesis of 10-2-10 I

10-2-10 I was synthesized according to the same procedure used for the synthesis of 10-2-10 Br. TMEDA was dialkylated with 1-iododecane in acetonitrile. The compound was obtained with product yield ~80%. Its purity was checked by 1 H NMR analysis.

Counterion exchange for the ions of weak acids (pKa > 3), using the synthetic route A

The synthetic approach used for anion exchange with weak acids is summarized in Scheme II-2. As mentioned before, for counterion exchange in the case of weak acids (HX, pKa > 3), a one-pot reaction was carried out, where Ag 2 CO 3 was used as a reagent providing the counterion exchange between HX and 10-2-10 Br. Intermediate products were not isolated. To obtain desired 10-2-10 carboxylate, Ag 2 CO 3 (0.75 eq.) and acid (1 eq.) were mixed in MeOH (75 mL) under continuous stirring at 40 °C during 30 min. After 10-2-10 Br (0.4 eq.) solubilized in MeOH (75 mL) was added. The reaction mixture was stirred at the 40 °C for 2 days. Aluminum foil was used to protect the reaction mixture from light in order to prevent photoreaction. The advantage of this method is that there is no need to isolate the silver salt before the exchange reaction with gemini salt. Recycling delay was increased to obtain good integration of the peaks corresponding to the carboxylate moieties. It is during this delay that the excited sample returns to its equilibrium state and the optimum delay is sample dependent. Compared with the gemini moiety, the carboxylate counterions are small molecules and their relaxation time is higher due to their fast motion. One can see that with increasing the size of the anion (increasing the length of the hydrophobic chain), less time for the recycling delay is required.

The completion of the reaction was checked using a method that is based on the photoreduction of AgBr to silver metal and molecular bromide as follow: 25 mg of obtained powder for C1-C6 and 10 mg for C8 was dissolved in 0.5 mL of MeOH and 20 mg of AgAc was added in this solution, and the mixture was irradiated with the natural light for 1 day. If the ion exchange is not complete, mixture will become black due to the reduction of AgBr (formed by ion exchange between 10-2-10 Br and silver acetate) to black silver metal and molecular bromide. If the exchange reaction was complete, the test will be negative, as no black silver metal can be obtained due to the absence of AgBr in the mixture.

Counterion exchange for the ions of weak acids (pKa > 3), using the synthetic route B

The synthetic procedure for the counterion exchange of week acid ions, made with synthetic

Synthesis of 10-2-10 Ac

To obtain 10-2-10 Ac, a counterion exchange was performed by the reaction between 10-2-10 Br and silver acetate. Formation of AgBr precipitate drives the reaction and provides the formation of 10-2-10 gemini salt of Ac, in about 70%.

Synthesis of 10-2-10 F

To

Counterion exchange for the ions of strong acids (pKa > 3), using the synthetic route C

For exchange reactions with strong acids 10-2-10 Ac was used instead of 10-2-10 Br as shown in Scheme II-5. Since acetic acid is a weak acid, it is easy to replace the acetate anion with an 64 anion from a strong acid (HNO 3 , H 3 PO 4 , HCl, TFA, MeSO 3 H and even HCOOH). The general procedure for all the anions, with exception of H 2 PO 4 -and HCOO -, is described below.

Scheme II-5. Synthesis of 10-2-10 gemini with counterions originated from a strong acids.

To a methanol solution of 10-2-10 Ac, 2 or 3 equivalents of HX were added, and the mixture was stirred at 40 °C for 2 h. The reaction was monitored by 1 H NMR by the disappearance of the signal attributed to acetate at 1.90 ppm. The final product was precipitated by adding less polar solvent in the methanol solution (Table II-5) and dried under vacuum.

The purity of compounds was checked by 1 H NMR. No significant change was observed by 1 H NMR after the ion exchange. The anion cannot be observed by 1 H NMR, and the signals attributed to the gemini amphiphile are similar to those of the corresponding bromide salt (Figure II-3). Thus, the 1 H NMR spectra of these are not shown here.

Synthesis of 10-2-10 H 2 PO 4 .

Since 10-2-10 H 2 PO 4 was poorly soluble in MeOH the procedure described above was modified. Thus, a methanol solution of H 3 PO 4 85% wt was added dropwise to a methanol solution of 10-2-10 Ac under continuous stirring at room temperature. A white precipitate of 10-2-10 H 2 PO 4 was formed immediately. The reaction mixture was stirred overnight to complete the reaction. The completion of the reaction was checked by 1 H NMR using D 2 O as a solvent. The disappearance of the signal attributed to acetate anion peak at 1.90 ppm indicated that reaction was complete. The precipitate was filtered and dried under vacuum line and by lyophilization.

The compound was characterized by 1 H, 13 C and 31 P NMR. By phosphorous NMR it could be determined if the phosphate was mono or dianionic. For this experiment we took monosodium and disodium phosphate salts which give a signal at 3.1 and 5.7 ppm in 31

Synthesis 10-2-10 C1 (formate)

The pKa of formic acid is 3.75. It is a weak acid, but stronger than acetic acid (pKa = 4.75).

Hence method C can be used for an anion exchange with some modifications of the typical synthesis.

To the methanol solution of 10-2-10 Ac was added formic acid in molar ratio 1:20; this large excess of formic acid was deliberately used to carry out the complete exchange of Ac. The reaction mixture was stirred at 40 °C during 24 hours, and monitored by 1 H NMR. The disappearance of the acetate peak at 1.90 ppm indicated that exchange reaction was complete. The solvent was removed under vacuum and the excess of formic acid was removed by heating the mixture at 55 °C under high vacuum over 5 days. The precipitate was solubilized in a small amount of acetone at 60 °C and kept at -20 °C for 1 day. The obtained crystals were filtered and dried under vacuum. The yield was 87%.

The stoichiometric ratio and purity of the compound was checked by proton NMR. It is important to notice that the recycling delay (D 1 ) was increased up to 25 s in order to obtain the correct integration of the peaks. The need to increase the recycling delay suggests that the relaxation time is higher for formate anion than for gemini amphiphile molecule. It can be explained by the small size of the anion that makes it quite mobile and request more time for nuclei to return from excited to ground state. We have investigated tertiary ammonium gemini surfactants with iodide, bromide and chloride as counterions. The synthesis of these three compounds was performed using different procedures. Tertiary 10-2-10 I was synthesized in a similar manner to its quaternary ammonium counterpart. N,Nʹ-dimethylethylendiamine (DMEDA) was used instead of TMEDA, and dialkylated with 1-iododecane. The synthesis of 10-2-10 ter Br and 10-2-10 ter Cl proceeded through the tertiary amine formed in the first step with 1-bromodecane, which was then protonated with the corresponding acid in a second step.

Synthesis of 10-2-10 ter I

Since the iodide ion is a good leaving group, the synthesis of 10-2-10 ter I could be performed by dialkylation of DMEDA with 1-iododecane in acetonitrile at 40 °C as shown in Scheme II-6. strongly indicates that most of nitrogen is in ammonium form, the small peak around 399 eV indicates that there is also some unprotonated form that could be a result of incomplete alkylation Chapte II. Synthesis and characterization of cationic gemini surfactants [START_REF] Jungwirth | Specific Ion Effects at the Air/Water Interface[END_REF] with DMEDA, or from diprotonation of the tertiary gemini. However this compound could not be determined by proton NMR, hence its ammount is less than 3%.

Table II-

Synthesis of 10-2-10 ter Br

For the initial synthesis of 10-2-10 ter Br we applied the same method as was used for 10-2-10 ter I, consisting of the dialkylation of DMEDA by 1-bromodecane. Unfortunately, the 1 H NMR indicated that the product contained an impurity with signals at 3.42 and 2.81 ppm. The compound was finally purified by repetitive recrystallization from CH 3 CN/MeOH, however the yield of the pure 10-2-10 ter Br was low (about 30%). Low yields of pure product and timeconsuming repeated recrystallizations convinced us to modify this synthetic procedure.

In a new procedure, the 10-2-10 amine compound was first prepared, and protonated with HBr.

Synthesis

Synthesis of 10-2-10 ter Cl

10-2-10 ter Cl was synthesized in a similar way to 10-2-10 ter Br by protonation of 10-2-10 amine with HCl. However, we considered that the starting material could contain traces of Br -ion from the initial synthetic reaction (Scheme II-7). Since HBr is a stronger acid than HCl and thus Br -ions cannot be easily replaced by Cl -anion, an additional method was used for purification.

To remove all traces of bromide, AgAc was used to precipitate any residual Br -ions. Formed AgBr was removed by filtration through celite. Being derived from a weak acid, the acetate anion will not react with 10-2-10 amine, as easily shown with proton NMR. The position of the proton peaks α to the nitrogen atom corresponds to those of the 10-2-10 amine: 2.82, 2.65 and 2.46 ppm for the spacer, α-CH 2 and α-CH 3 respectively. This in addition to the absence of acetate peak, allows us to conclude that the surfactant molecule stays in amine form.

The synthesis of 10-2-10 ter Cl was performed by adding HCl 37 wt% to a solution of 10-2-10 amine in acetone under stirring. The obtained powder was purified from silver salts by filtration over celite in MeOH and then crystallized from MeOH.

Proton NMR and XPS were used to check the structure and purity of the compound.

Obtained XPS results are compatible with theoretical values within experimental error (Table II-4). Chapte II. Synthesis and characterization of cationic gemini surfactants [START_REF] Horinek | Rational design of ion force fields based on thermodynamic solvation properties[END_REF] Silica and oxygen were the result of surface and atmospheric contamination and not from the compound. We can see that the amount of Br in the system is insignificant. On the Figure II-12 XPS spectra for N1s region is presented. One can see that the large peak at 402 eV corresponds to the ammonium in the gemini molecules indicating that 10-2-10 amine was protonated. However, similar to 10-2-10 ter I there is small peak at 399 eV indicating that small amount of the compound was left in amine form. Even so this compound cannot be detected by NMR that means that its amount is less than 3%.

2.Experimental section 2.1. Materials

Reagent grade solvents for syntheses were used as received. All chemicals were of analytical grade and used as received. Methanol (≥99.6%), acetonitrile (≥99.9%), acetone (≥99.5%) and diethyl ether (≥99.5%) were purchased from Sigma-Aldrich, deuterium oxide (99.96%) and methanol-D 4 (≥99.8%) purchased from Euriso-top. All other compounds were purchased from Sigma-Aldrich and used without further purification.

The 1 H and 13 C NMR spectra were recorded at 25 °C on a Brucker AC 300 FT at 300 MHz ( 1 H) and 75 MHz ( 13 C). Chemical shifts (δ) are reported in parts per million against referenced solvent signals.

X-ray photoelectron spectroscopy (XPS) was carried out on a ThermoFisher Scientific K-ALPHA spectrometer. Surface analysis was done with a monochromatized AlKα source (hν = 1486.6 eV) and a 200 micron spot size. A pressure of 10 -7 Pa was maintained in the chamber during analysis. The full spectra (0-1350 eV) were obtained with constant pass energy of 200 eV and high resolution spectra at constant pass energy of 40 eV. Charge neutralization was activated even for those conductive samples. High resolution spectra were fitted and quantified using the AVANTAGE software provided by ThermoFisher Scientific. Since XPS is only a semi-quantitative analysis the error of the experiment can be count as 5 to 10% depending on the sensitivity factors which were used to reach quantification.

Synthetic procedures for quaternary 10-2-10 gemini surfactants

10-2-10 Br. To an acetonitrile solution (120 mL) of TMEDA (15 mL, 99.4 mmol) was added 1-bromodecane in molar ratio 1:3 (62.2 mL, 298 mmol). The reaction mixture was stirred at 80 °C under reflux for 48 h. After completion, the reaction mixture was cooled to 4 °C and the obtained precipitate of 10-2-10 Br was washed with acetone (1x500 mL), recrystallized from CH 3 CN and dried under vacuum and lyophilized. 10-2-10 Br was obtained as a white solid in 85% yield. (50 mL) in molar ratio 1:2.2. The reaction mixture was stirred at 80 °C under reflux for 48 h. The reaction mixture was cooled to 4 °C and the obtained powder was washed with diethyl ether and recrystallized from acetonitrile as follows: the compound was dissolved in 10 mL of CH 3 CN at 60 °C and cooled to room temperature, providing crystals. These crystals were filtered and dried under vacuum. The purity of 10-2-10 I was checked by 1 H NMR analysis.

H NMR (300 MHz, MeOD, 25˚C):

δ = 4.06 (s, 4H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 3.51 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.28 (s, 12H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 1.85 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + ), 1.42-1.31 (m, 28H, CH 3 - (CH 2 ) 7 -CH 2 -CH 2 -N + ),
1 H NMR (300 MHz, MeOD, 25˚C): 3 mmol) in molar ratio 1:2.5. The reaction mixture was protected from light with aluminum foil, and stirred at 40 °C until the completion of ion exchange. Reaction was followed by 1 H NMR, by the appearance of a signal at 1.90 ppm (integration 6H), assigned to the methyl groups of the acetate anion. Reaction completion was assessed by photoreduction of the obtained AgBr to silver metal and molecular bromide, as described in section 1.1.3. After completion, silver bromide and excess of silver acetate were removed by filtration through celite.

δ = 4.08 (s, 4H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 3.53 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.30 (s, 12H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 1.86 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + ), 1.43-1.31 (m, 28H, CH 3 - (CH 2 ) 7 -CH 2 -CH 2 -N + ), 0.90 (t, 6H, CH 3 -(CH 2 ) 9 -N + ).
δ = 3.94 (s, 4H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 3.42 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.21 (s, 12H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 2.14 (t, 4H, CH 3 -CH 2 -CH 2 -COO -), 1.83 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 - CH 2 -N + ), 1.68-1.56 (m, 4H, CH 3 -CH 2 -CH 2 -COO -), 1.41-1.31 (m, 28H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + ), 0.92 (m, 6H, CH 3 -(CH 2 ) 9 -N + and 6H, CH 3 -(CH 2 ) 2 -COO -).
δ = 3.95 (s, 4H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 3.42 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.21 (s, 12H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ),
After evaporation of the solvent under vacuum, the residue was dissolved in MeOH and filtered through celite, and the solvent was evaporated under vacuum until the dry residue was formed (to avoid the concentration of water from the solvent). The residue was dissolved in MeOH (5 mL) at 60 °C and the product was crystallized by adding 700 mL of acetone. 10-2-10 Ac was obtained as white crystals in 70% yield. 10-2-10 F. For 10-2-10 F synthesis 1 eq. of 10-2-10 I (2 g, 3.06 mmol) was mixed with 2.05 eq. of AgF (0.78 mg, 6.13 mmol) in MeOH (40 ml) and stirred at 40 °C for 2 hours. Proton NMR spectroscopy was used to check the purity of obtained compound.

H NMR (300 MHz, MeOD, 25˚C):

δ = 3.94 (s, 4H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + -(CH 3 ) 2 ), 3.42 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.20 (s, 12H, (CH 3 ) 2 -N + -(CH 2 ) 2 -N + -(CH 3 ) 2 ), 1.90 (s, 6H, CH 3 -COO -), 1.82 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 - N + -), 1.41-1.30 (m, 28H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 0.90 (t, 6H, CH 3 -(CH 2 ) 9 -N + -).
The purification of 10-2-10 F was performed by filtering the reaction mixture through celite and evaporation of methanol solution under vacuum. This step was repeated several times until the residue of 10-2-10 F will become white. This white precipitate was dissolved in 3 mL of acetone at 50 °C and cooled to 4 °C. Crystals were formed, filtered and dried under vacuum. It is important to note that, this compound cannot be dried by lyophilization, due to its decomposition under strong vacuum.

1 H NMR (300 MHz, MeOD, 25˚C): II-5). The product was precipitated by adding a low polarity solvent as described in Table II-5. The solid was washed (3 times) with the same solvent used to precipitate the product and dried under vacuum and by lyophilization. 10-2-10 X was obtained as white powder.

δ = 3.90 (s, 4H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 3.41 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.20 (s, 12H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 1.83 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + ), 1.42-1.31 (m, 28H, CH 3 - (CH 2 ) 7 -CH 2 -CH 2 -N + ), 0.90 (t, 6H, CH 3 -(CH 2 ) 9 -N + ).
Table II-5. Detailed information for the synthetic procedure for 10-2-10 X (X = NO 3 -, Cl -, TFA, MeSO 3 -).

Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level Introduction

In the previous Chapters we overviewed the studies on the Hofmeister effects of the different colloidal systems. Here, we describe the impact of the nature of counterions to the self-assembly of the cationic gemini surfactant molecules in aqueous solutions. Even though a similar study was done previously on the 14-2-14 gemini systems [START_REF] Manet | Counteranion Effect on Micellization of Cationic Gemini Surfactants 14-2-14: Hofmeister and Other Counterions[END_REF], the determination of the physical properties for 10-2-10 gemini systems was necessary in order to provide the information that is important for further investigation of the systems by other techniques (chemical trapping, Chapter IV; molecular modeling, Chapter V). Two types of counterions: inorganic counterions and alkyl carboxylates, along with two types of 10-2-10 gemini: quaternary ammonium gemini (10-2-10 X) and tertiary ammonium gemini with the same chain length (10-2-10 ter X, in which one of the methyl at the headgroup is replaced by a proton) were studied in order to investigate both the ion specific effects and the contribution of the structural properties of the headgroup to the self-assembly behavior.

Indeed, we expect that in addition to the properties of the counterions, the properties of the cation of gemini headgroup would influence the balance of the forces that determines the association between the ions (Figure III-1). More precisely, exchanging the methyl group by a proton would have an important effect on the delocalization of cationic charge of the headgroups and thus affect the interaction between the headgroups and the counterions.

To characterize the micellization process as well as micellar structure we used conductivity measurements to determine critical micelle concentration (CMC), ionization degree (α), and the free energy of micellization (ΔG°M). The aggregation number of the surfactant molecules (N) indicates how many monomers (gemini molecules) form a micelle on average and is measured using a fluorescence quenching technique in collaboration with Dr. Dario Bassani. In order to study the properties of 10-2-10 ter X we also used NMR and pH measurements.

Conductivity measurements

In this section we present the determination of the micellisation parameters such as CMC, ionization degree and free energy of micellization from the conductivity measurements.

Below the CMC, surfactants in solution are solubilized without forming persistent aggregates. Contribution to the conductivity comes from the mobility of cations and anions and increase linearly with surfactant concentration. Above the CMC, conductivity changes due to the increase of the concentration of charged micellar aggregates that have much lower mobility compared to non-cooperative ions. Thus, we obtain two different slopes in the plot of conductivity versus surfactant concentration that reflect the binary behavior of the system (before and after CMC). This allows us to estimate the ion mobility which decreases whin micellar aggregates form.

Surfactant properties estimated by conductivity measurements

Critical micelle concentration

The determination of the CMC was performed by two methods. First one is the Williams and

Phillips method [START_REF] Williams | The critical micelle concentration of sodium lauryl sulphate at 25°[END_REF], that is individuating the intersection of two straight lines in the graph of ). Among other methods to estimate the CMC [START_REF] Fujiwara | A temperature study on critical micellization concentration (CMC), solubility, and degree of counterion binding of α-sulfonatomyristic acid methyl ester in water by electroconductivity measurements[END_REF][START_REF] Garcia-Mateos | Critical micelle concentration determination in binary mixtures of ionic surfactants by deconvolution of conductivity/concentration curves[END_REF][START_REF] Manabe | Effect of alkanols on intermicellar concentration and on ionization of micelles[END_REF][START_REF] Pérez-Rodríguez | A Comparative Study of the Determination of the Critical Micelle Concentration by Conductivity and Dielectric Constant Measurements[END_REF], we chose the method proposed by Carpena et. al [START_REF] Carpena | Problems Associated with the Treatment of Conductivity-Concentration Data in Surfactant Solutions: Simulations and Experiments[END_REF], based on fitting the conductivity data to a nonlinear function obtained by direct integration of a Boltzmann type sigmoidal function. It was shown that this aproach is more adequate for the analysis of conductivity data than conventional and differential conductivity methods [START_REF] Carpena | Problems Associated with the Treatment of Conductivity-Concentration Data in Surfactant Solutions: Simulations and Experiments[END_REF][START_REF] Galgano | Micellar properties of surface active ionic liquids: A comparison of 1-hexadecyl-3-methylimidazolium chloride with structurally related cationic surfactants[END_REF][START_REF] Kumar | An electroconductivity study on degree of counterion binding or dissociation of a-sulfonatomyristic acid methyl ester micelles in water as a function of temperature[END_REF].

The fitting equation that was used to estimate experimental conductivity data is presented below:

( ) = ( ) + + ( -) (III-1)
Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level [START_REF] Lima | Effect of Counterions on the Shape, Hydration, and Degree of Order at the Interface of Cationic Micelles: The Triflate Case[END_REF] where x is the concentration of the compound, and F(0) is the conductivity at x = 0, A 1 and A 2 are the slopes of the pre and post-CMC regions of conductivity versus concentration (x) respectivly, x 0 is a centeral point of the transition and Δx is a width of this transition.

Fitting was done using Origin 9.1. For each individual set of experimental data, parameters F(0), A 1 , A 2 and x 0 were chosen as follows: F(0) = κ for water, A 1 = S 1 , A 2 = S 2 , x 0 = CMC and Δx was chosen from the type of the transition in the slope. The low (hight) values means an abrupt (gradual) transition. In our case Δx was chosen as around 0.1x 0 in the fitting parameters [START_REF] Carpena | Problems Associated with the Treatment of Conductivity-Concentration Data in Surfactant Solutions: Simulations and Experiments[END_REF][START_REF] Galgano | Micellar properties of surface active ionic liquids: A comparison of 1-hexadecyl-3-methylimidazolium chloride with structurally related cationic surfactants[END_REF].

.2. Ionization degree

In this work, we used two approaches to estimate the degree of counterions dissociation, or ionization (α). The most common way to determine ionization degree was initially proposed by Zana [START_REF] Zana | Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications[END_REF], α is a ratio in the slopes of the plot of κ versus C above and before the CMC:

= / (III-2)
However, it was shown in literature that this method overestimates the real values due to the approximation that a micellized surfactant ion, whose electrical charge is not compensated by a bound counterion, contributes to the conductivity of the solution to the same extent as if it were free [START_REF] Zana | Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications[END_REF]100].

In Before CMC the concentration of cations and anions equals:

= (III-3) [ ] = (III-4)
where C is the surfactant concentration.

Beyond the CMC the concentration of free amphiphilic cations in solution remains constant:

[ ] = (III-5)
the concentration of the micelles equals:

[ ] = (III-6)
where N is the number of monomers that form a micelle.

The concentration of counterions is thus given by:

[ ] = ( + ( - ) ) = ( + ( -) ) (III-7)
The conductivity of the solution of the amphiphilic salt A 2X at the concentration C before the CMC consists of the sum of the conductivity from cations and anions at this concentration:

= + (III-8)
where λ A and λ X are the equivalent conductivities of the amphiphile ions and counterions and

= [ ], = [ ] (III-9)
Hence, it follows that:

= ( + ) (III-10) = ( + ) (III-11)
where S 1 is the slope of the specific conductivity-concentration curve below the CMC.

Above the CMC specific conductivity can be presented as a sum of three components: conductivity due to the single amphiphilic ions at the CMC, micellar ions and free counterions: Since N was independently determined from fluorescence quenching (for more details see subsection 2), λ X is a literature value and S 1 and S 2 -slope of the specific conductivity-concentration curve below and above the CMC respectively, S 2 can be defined as:

= ( - ) + (III-16)
Although α is usually defined as a value at the CMC, in reality, the aggregates are not well formed just at the CMC, and it is difficult to measure N at this concentration. That is why in our study we measured α at 2xCMC.

Hereafter, we present ionization degrees calculated using Zana's (α Z ) and Evans' method (α E ) using Equations (III-2) and (III-16).

Free energy of micellization

To study a colloidal system, it is important to have information about its thermodynamic properties, in particular the Gibbs energy of micellization ΔG°M as it is one of the key factors in micelle formation.

Free energy of micellization for ionic surfactants (one chain connected to one monovalent head that has monovalent counterion) is usually described by the equation [START_REF] Zana | Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications[END_REF]103]:

°= ( + ) (III-17)
However, for ionic surfactants some molecular factors should be taken into account, such as number of hydrophobic and hydrophilic moieties per molecule, valence of counterions as well as headgroups, added salts, etc. Hence it is important to use the proper relationship between ΔG°M and the CMC for a correct analysis of the results. R. In this equation, β is the degree of counterion binding (β = 1 -α) and the cmc is expressed in mol of alkyl chain/L. In this study we are using CMC that is critical micelle concentration in mol of gemini surfactant per liter. Thus, cmc = 1/2•CMC.

To calculate the free energy of micellization, we used ionization degree that was estimated using Evans' method (Equation (III-16)).

Results and discussions

In this section, the results obtained by conductivity measurements for quaternary 10-2-10 gemini with two groups of counterions: inorganic and alkyl carboxylates as well as for 10-2-10 ter gemini with some halides as counterions are presented.

Quaternary 10-2-10 with inorganic counterions

Table III-1 reports the results obtained by conductivity measurements for quaternary ammonium gemini (10-2-10 X) with inorganic counterions, X = halides, NO 3 -, MeSO 3 -, PH, C2 and TFA. As mentioned in paragraph 1.1.1, the CMC values were calculated by two different methods:

CMC a was obtained by intersection of two straight lines (S 1 and S 2 ) in the graph of conductivity (κ)

versus concentration (C) [START_REF] Williams | The critical micelle concentration of sodium lauryl sulphate at 25°[END_REF] and CMC b was obtained by fitting the experimental conductivity data based on integration of a Boltzmann type sigmoidal function [START_REF] Carpena | Problems Associated with the Treatment of Conductivity-Concentration Data in Surfactant Solutions: Simulations and Experiments[END_REF]. From the Analyzing the CMCs for 10-2-10 gemini, we can see that the micellization of the amphiphiles follows the Hofmeister series:

I -< TFA ~ NO 3 -~ Br -< Cl -< MeSO 3 -< C2 < F -< PH,
except F -anion. A similar trend was found for the 14-2-14 gemini with different counterions performed by S. Manet previously in the group [START_REF] Manet | Counteranion Effect on Micellization of Cationic Gemini Surfactants 14-2-14: Hofmeister and Other Counterions[END_REF].

In Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 90 correlation of micellization process with physical properties for these anions can be due to two main reasons: these are monoatomic chemically related elements (halogens) and they have the same number of electrons of the external electron orbitals, thus ion size (density) is the main parameter that changes significantly from F to I. Since these properties of the ions vary monotonously from F to I, it is indeed expected that their aggregation behavior and CMC also vary monotonously.

For the polyatomic counterions, their non-spherical shape and inhomogeneous distribution of electron density make a significant contribution to the ion and aggregation behavior. shows the trend of the CMC values as a function of -ΔG hyd . For more hydrophobic ions (lower -ΔG hyd ) the CMC is lower, whereas for more hydrophilic (higher -ΔG hyd ) the CMC is higher.

However, for some of the ions, this trend is not as simple, indicating that the CMC does not depend on one parameter only, but is presumably a result of cooperative effects of several parameters. The ionization degree, or fraction of free counterions, characterizes the association strength between counterions and headgroups. On Figure III-4, α E versus CMC shows that as the CMC increases, so does the ionization degree. This implies that more hydrophilic ions are more dissociated, and that micellization is less favorable, than for ions that make strong ion pairs with the headgroups due to their hydrophobicity. Surprisingly, PH has a lower ionization degree than C2, which may be due to its low hydration number (1.8) with respect to C2 (2.2), or due to its slightly higher polarizability (5.79 Å 3 ) compared to C2 (5.50 Å 3 ). with increasing chain length of the counterions. This trend is expected as hydrophilicity decreases for the counterions of longer chain lengths. However, an exception is observed for C1. In spite of its higher hydrophilicity (-ΔG hyd = 403 kJ•mol -1 ) in comparison to C2 (-ΔG hyd = 373 kJ•mol -1 ), its CMC is slightly lower. Hence, other properties of the ion also contribute to the self-assembly process. This was already observed with 14-2-14 gemini [START_REF] Manet | Counteranion Effect on Micellization of Cationic Gemini Surfactants 14-2-14: Hofmeister and Other Counterions[END_REF], although C1 is more hydrophilic than C2, it is less hydrated (n H = 2.1) than C2 (n H = 2.2). Moreover, C1 is the only counterion of the series that does not have an alkyl group attached to the carbonyl moiety, which has a strong impact on electron density. This is reflected by the pKa value (3.75) which is more than one unit lower than that of other n-alkyl carboxylates (around 4.8). The free energy of micellization has the same trend as the CMC and α. With increasing chain length, -ΔG°M increases in the following order C2 < C1 < C3 < C4. For the counterions with longer chain lengths we would expect a further increase in -ΔG°M that would indicate their favorable contribution to the self-assembly process.

In this system therefore, the variation of just one parameter of the system (hydrophilicity of the counterion) has a predictable effect on the micellization properties. An increase in hydrophobicity (increase in of the chain length) favors micelle formation by decreasing CMC, α and increasing -ΔG°M, C1 being the exception in this tendency.

Tertiary 10-2-10 gemini with halide counterions

In order to estimate the impact of the nature of cation-anion interactions on the Hofmeister series, we changed the properties of the headgroup of the gemini surfactant by substituting the quaternary ammonium group with a tertiary one, see Figure III-7. We denote tertiary ammonium gemini as 10-2-10 ter X. The objective is to investigate the effect of the cation nature on the counterion effect. In the ammonium headgroups, replacing a methyl by a proton induced a significant change in the properties of the gemini and it was observed that the CMC values for 10-2-10 ter X are an order of magnitude lower than for 10-2-10 X. A much stronger propensity of the 10-2-10 ter X to form micelles in comparison to 10-2-10 X is clearly observed. The conductivity slopes for tertiary and quaternary gemini were then compared. with Cl -, Br -and I -anions. 1-2-1 and 1-2-1 ter were used as analogs of the headgroups of quaternary and tertiary 10-2-10 to simplify the system. In Table III-5 the free energies of binding (ΔG B ) between counterion and headroup are presented. In order to analyze this data, the notion of binding energy is defined as follows: it is the free energy released when the complexes are formed.

For example, the free energy of the complex between chloride ion and 1-2-1 is 3.17 kcal/mol lower in free energy than the sum of the components (Figure . Therefore, the binding energy for the 1-2-1 Cl system is 3.17 kcal/mol. We first observe that ΔG B is significantly lower for the 1-2-1 ter X than for its quaternary analog. A more negative value of binding energy implies that the complex is strongly bound. These results show therefore that association between anion and headgroup is much stronger for the 1-2-1 ter analog, which has a proton instead of methyl group per each ammonium. Second, regardless of tertiary or quaternary 1-2-1 cations, the DFT results suggest that ΔG B is lowest for Cl - (the strongest association with the headgroup) and highest for I -(the weakest interactions with the headgroup). Whereas, experimentally we observe the opposite behavior for the quaternary 10-2-10 X complexes. Such a discrepancy can be due to the cooperative effect that the 10-2-10 X surfactants exhibit by self-assembling into micellar aggregates. The much stronger association between halides and cations for the tertiary 1- It is interesting to discuss the counterion effect on the CMC values of tertiary gemini by comparing Br -and Cl -counterions. In contrast to the quaternary analogs where the CMC for Cl -is twice higher than for Br -(12.4 and 6.4 mM respectively), in the case of tertiary gemini the CMC for Cl -is slightly lower than for Br -(0.33 and 0.40 mM respectively). These results suggest that the variation of the headgroup properties has an important effect on the counterion-surfactant interactions and consequently on their aggregation behavior. Decreasing softness (increasing hardness) of the cation by replacement one methyl to proton changes the interaction between cations and anions. As it was discussed by Collins and coworkers [START_REF] Collins | Ions in water: Characterizing the forces that control chemical processes and biological structure[END_REF], and is schematically shown in properties. Meanwhile, the strong decrease of CMC for tertiary gemini compared to quaternary gemini was not expected, since the fact that the tertiary ammonium headgroups are harder cations than the quaternary ammonium ones should lead to the decrease the strength of the interaction (increase the CMCs) with the soft anion. As it was discussed, the possible decrease in the ionic interactions is compensated by the formation of hydrogen bonds as suggested by the DFT calculations.

Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 103

Aggregation number. Fluorescence quenching technique

The aggregation number of a surfactant gives the number of surfactant molecules making up an aggregate at a given concentration and temperature. This is related to the size of the aggregate, and this information is required for α E determination and computer modeling that were applied in this work. There are several methods that can be used to estimate the aggregation number [START_REF] Zana | Surfactant solutions: new methods of investigation[END_REF]108]. However probe (P) and quencher (Q), applied for the system, have to be chosen very carefully and their concentration must be kept low to prevent the significant influence on the micelle size.

Addition of a quencher to the system will result in a decrease of the probe's emission intensity due to the distribution of the quencher inside the micelles. The decrease in intensity is related to the probability of a micelle containing the probe and the quencher, the lifetime of the excited probe, and the quenching rate of the probe by the quencher. There are two methods of fluorescence quenching (FQ): steady-state fluorescence quenching (SSFQ) and time-resolved fluorescence quenching (TRFQ), which are compared below.

Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 104

Theoretical aspects of fluorescence quenching techniques as a method to determine aggregation number

Fluorescence quenching in homogeneous media

If the reaction under investigation occurs in homogeneous solution and contains the hydrophobic fluorescent probe (P) and a solute that quenches the probe excitation (Q), dynamic For a dynamic quenching process, the ratio τ 0 /τ equals to the ratio I 0 /I Q of the fluorescence intensities measured in the absence and presence of quencher, I 0 and I Q , respectively, giving:

= + [ ] (III-24)

If the fluorophore forms a stable complex with Q that is non-fluorescent, then the quenching process is termed static quenching and the slope of the Stern-Volmer equation gives the association constant:

(III-25)
where K s is an association constant for the quencher-fluorophore complex that depends from the nature of probe/quencher pair and on its microenvironment.

P + hv P* τ 0 =1/k P* + Q P + Q k Q P* P P* + Q PQ K S
Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 105

In the cases where both static and dynamic quenching are operational, the Stern-Volmer equation becomes:

= ( + [ ])( + [ ]) (III-26)

Steady-state fluorescence quenching (SSFQ) in micelles

In the case of micellar solutions, the system becomes heterogeneous and requires an appropriate kinetic model that is based on a set of assumptions. We consider a system that includes only dynamic quenching and for simplification assume that P and Q do not exchange with the bulk during the photochemical process (immobile P and Q) [119]. This implies that P and Q stay in the same micelle significantly longer than the probe's excited-state lifetime, which is on the order of tens of nanoseconds. The model also assumes that the distribution of the probes and quenchers in the micelles follows Poisson's statistics. One important consideration is that the concentration of P has to be much lower than the micelle concentration, [M], to avoid placing two probes within the same micelle. This could lead to collisional quenching of the probe's emission, or to the formation of an eximer possessing different emission properties.

If all these assumptions are satisfied, with increasing the quencher concentration, [Q], as The main assumption of the model used to obtain Equation (III-27) is that the ratio k Q /k >> 1, that means that τ << τ 0 and indicates that the quenching of P* is much faster than the other decay processes.

In practice, SSFQ involves measurements of the fluorescence emission intensity with increasing ratio of [Q]/[M] using a spectrofluorometer. However, this only gives the difference between I 0 and I Q and does not provide information concerning the fluorescence decay. As a consequence, it is impossible to distinguish micelles that have one or more molecules of quencher.

When τ << τ 0 this difference between micelles with one or more Qs is negligible. where the difference in total surfactant concentration, C, and CMC will give concentration of the amphiphile molecules that form micelles. It was shown by Zana and others [START_REF] Zana | Surfactant solutions: new methods of investigation[END_REF] that when k Q is not much higher than k, the values of N became underestimated.

Tim-resolved fluorescence quenching (TRFQ) in micelles

The main benefit of TRFQ lies in the use of fluorescence decay rates to determine the proportion of micelles containing both probe and quencher [120]. The basic experiment is the same as for SSFQ, but the decay curves of the probe's emission rather than its intensity are recorded in the absence and presence of increasing concentrations of Q. Emission decay rates can be recoded using a single photon counting setup, a high-speed streak camera, or a phase-shift technique.

Assumptions used in the TRFQ analyses are as follows:

(i)

The system includes dynamic quenching and P remains confined to the aggregate during the photochemical process. However, it is not required that Q remains immobile in the micelle during the process.

(ii)

The distribution of the probes and quenchers follows Poisson's statistics.

(iii)

The probe concentration should remain low to avoid eximer formation.

(iv) Quencher molecules do not interact with each other, and the quenching rate of the excited probe in a micelle containing more than one quencher is equal to the sum of the quenching rates of the individual quenchers.

The probe's lifetime in micelles without Q gives a monoexponential decay (k 0 = 1/τ 0 ). It should be noted that in cases where the excited probe is quenched by oxygen, the lifetime of the probe inside a micelle is often comparable to that of the probe in de-oxygenated solution due to the lower oxygen concentration inside the micelles. Upon addition of Q, the emission decay is no longer monoexponential due to the quenching of some of the probes. The time dependance of the fluorescence intensity is described using the Infelta-Tachiya model[118, 119, 121, 122]: so that <n> can be determined. Subsequently, the aggregation number (N) at each [Q] can be calculated as:

( ) = [- -{ - (- )}] (
= [ ] =< > [ ] (III-30)
For a Gaussian micelle size distribution, the quencher averaged aggregation number N av , for any particular [Q] derived by a fit of the decay to the Equation (III-29), is written as a series in the quencher concentration. This N av can be presented as weight averaged aggregation number N W with some deviation[124]:

= - [ ] (III-31)
where is the standard deviation in the aggregation number.

Weighted averaged aggregation number N W is defined as:

= ∑ ∑ = ∑ (III-32)
where n is the aggregation number and X n is the distribution function. The intercept of N av vs It is important to highlight that in comparison with SSFQ, TRFQ does not rest on the assumption that k Q /k >> 1 and that this technique can account for molecules of Q that are exchanged between the micelle and the environment during the excited state lifetime of P*. Furthermore, as it was mentioned in paragraph 2, additional information can be obtained from the decay constant of

(1/2)([Q]/(C -CMC))

Results and discussions

In this paragraph we focus on the contribution of counterion properties to the aggregation number (N) of the micelles determined by TRFQ. The investigated systems included quaternary ammonium 10-2-10 X gemini with inorganic counterions (X = Br -, Cl -, I -, NO 3 -, MeSO 3 -, PH, C2

and TFA) and carboxylate counterions (C1-C6). N was also determined for tertiary ammonium 10-2-10 ter Br. As TRFQ provides information on micellar aggregates properties. Analyzing it, the impact of counterion nature, as well as contribution of the headgroups properties (for 10-2-10 ter Br)

on the morphology of aggregates can be estimated.

Quaternary ammonium 10-2-10 with inorganic counterions and tertiary ammonium 10-2-10 ter Br

Time-resolved fluorescence quenching experiments were done for 10-2-10 X gemini, where X = Br -, Cl -, F -, NO 3 -, MeSO 3 -, PH, C2 and TFA at concentrations 2xCMC and 3xCMC as well as for 10-2-10 ter Br at 2xCMC. Averaged aggregation number, N av , as well as the weight averaged aggregation numbers, N W , (determined from the intercept of a linear fit to N av vs [Q]/(C -CMC)) Comparing N av and N W we can see that for a concentration of 2xCMC, the difference in values for the different counterions is small. However, at 3xCMC for the hydrophilic F -, PH and C2

the N W has larger values than N av . We attribute this to the presence of polydispersity in the system, as estimated from the standard deviation in the N W aggregation number, .

At 2xCMC, most of the investigated surfactant systems show low polydispersity, with values of lower than 20. However, at higher surfactant concentrations (3xCMC), the situation is different.

The most polydispersed micellar system is the one with the PH counterion ( = 78) as we can see from Figure III-15, where N varies from 33 to 66. A relatively high polydispersity is also observed for F -and C2, where are 59 and 49, respectively. In contrast, micelles formed by 10-2-10 gemini associated with Cl -, Br -, TFA and NO 3 -are monodisperse at this concentration. Interestingly, polydispersity is pronounced for the gemini amphiphiles with hydrophilic counterions. We can assume that more hydrophobic counterions, when they are small and do not integrate the micellar core (like alkyl carboxylates), will better stabilize the micelles and thus aggregates of similar numbers are built. the N av for 3xCMC is larger than that for 2xCMC, and the difference is more significant for the most hydrophilic ions such as F -and PH. Interestingly, an N av for 10-2-10 ter Br ( 16) at 2xCMC is ca.

half that of its quaternary analog 10-2-10 Br [START_REF] Gurney | Ionic processes in solution[END_REF]. This may be because of its smaller headgroup size compared to 10-2-10 Br, which allows it to form stable smaller aggregates.

Quaternary ammonium 10-2-10 with carboxylate counterions

Aggregation number for the quaternary 10-2-10 with carboxylate counterions (from C1 to C6) were obtained by TRFQ at 2xCMC and 3xCMC to investigate the contribution of hydrophobicity of the counterions to the aggregation behavior of micellar solution. The same approach to estimate averaged aggregation number, N av , weight averaged aggregation numbers, N W , and the standard deviation in micelle aggregation numbers, , as for inorganic counterions was used.

The results are presented in Table III Similarly to the inorganic counterions at 2xCMC, the difference in values between N av and N W is insignificant. However, with increasing concentration, the change becomes more visible. At 3xCMC the estimated N W values are higher than those of N av . This difference comes from the polydispersity of the systems. In presented. From this we can see that the highest variation of N av is for C1 and C2 counterions, as also confirmed from the dispersion coefficient, , determined to be 42 and 49, respectively. For the counterions with longer chains decreases indicating that the polydispersity decreases. Similar results were obtained for inorganic counterions, where larger polydispersities were observed for more hydrophilic couterions (F -, PH, Figure III-15). However, it is noteworthy that for alkyl carboxylates and inorganic counterions having similar CMCs and hence hydrophobicity (i.e. C6 and Br -or C4 and Cl -), the organic anions exhibit a higher polydispersity than the inorganic ones. In the same way as for the 10-2-10 gemini with inorganic counterions, the values of N av at twice the CMC are very similar for all carboxylate counterions despite that their values of CMC vary from 22.7 to 5.7 mM for C2 to C6. Taking into account the polydispersity, we can say that for Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 114 all carboxylates (except just C1) N av does not vary much and equals ~ 24 independently from the ionization degree (Figure III-20, note we use α Z in order to estimate the ionization degree for all counterions) due to the presence of ordered counterion layer. For C1, the aggregation number is slightly higher even though its α Z is lower than for C2. In comparison with inorganic counterions, almost no increase in N av for the more hydrophobic counterions C4, C5 and C6 was observed. We propose that this is due to the fact that the longer hydrophobic chains may influence the packing parameter of the micellar aggregates by penetrating into the micellar hydrophobic core, thus limiting the increase of their N av . In order to estimate how the hydrophobicity of the counterion influences the N av, we estimated the carbon content of the micelles. Knowing the aggregation number and ionization degree of the micelle we can recalculate how many carbons contain the micellar core according to the equations:

= + (III-33)
Where Carb S and Carb C are respectively:

= (III-34) = ( -) (III-35 )
where L is the number of carbons in the counterion chain. Indeed, it is clear that the carbon content of the micelle increases for the more hydrophobic counterions. Therefore, penetration of the micellar core by the hydrophobic chain of longer alkyl carboxylates prevents an increase in the number of aggregates, in other words, the integration of extra gemini molecules, and simultaneously augments the micellar volume.

Conclusions

In this Chapter we described our study of ion specific effects of counterions on the 10-2-10 X gemini surfactant self-assembly process. The physical properties in bulk solution for quaternary ammonium gemini surfactant salts with different counterions and tertiary ammonium gemini with halides were investigated. Conductivity measurements were used to study the properties of ionic solutions to estimate the CMC, ionization degree (determined by two methods: Zana's (α Z )

and Evans' (α E )) and free energy of micellization (-ΔG°M). Time-resolved fluorescence quenching was used to investigate the aggregation number of the micelles (N).

In summary, the CMC values for 10-2-10 X follow the Hofmeister series I -< TFA ~ NO 3 -~ Br -< Cl -< MeSO 3 -< C2 < F -< PH. Self-assembly of quaternary gemini surfactants depends on the hydrophilic/hydrophobic properties of the counterion (-ΔG hyd ). For 10-2-10 X with monoatomic counterions (X = halides), the CMC depends monotonously on all examinated ion properties. In the case of polyatomic counterions, however, it is difficult to correlate the behavior of the surfactant to just one ion property. Various properties determine cooperatively the forces that drive micelle formation. Meanwhile, as was mentioned before, free energy of hydration best predicts the behavior of the surfactant molecules in solution. To follow the contribution of just hydrophobic component to the micellization, aliphatic carboxylates were chosen as counterions that have the same chemical nature. The main trend indeed suggests that increase of the chain length (hydrophobicity) of the counterion promotes micellization of the surfactant.

Ionization degree determines the fraction of free ions. Gemini with more hydrophilic/hydrated counterions have higher ionization degrees, indicating that they have lower interactions with headgroups of the gemini surfactants. Instead, amphiphiles with poorly hydrated hydrophobic counterions have lower ionization degrees and tend to associate strongly to form ion pairs. The same behavior is observed for 10-2-10 with carboxylate counterions: the more hydrophobic the counterion, the lower ionization degree α is. Free energy of micellization ΔG°M is a characterization of the thermodynamic properties of the system. It increases with increasing hydrophobicity of the ion which favors micelle formation.

The aggregation numbers of the micelles were not found to depend significantly on the counterion properties, and are around 30. However there is still a small dependence of the N values on the ionization degree of the system. Micellar systems with counterions which are characterized

Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 117 by higher ionization degree tend to have smaller values of the aggregation number. Whereas larger aggregation numbers are observed for the micellar systems with counterions having lower α. This tendency may be due to the electrostatic repulsion of the gemini headgroups in the case of high α that is disfavors the integration of the new monomers to the micelle. For the surfactants with low α, this effect is screened by counterions.

In order to understand better the balance of forces that drives the self-assembly of cationic gemini surfactants, we employed other techniques such as chemical trapping, molecular dynamics (interfacial properties), rheology and crystallography described in the following chapters.

In addition, we showed that the changes in the properties of the headgroup (substrate) would affect the order of anions in Hofmeister series. Increasing the hardness of the cationic headgroup for gemini by replacement one methyl by a proton, we could reverse the Hofmeister effects. For 10-2-10 ter X, we observed that Cl -, having slightly smaller CMC (0.33 mM), can be considered to be more associated with the headgroup than Br -(CMC is 0.40 mM).

Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 118 4. Experimental section 4.1.

Conductivity measurements

Methodology. Conductivity measurements were performed using benchtop meter CONSORT C860 (Belgium) with platinum electrode SK10T (Belgium). The temperature was maintained at 30±0.1 °C using a thermostat Huber Ministat CC. All solutions were prepared with milliQ water (18.2 MΩ•cm). The technique is very sensitive to the presence of ions, so the electrode as well as glassware should be rinsed several times with ultrapure water before usage.

For quaternary 10-2-10 X gemini the conductivity measurements were performed through the increase of the concentration of amphiphile molecules in the pure water. The stock solution of the surfactant was prepared at concentration much higher CMC (if the solubility of surfactant molecule allows). And after, the aliquots of this stock solution were added to the same sample of pure water increasing the concentration of amphiphiles. For tertiary 10-2-10 ter X gemini conductivity measurements were carried out by dilution the concentrated surfactant solution (usually b -data from conductivity measurement estimated by fitting based on the integration of Boltzmann function c -data taken from ref. [128] d -data taken from ref. [START_REF] Yitzhak | Ion Properties[END_REF] Note that values for the CMC calculated using fitting model are slightly smaller (or almost equal) the ones calculated using the intersection point. As it was already discussed before in this chapter (section 1.1.1) the method proposed by Carpena et al. consider to be more accurate for the CMC determination. In our particular case discrepancy between results obtained from two methods is negligible.

S 1 b S 2 b λ X c (25°C) λ X (T) α Z a α Z b S•cm 2 •mol -1 S•cm
For 10-2-10 Br, as the compound with better purity due to the simples way of synthesis, the conductivity measurements were done 10 times to see the discrepancy between the measurements.

From the obtained data: CMC a = 6.54 ± 0.24 and CMC b = 6.43 ± 0.27 we can say that both methods give statistically coherent results. An error of the experiment with 10-2-10 Br is less than 5%.

Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 120 However it is possible that it can slightly increase for the other systems due to more complex synthesis (see the Chapter II) and possible impurities that can influence to the real concentrations and conductivity values.

Molar ionic conductivity was taken from the CRC Handbook of Chemistry and Physics (85 th edition)[128] and Marcus Yizhak, Ion properties [START_REF] Yitzhak | Ion Properties[END_REF] for MeSO 3 -. The calculation of the ionic conductivity at given temperature were done according to the equation[129]:

, °= , ° °[ + . ( -° )] (III-36)
where λ°i ,T is a molar ionic conductivity at given temperature T and λ°i ,25°C is a molar ionic conductivity at 25 °C.

Theoretical S 1 values consist of twice ionic molar conductivity of the counterion (2 anions per one gemini) plus conductivity from the amphiphilic cation. In our study S 1 values correspond to the expected ones within error that could come from the experiment due to manual performance of the measurements. The exception is F -and I -where S 1 is lower than 2λ X (T). The conductivity was measured just after synthesis though we have

S 1 = 99 S•cm 2 •mol -1 instead of ~ 140 S•cm 2 •mol -1 that
means there are less mobile species. This may be because 10-2-10 F is known to decompose quite fast, therefore there are less 10-2-10 in solution than expected.

As for 10-2-10 I the error comes from the measurements at high temperature. Machine was calibrated at the room temperature and experiment was carried out at 50 °C that caused some error in the slope. This chapter begins with a brief overview of selected arenediazonium chemistry. The principle of the method is described in part 1 and part 2, and will be dedicated to the results and discussions. little effect on the measured rate constant. The loss of N 2 generates a highly reactive intermediate.

Spectrofluorometric measurements

Methodology

Therefore, for an arenediazonium ion whose reactive group is located within the micellar interface, the final product yields primarily depend on the interfacial nucleophile concentration and only modestly on the selectivity of the reaction towards the different nucleophiles [134]. Finally, many of the reaction products are stable and can be analyzed quantitatively by HPLC.

Indeed, as shown in Scheme IV-1, the rate-determining step is the loss of N 2 with formation of a highly reactive aryl cation that reacts almost immediately with the nucleophiles in its vicinity.

This stepwise mechanism[140] combined with a low selectivity toward weakly basic nucleophiles, makes arenediazonium a good probe for examining the interface composition of amphiphile assemblies.

Arenediazonium ion as a probe of the interfacial region of the micelles

The CT method uses an arenediazonium ion as a probe that is assumed to be located in the interfacial region of the surfactant aggregates where it reacts with nucleophiles to give products that provide information about their concentrations within the interfacial region. Due to its amphiphilic nature and because ions and molecules are moving at near diffusion controlled rates, low concentrations of 16-ArN 2 + easily integrate into the micelles without significantly affecting the structures and with the reactive diazonium group positioned within the interface of the surfactant aggregate (Figure IV-1). We will present evidence supporting this assumption from molecular dynamic simulations described in Chapter V. The surfactant solution is assumed to be in dynamic equilibrium, the distribution of its components is described by pseudophase model in which the association colloidal solution is treated as two pseudophases: aqueous phase and the totality of aggregated surfactant as the second phase [142,143]. As it was discussed by Larry Romsted in the ref.

[134], transfer rates for the elements of the system, including probe, between pseudophases are much higher than dediazoniation reaction rate. We can therefore assume that the reaction yield after dediazoniation of the probe comes from sampling the entire interfacial region of the aggregated surfactant (16-ArN 2 + )

or aqueous region in case of short-chain analog (1-ArN 2 + ).

Thus, 16-ArN 2 + is well-suited for quantifying the molecular populations at interfacial region of colloidal systems[131-133, 141, 144].

Main assumption of the chemical trapping

The CT method assumes that the selectivity of the dediazoniation reaction, S W X , is the same in surfactant aggregates' interface and in aqueous solution [132,139,141,145] (Equation (IV-1)).

This assumption is supported by the particularly small selectivity for the type of nucleophile and insensitivity to the solvent polarity,

= (% )[ ] (% )[ ] = (% ) (% ) (IV-1)
where square brackets refer to total (stoichiometric) concentration in solution in moles per liter; the subscript m to interfacial molarity of interfacial volume in moles per liter; % indicates percent yield of a product determined by HPLC; and parentheses refer to product yields.

In micellar solutions the resulting product yields are proportional to the concentrations of ions or water within the interfacial layer and not to their stoichiometric concentrations in solution [141].

To estimate the concentration at the interface, the model arenediazonium reaction in water is used as Using this assumption, we can easily estimate and quantify the interface concentrations by referring to the dediazoniation yields in the aqueous solution, where concentration of surfactant and nucleophiles are known.

Results and discussions

In this part, we present the results obtained from chemical trapping for 10-2-10 gemini with some of Hofmeister anions, such as Br -, Cl -, I -, MeSO 3 -, C2 and carboxylate counterions:

propionate (C3) and butyrate (C4). Observed interfacial concentrations of counterions as well as water molarity allow us to estimate the interfacial components of the micellar aggregates and correlate them to the nature of the counterions and to the surfactant self-assembling process. We will also summarize briefly the limitations and problems we faced with applying this technique to different anions.

Inorganic counterions

The CT experiments were done for 10-2-10 X gemini with counterions such as: X = I -, Br -, Cl -, C2 and MeSO 3 -, in the concentration range from around 2xCMC to 250 mM, except 10-2-10 I due to its low solubility (25 mM). The concentration range was chosen to follow the relationship between the interfacial counterion and water concentrations as well as the change in aggregate morphology [131,141].

In the Table V-1 we report the results obtained by HPLC measurements for dediazoniation reaction of 16-ArN 2 + in 10-2-10 Br micellar solutions (was chosen as an example) at 25 °C in the concentration range from 10 to 250 mM (1 mM HBr was added in each sample to lower the pH to about 3). Calibration curves for each observed compound were used to transform the peak areas to product concentrations and then to the observed yields, that were normalized to 100% by counting 16-ArH in a total yield [132]. Results for all others counterions were produced but not shown here. The interfacial molarities are higher for hydrphobic I -and Br -, e.g., at ~2 x CMC they are 3.8 and 2.1 respectively. They decrease with respect to the anion hydrophobicity: 1.5, 1.2 and 1.0 for Cl -, MeSO 3 -and C2 respectively. Note that for 10-2-10 C2, the solution contains a significant amount of acetic acid that was added to hold the pH constant, however, C2 m still has one of the lowest interfacial molarities. The H 2 O m has an opposite trend: the more hydprophilic counterion is, the higher the interfacial water molarity is.

1-ArOH (%) [X t ] (M)

1-ArOH (1-2-1 I) 1-ArOH (1-2-1 Br) 1-ArOH (1-2-1 Cl) 1-ArOH (1-2-1 C2) 1-ArOH (1-2- 1
With increasing surfactant concentration, the interfacial concentrations of the X m increases and H 2 O m decreases, more or less monotonously for hydrophilic counterions, but the slope for Br -

shows a break at about 50 mM. The reasons for the break in the values for Br m and the downward slope for I m are currently unknown and must await future research. These results correlate well with physical properties of the surfactants, such as solubility, CMC and ionization degree, which significantly depend on the ions and reflect their hydrophobic/hydrophilic properties. For example, values of CMC for 10-2-10 gemini follows the trend I -< Br -< Cl -< MeSO 3 -< C2 and their ionization degree (α Z ) is respectively 0.12; 0.21; 0.43; 0.44 and 0.52. These results describe the interactions between counterions and headgroups, low ionization degree typical for the ionic micelles in the case when anion and cation are strongly bound and prone to ion pairs formation.

High degree of ionization is a result of weak screening of the micelle. Counterions are highly hydrated and interact with polar headgroups weakly (e.g., F -, PH) [START_REF] Kunz | Specific ion effects in colloidal and biological systems[END_REF]147].

The results obtained by the different methods (conductivity and CT) correlate very well in that both show that the behavior of the amphiphile self-assembly depends on ion specificity and tends to follow the Hofmeister series.

Carboxylate counterions

To investigate the influence of counter anion hydrophobicity on the interfacial micellar composition, chemical trapping was carried out for 10-2-10 gemini with alkyl carboxylate counterions: C2, C3 and C4. The selectivity of arenediazonium ion towards carboxylate counterions comparing to water, S W X , was assumed to be the same as for acetate (paragraph 2. CT results fit the expected tendency, the interfacial concentration of the counterions in micellar solution increase for more hydrophobic ions and correlate with measured ionization degree (α Z ) that is 0.52, 0.38 and 0.33 for C2, C3 and C4 respectively (Chapter III), demonstrating a correlation between the counterion hydrophobicity and association degree in gemini interfaces.

Complications

The chemical trapping method has certain previously described limitations [134]. Strongly basic nucleophiles cannot be used in chemical trapping due to their extremely rapid reaction with the terminal nitrogen resulting in undesired side products that make quantification of counterions very difficult. The mechanism of this type of reaction are well described by Hegarty[148]. Other competing reactions include formation of an indazole side product at high pH[149] and reduction of the arenediazonium ion by the phenol product. Finally, unfavorable physical properties, such as high solution viscosity may cause complete mixing problems.

Here, we will briefly overview some other problems and limitations that were observed during chemical trapping experiments for some of the anions, such as NO 3 -, TFA, PH (H 2 PO 4 -) and carboxylates which made the determination of the interfacial concentration much more difficult.

1. Instability of the reaction product e.g., by fast hydrolysis. Dediazoniation with NO 3 -in 10-2-10 NO 3 micelles by the HPLC revealed only a phenol product (z-ArOH). This indicates that the expected phenol nitrate ester (very good leaving group) hydrolyzes in the time frame of these experiments and requires a faster method for following the reaction. Chapter IV. Interfacial properties of micellar aggregates via the chemical trapping experimental method 137 2. TFA is also a good leaving group, however not as good as nitrate. The z-ArTFA product was observed, however it hydrolyzed rapidly during the HPLC analysis (Scheme IV-2) and estimation of the product yield was difficult.

Scheme IV-2. Reaction product of heterolytic dediazoniation of arenediazonium ion in presence of triflouroacetate and its hydrolysis.

One possible solution to such problems is to run these reactions in isotopically labeled water. In this case phenol formed from the reaction between the probe and H 2 O 18 (z-ArO 18 H) can be distinguished from the phenol which is the secondary product from phenyl nitrate ester or from z-ArTFA after hydrolysis (z-ArO 16 H). Analyzing products using mass spectrometry, the product yield for both product (z-ArO 18 H and z-ArO 16 H) should be estimated.

3. Another counterion that requires adjustment of the conditions for HPLC product analyzes is

H 2 PO 4 -
. It gives the stable phosphate ester as a product of dediazonium ion reaction. However, z-ArH 2 PO 4 has two protons and the phosphate ester will dissociate into its monoanion form, which will require the development of new HPLC conditions to quantitate the ester and other products. The work still in progress to find the proper conditions for the separation of charged molecules.

4.

A new big challenge appeared during the CT experiments on gemini micelles with carboxylate counterions. An unidentified peak with a long retention time and therefore hydrophobic nature was observed on the HPLC chromatograms of the long chain products formed in micelles, but not in the reaction with short chain analog. The probable site of the formation of the "mystery" peak is within the micellar interface because of the high concentrations of reactants. No dependence on the chain length of the carboxylate counterion was observed. Attempts to isolate the unknown compound and analyze it by NMR and HPLC failed due to its instability. The size of the "mystery" peak decreases with increasing pH indicating that the formation and breakdown of the product is acid catalyzed. After long characterization process of this undefined compound by one of our collaborators, Xiang Gao, Larry Romsted and his students decided that the mystery peak is a diazoether formed in a reaction between the arenediazonium ion, the phenolic product that is general base catalyzed by carboxylate groups. The very reactive phenolate reacts with arenediazonium ions concentrated in the interfacial region of the micelles giving a uncharged very hydrophobic diazoether (two 16 carbon tails attached to two aromatic rings (44 total carbons), Scheme IV-3. Although stable in neutral solution, the ether decomposes in acid and is sensitive to temperature increases. The compound representing the "mystery" peak has not be isolated yet, its expected properties are consistent with the above observations. Scheme IV-3. Formation of diazoether as a reaction between phenate and diazonium ion in presence of general base, in our case -carboxylates.

Conclusions

For the first time, the CT method was applied to micellar solutiond of 10-2-10 gemini amphiphiles with variety of counterions to study ion specific effects through quantification of interfacial counterion, X m , and water, H 2 O m , molarities. The results show that X m strongly depends on ion nature and follows the trend I -> Br -> Cl -> MeSO 3 -> C2 for the small anions, and C4 > C3 > C2 for carboxylate counterions. Poorly hydrated and strongly associated counterions with the polar headgroup are located primarily in the interfacial region of the micelle and hence give high X m monitored by arenediazonium ion. Contrary, interfacial concentration of highly hydrated ions is low and indicates low degree of counterion-headgroup binding. The CT results are consistent with investigated physical properties of the 10-2-10 surfactant in bulk solution such as the CMC and ionization degree, and all correlate with the Hofmeister series. We demonstrated quantitatively that hydrophobicity/hydrophilicity of the anion plays a crucial role in determining interfacial properties of micellar aggregates.

Dediazoniation reaction

Dediazoniation reactions in reference aqueous solution and in gemini micellar solutions were carried out by adding stock acetonitrile solutions (ice cold) of 1-ArN 2 BF 4 or 16-ArN 2 BF 4 (final concentration was 10 -4 M) to 1-2 mL volumetric flasks containing short chain analog 1-2-1 X or gemini surfactant 10-2-10 X respectively and thermostated at 25 °C. Note, the reaction with iodide counterion was performed at 50 °C due to its low solubility. Because the arenediazonium ion, due to its high reactivity, reacts with MeCN in the stock solutions, it must be prepared fresh and used fast.

Since dediazoniation is the 1 st order reaction, after around 10 half-lives (48 h) the reaction can be assumed to be complete and the products can be separated and analyzed. In each reaction with 1-ArN 2 + cyclohexane (50 µL) was layered on top of each solution to minimize loss of 1-ArOH, and 1-ArX by evaporation. In order to minimize the formation of the reduced product 1-ArH[132],

diazoether and indazole[134, 136], the pH was kept below 5 by adding corresponding acid, 1 mM in case of strong acids such as HCl, and a concentration close or equal to the surfactant concentration in solution in case of weak acids, such as acetic acid.

HPLC analysis

Product yields were determined on a Perkin-Elmer Series 200 equipped with a UV/Vis detector, a Varian Microsorb MV C 18 columns (length, 25 cm; particle size, 5 μm), and a computer-controlled Perkin-Elmer 600 Series Interface. Calibration curves for calculating product yield from peak areas for all the products are prepared from independently synthesized compounds at concentration range that correspond to the expected one from chemical trapping experiment. Products of the dediazoniation reactions were separated and analyzed by the HPLC under conditions summarized in

Table IV-2.
Table IV-

Chapter V. Structural and interfacial properties of micellar aggregates via computer simulations Introduction

In previous chapters we described the investigated systems of cationic 10-2-10 gemini with inorganic and alkyl carboxylate counterions using different experimental techniques: conductivity, 1 H NMR, fluorescence quenching, chemical trapping. In this chapter, we present the results obtained by molecular dynamics (MD) simulations on the same systems, which give the opportunity to follow the mechanisms and dynamics of the molecular assembly systems at an atomistic level. MD simulation technique is a powerful tool to investigate the structure and dynamics of supra-and macro-molecules and nanoobjects[150-153]. In few words, during an MD run, the phase space of the system under investigation is sampled within the framework of classical mechanics. By integrating the equations of motions of Newton of each present atom, trajectories of the system evolving in time can be obtained. Assuming that the convergence has been reached and that the ergodic hypothesis is valid, physical properties of the system can then be evaluated which can explain, either complement or support experimental observations. The forces acting on the atoms are derived from a so-called force field, a collection of functional forms and parameters that is supposed to best represent the (electronic) potential energy surface, within which the nuclei move. Therefore, accuracy of the calculations strongly relies on how the chosen force field correctly describes the interaction forces. The other type of MD simulations that gives more accurate results due to the accounting the quantum effects of the electrons is ab-initio approach. However it can be applied just for small systems due to the time required for the calculations.

On the other hand, if the size of the system is relatively small and the effect of the electrons on the structure is sought, one among all the ab-initio methods can be implemented, in which the quantum effects of the electrons are taken into account.

Being a suitable tool for simulating the structures and dynamics of molecular assemblies, MD simulations have already been widely applied for the study of the Hofmeister effects [START_REF] Lund | Specific Ion Binding to Macromolecules: Effects of Hydrophobicity and Ion Pairing[END_REF][START_REF] Vrbka | Specific Ion Effects at Protein Surfaces: A Molecular Dynamics Study of Bovine Pancreatic Trypsin Inhibitor and Horseradish Peroxidase in Selected Salt Solutions[END_REF]154,155] as well as for the investigation of micellar solutions [156][157][158]. In particular, it has been used to investigate the Hofmeister effects on the air/water interface [START_REF] Boström | Surface Tension of Electrolytes: Specific Ion Effects Explained by Dispersion Forces[END_REF], surfaces[154, 159] and biological macromolecules or their models [START_REF] Lund | Specific Ion Binding to Macromolecules: Effects of Hydrophobicity and Ion Pairing[END_REF][START_REF] Vrbka | Specific Ion Effects at Protein Surfaces: A Molecular Dynamics Study of Bovine Pancreatic Trypsin Inhibitor and Horseradish Peroxidase in Selected Salt Solutions[END_REF]155].

Concerning simulations on surfactants systems, it is worth mentioning the work of C. Bruse et al. [157], where MD was used to characterize sodium dodecyl sulfate (SDS) micelle built from 60 units and solvated in water. They focused on shape, roughness of the micelle and interfacial sodium distribution. S. Storm et al. in their paper[158] studied the aggregation of SDS and cetyltrimethylammonium bromide (CTAB) in micellar structures, in particular, their size, shape and density profile were analyzed. Our group has already investigated gemini cationic surfactants with MD[156], specifically we studied the orientation of the amphiphile molecules in cylindrical micelles that was driven by the balance of electrostatic interaction between headgroups, as well as headgroups and counterions, van der Waals interaction and steric repulsion between hydrophobic tails.

In the present work we used MD simulations to characterize the interactions and concentration of headgroups, halides, polyatomic and carboxylate counterions and water within the interfacial region of ionic micelles (cationic gemini surfactant) and to study their aggregates morphology (sphericity, roughness and counterions' orientation). The main difficulty is to choose the parameters for the system which mimic best the interactions between surfactants, counterions and water molecules. The choice of parameters for monoatomic ions, i.e. the Lennard-Jones parameters describing the van der Waals forces, is even larger if one considers the wealth of different experimental and theoretical data available in literature [START_REF] Horinek | Rational design of ion force fields based on thermodynamic solvation properties[END_REF]160,161]. In this work we implemented CHARMM force field (as it is considered to be the best all-atom force field for surfactant molecules). However, since it lacks a consistent set of parameters for all the halides, we opted to use those recently developed by Jorgensen and co-workers in the framework of OPLS force field[161] for the halides and acetate, whereas the parameters for all other counterions were used from CHARMM force field.

Results and discussions

In the present paragraph we will overview the results from the computational modeling focusing on the aggregate morphology and interfacial concentrations for 10-2-10 micelle with the following counterions: halides, PH, NO 3 -, TFA and a series of carboxylates. As it was described in Chapter III, the aggregation number of micelles at twice the CMC was relatively similar 30 ± 5. For the MD simulations, we chose the same aggregation numbers for all the gemini systems (27 monomers), so that differences in the calculated properties will be only due to the ion properties and not to the morphological effects. We then compared nucleophiles interfacial concentrations computed from simulations with those measured with chemical trapping technique using the arenediazonium probe (Chapter IV).

Contribution of ion specific effects to the micelle structure

As it was discussed previously, ion specific effects have a significant influence on the micellar systems, and in this paragraph we sought to estimate how properties of an ion such as its hydrophobicity, size and morphology influence the size and shape of the modeled micelle and its ion distribution. Structural parameters such as radius of gyration (R g ), solvent accessible surface area (SASA) of the hydrophobic tails and eccentricity (ε) were calculated with different counterions in order to estimate the compactness, the roughness and the sphericity of the micelles respectively. The ion association between polar heads of the micelles and counterions was analysed with the evaluation of the radial distribution function (RDF) of each counterion with respect to the nitrogen atoms of the micellar headgroups.

(i)

The radius of gyration is the mass weighted scalar length of each atom from the center-ofmass (COM). The equation is as follows :

= ∑ ∑ (V-1)
where r is the radius and m is the mass of the atom. R g was analyzed using AmberTools13

( ) = (V-2)
where ρ is a number of particles per unit volume. RDF was analyzed using GROMACS analysis tools [165]. (iv) Eccentricity is a measure of sphericity and can be defined as:

= - (V-3)
where I min is the moment of inertia along the x, y, or z axis with the smallest magnitude and I avg is the average of all three moments of inertia. For the perfect sphere ε = 0 and for the ellipse 0 < ε < 1.

Halides

Halides are monoatomic ions and their impact on the amphiphile self-assembly process can be explained by ion properties such as hydration free energy, size, acidity and polarizability. Note, Chapter V. Structural and interfacial properties of micellar aggregates via computer simulations 149 that the last one was not accounted in the MD simulations. It is well known that for the halide series hydrophobicity and atom size decrease from iodide to fluoride, I -> Br -> Cl -> F -; physical properties (like solubility, CMC and ionization degree) of bulk surfactant solutions reflect this nature very well, see Chapter III for more details on this matter.

Simulations clearly show that the various ions affect differently the micelle structure. In The RDFs presented on Figure V-3 D describe counterion density with respect to the nitrogen atoms of the headroups: interestingly, the highest value was found for I -and the lowest for F -. The first RDFs peaks for the counterions is around 4.5 Å and represents the so-called contact ion pair; from the height of the peaks we can see that the cationic heads of the 10-2-10 micelle are capable of attracting around three times more I -or Br -than F -. The second peaks, corresponding to solvent-shared ion pairs, are located at around 7.5 Å for Cl -, Br -, I -respectively and at 6 Å for F -.

Finally, the third peak at around 12 Å, which represents the fully-solvated ion pairs, is very weak.

Polyatomic counterions

Polyatomic ions have a complex morphology and cannot be described simply by one RDFs for polyatomic counterions were calculated as a distance from the nitrogen atom of the headgroup to the central atom of the anion (nitrogen for NO 3 -, phosphorus for PH, carboxyl carbon for C2 and TFA). RDF of each counterion with respect to the nitrogen atoms of the micelle was estimated as the distance from the gemini nitrogen to the oxygen of the carboxylate group. These results strongly suggest that hydrophobic alkyl carboxylates, penetrating the micelle, change its micellar structure, making it more swollen and rough, and less spherical. Whereas, halides and polyatomic counterions of similar hydrophobicity, favor the compression of the micelle, making it less rough and more spherical.

Interfacial concentrations

In order to give a better estimation of the interfacial counterions and water molarities of the micelles, we compared the results deriving from two completely different approaches: chemical trapping technique (described in Chapter IV) and MD simulations of 10-2-10 with three different counterions: Br -, Cl -, and C2. This is the first work in which experimental and computed interface ion concentrations are compared quantitatively. We will show that the two approaches agree remarkably well one with another. The calculated interface concentrations of water are overall lower than the experimental values. This might be an overestimation due to the assumptions used for the chemical trapping results analysis. The chemical trapping treats the interfacial region and bulk aqueous solution as the same when the product yields in both regions are the same (see Chapter IV). However, this is not necessarily true because the interfacial region has an adjacent hydrocarbon core that the bulk solution does not. Moreover, the headgroups in the interfacial region have a radial orientation that is not present in bulk solution. At the same time the underestimation can come from the MD simulations. For example, the interfacial water concentration was estimated at a distance from the COM that correlated with the maximum of the peaks for the counterions. However, we cannot be absolutely sure that chemical trapping reaction took place exactly at the same distance from COM.

According to the Hofmeister series, the counterion association with cationic micelle is the highest for the Br -and lowest for the C2, this correlates well with the experimental measurement:

CMC and ionization degree α (Chapter III). Deriving an absolute value for α from atomistic simulations, on the other hand, is not trivial. One has to define the boundary, beyond which the counterions are considered dissociated with the micelle. Indeed the distinction between "bound" and "free" ions is somewhat arbitrary[169] and the experimental measurements of this factor strongly depend on the technique used [170].

In The RDFs show that the counterion density around the surfactants' headgroups is the highest for Br -and the lowest for C2. The first peak in the RDFs of the 10-2-10 gemini is ca. 21Å, which represent the contact ion pair distance. The second peak, the water-shared or water-separated ion pairs, is at ca. 25 Å. The third peak at ca. 30 Å, represents the fully-hydrated ion pair. This signal is very weak and only observed with gemini micelles with Cl -and Br -counterions. In order to estimate the ionization degree via MDs, we delivered the fraction of ions at the distance from the COM corresponding to the three troughs on the RDFs profiles, considering that counterions in these regions are not associated with thr micelles Table V-2. In all cases, the computed ionization degrees from MD are very close to the experimental values with all three peaks included.

The largest boundaries located just beyond the third peak of the RDFs, i.e. at ~ 32 Å

Conclusions

Ion specific effects were studied using MD simulations. We investigated aggregates organization of the micelles of 10-2-10 gemini with different counterions. We investigated the interfacial properties of the micellar assemblies as well as structure of the micellar aggregate as a function of the type of monoatomic or polyatomic counterions.

Analyzing the Rg, SASA, ε and RDF, we showed that, due to their strong association and ion pair formation with quaternary ammonium headroups of the amphiphilic molecule, the more hydrophobic counterions (I -, Br -and NO 3 -) produce more compact and stable micelles, where tight packing of the micellar surface protect the hydrocarbon core of the micelle from the solvent molecules. Conversely, hydrated and hydrophilic ions (F -, PH, C1, C2), interact in a weaker fashion with positively charged headgroups, have a lesser degree of contact ion pairing and compress the micelle to a lower extent, thus increasing roughness. The exception being TFA. This counterion is considered hydrophobic, with -ΔG hyd = 251 kJ•mol -1 , and in solution it affects the surfactant properties in the same way that Br -or NO 3 -do. However, MD calculations showed that its properties were closer to that of C2. Such discrepancy can be explained by use of not optimized parameters for this counterion.

The ion specific effects of the mono-and polyatomic anions, with the exception of TFA, calculated by MD simulations exhibit the same trend as was observed experimentally (see Chapter III). This trend follows the Hofmeister series where the lyotropic properties decrease according to the follower order: I -> NO 3 -~ Br -> Cl -> C2 > F -~ PH.

Simulations for the gemini associated with carboxylate counterions showed that the chain length of the anion has a strong impact on the micellar properties. We found that the longer the counterion chain length, the more Rg, SASA, ε and g(r) increased. This is due to the orientation of their aliphatic moieties pointing towards the hydrophobic core of the micelle and hence penetrating into the micelle for the longer chain lengths. In this context the micelle of 10-2-10 with C6 as counterion is behaving as 16-2-16 gemini.

By deriving the boundaries of interfacial region and diffuse layer of the aggregate, we could calculate by MD simulations the interfacial concentrations of counterions and water molecules, which showed a remarkable agreement with the chemical trapping values. Surprisingly, in comparison with Cl -and Br -, we found that due to its highly "diffuse" trend of concentration Chapter V. Structural and interfacial properties of micellar aggregates via computer simulations 165 distribution, the C2 counterions is simultaneously the most penetrating anion into the hydrophobic core of the micelle and the one with the highest ionization degree. Within the acetate molecule, the methyl moieties have a relatively hydrophobic nature while the carboxyl group displays a more hydrophilic one. We envisage that this ambivalent nature of acetate could explain for its observed behavior within the micelle. The different degree of hydrophobicity between Cl -and Br -is also shown: Br -, less hydrophilic, is more concentrated at the interfacial region than Cl -.

Moreover, it was shown that the arenediazonium probe, used for the chemical trapping measurements, is well accommodated within the micelle and its reactive part colocalized with gemini headgroups and counterions at the interfacial region.

In addition, it is noteworthy that, even though polarizability of the anions was not counted in MD simulations, the obtained results that are in good agreement with experimental data. This therefore strongly suggests that the parameters used for the molecular dynamic simulations were chosen correctly.

Experimental section

Materials

Parameters validation

Given that monoatomic cations and anions parameters differ considerably among all the published force fields and a consistent set of parameters in missing for the halides series in CHARMM36 force field, we implemented the set of parameters from OPLS force field[161] For consistency, OPLS parameters were also used for the acetate anion [173] where the chemical trapping results were compared with Cl -and Br -.

There are different case studies in which OPLS parameters were used within CHARMM force field giving good results[174] and moreover some CHARMM22 non-bonded parameters were originally taken from OPLS force field [175]. In the present work we implemented OPLS parameters for the halide counterions and CHARMM36[176] parameters for the gemini surfactants that compose the micelle and all the other polyatomic counterions (together with the CHARMM modified TIP3P model for the water molecules).

We validated these simulation settings against two structural properties of Cl -and Br - immersed in water: 1) the position of the first peak of the radial distribution function (RDF) of water oxygen atoms with respect to the anion and 2) the interaction energy of the counterion with one water molecule. The first structural data was confronted with the related experimental value taken from Marcus work[160] (which is a review that contains ions data generally used as target in computational works); while the second one with ab-initio results taken from ref. [177]. Our finding shows that using OPLS parameters for counterions within CHARMM force field give results in good accordance with experimental data.

Methods

A micelle made up of 27 gemini surfactants 10-2-10 was build using Packmol software [178] and immersed in the box of water with a volume of ~ 83x80x82 Å 3 and ~ 17900 water molecules (Figure V-13). The system was then neutralized with 54 counterions randomly placed in the box; the resulting concentration of the counterion in the box is ca. 164 mM (stoichiometric concentration of the salt equal to ca. 82 mM). The whole system was minimized, then gradually heated up to 300 K at constant volume using Langevin dynamics with a dumping coefficient equal to 5 ps -1 . The density of the system was equilibrated at constant number of particles, pressure (1 atm) and temperature As for the evaluation of the observables, the first 5 ns of the simulations were discarded.

Indeed, looking at Figure V-14, we deem the systems converged just after the first 5 ns of simulation time; whereas, in the case of the diluted system, we omitted the first 10 ns of production run. It was shown that nature of the counterion strongly impacts the balance of forces that contributes to the formation of the micellar aggregates. Thus, for example, the thermodynamic parameter, hydration free energy -ΔG hyd , which determines required energy for transfer of an anion from the ideal gas phase to the infinitely dilute solution, has significant influence on the CMC of the surfactant. For the gemini with counterions that have low -ΔG hyd (e.g., TFA, Br -, NO 3 -), formation of micellar aggregates is more favorable compared to the gemini having counterions with high values of -ΔG hyd (C2, PH, F -). For more details, see

Chapter III.

In the present Chapter, we investigate how this ion specificity will impact the assembly at The question we ask here is the following: if the ion specific effects that we have observed at the councentration around CMC with spherical micellar solution will have the same contribution to the self-assembly process of gemini surfactants at the higher level of system complexity (entangled wormlike micelles), or whether it would differ due to the change in balance of forces that drives the self-assembly of the amphiphiles?

Wormlike micelles

Wormlike micelles of cationic gemini surfactants can be easily obtained without any additives due to the packing parameter [START_REF] Zana | Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications[END_REF]. The geometry of the surfactant aggregates is determined to a certain extent by packing parameter [START_REF] Israelachvili | Intermolecular and Surface Forces[END_REF], p = v/a 0 l, where v and l are volume and extended length of the hydrophobic part respectively, and a 0 is a surface area occupied by the surfactant headgroup. Gemini surfactants, having larger volume of the hydrophobic moiety than their single chain analogues, tend to form cylindrical micelles at much lower concentration [183]. Moreover, for the ionic amphiphiles the facility to form wormlike micelles would depend on the degree of ionization that determines the effective headgroup area. For the surfactants with a lower degree of ionization, the formation of wormlike micelles would be more favorable due to the screened electrostatic repulsion between the headgroups of the surfactant molecules and consequently smaller area per headgroup (a 0 ).

It was previously shown [187,190] that for the salt-free solutions in dilute regime (Ф < Ф*), the micellar length and viscosity increase slowly with increasing surfactant concentration. However, in the semi-dilute regime (Ф > Ф*), the length of the aggregates as well as viscosity increase rapidly [191]. Entanglement of the long micellar aggregates increases the viscoelastic properties of the system, and thereby the micellar network can be characterized as the one for flexible polymers.

Nevertheless, the micellar solutions are called "living" polymer in order to differentiate them from the macromolecular polymer, due to their ability to break and recombine reversibly.

To describe more precisely micellar growth of the surfactants, we used a model proposed by Mackintosh and co-workers [187,190]. With increasing the concentration of the surfactant in the solution (Ф), the average aggregation number ( ) of the micelles rises and can be described by a simple power law increase, with exponent 1/2:

~ Ф / (VI-1)
where Ф is a volume fraction. From the thermodynamic point of view, the growth of aggregates can be characterized by several energies. For the neutral or highly screened micelles there are two main terms. First one is end-cap energy. Being energetically costly, it would promote formation of long aggregates, reducing amount of the end-caps. The second term is entropy of mixing. It would favor formation of many small micelles. These two competitive terms will give rise to the polydispersity of the system. Moreover, the average aggregation number of the micelles can be described as exhibiting a power law growth and will depend on the concentration as follows:

~ Ф / [ ] ⁄ (VI-2)
where is average aggregation number, and E c is an end-cap energy. At the same time, viscosity according to the theory based on this growth law [192][193][194] is expected to be:

~ Ф ~ Ф . (VI-3)
In the case of ionic surfactants, self-assembly process is more complex due to the presence of an additional thermodynamic term: electrostatic repulsion between headgroups that would favor the disassembly of micelles. In addition, due to the presence of counterions in the system we should add a new parameter, Debye screening length (κ -1 ). This parameter would depend on the concentration and properties of the counterions that determine the degree of ionization of the micelle (α). The relationship between Debye length κ -1 and ionization degree in this case can be described by equation[183]:

= (VI-4)
where ρ is the number density of polar heads. strongly enhances the micellar growth with increasing surfactant concentration. The growth of average aggregation number could be derived from electrostatic energy as was described by Mackintosh et al.[187] and Safran et al. [195], giving:

≃ Ф ⁄ [ - * √Ф ⁄ ] (VI-5)
where l is Bjerrum length, a is radius of rodlike micelle and v* is an effective charge per unit length.

Important to note, that aggregate growth depends on E c and α. Due to the scaling of the micellar length, the viscosity of the system in semi-dilute regime (above Ф*) would grow rapidly. Hence, investigating it through zero share rate viscosity and determining overlap concentration, we are able to estimate the contribution of the ion specific effects on the formation of "living" polymer.

Results and discussions

To determine the overlap concentration, we measured zero shear rate viscosity ( 0 ) of the surfactant solutions at different concentrations at 20 °C. Figure VI-2 presents the dependence of 0 on the concentration of the surfactant in the solution for gemini with different counterions. The behavior of 0 is characterized by rapid increase at certain concentration that depends on the counterion of the gemini surfactant. This sharp increase in viscosity indicates the transition from dilute to semi-dilute regime. In the dilute regime, the distance between micelles is relatively high (more than Debye screening length) and the size of aggregates increases slowly. After the transition to semi-dilute regime, the rate of the growth of the aggregates length increases significantly inducing the entanglement of the wormlike micelles. The crossover of these two regimes is characterized by Ф*, that can be easily determined as an intersection of the two lines, the linear fit describing dilute regime and the linear fit describing semi-dilute regime.

It is interesting to note, that for such counterions as TFA, Br -and NO 3 -, the transition is characterized by abrupt change in the slope, whereas for Cl -, C2 and PH the transition is more gradual. The same behavior of the slope was observed for the conductivity measurements, where the transition from the monomers to the micellar aggregates (CMC) depends on the nature of the counterion and provides information about ionization degree of the formed micelles. As was mentioned in the section 1, Ф* strongly depends on the end-cap energy as well as the degree of ionization of the micelle. Since the cationic part of the salt is unchanged, E c would not vary much for the different counterions. At the same time, the counterion nature, that determines ionization degree, would influence the micellar growth. For example, Oda et al. [196] showed that for cetyltrimethylammonium salts, the observed value of Ф* is lower for more hydrophobic counterions (n-heptane sulfonate and mixture of Br -+ HCN -in molar ratio 1/2) than for more hydrophilic ones (n-hexane sulfonate mixture of Br -+ HCN -in molar ratio 1/1). Counterions with higher degree of binding (1-α) screen the micelle and favor the aggregate formation, whilst E c does not strongly dependent on α. Moreover, the influence of the salt effect on the formation of wormlike micelles was investigated [START_REF] Alkschbirs | Effects of some anions of the Hofmeister series on the rheology of cetyltrimethylammonium-salicylate wormlike micelles[END_REF][197][198][199]. Indeed, the Hofmeister effects have a significant contribution to the forces that provide aggregate growth and particularly to the electrostatic interactions. In their work, Oelschlaeger et al. [199] discuss the influence of added salts on the scission energy (energy required to create two end-caps). They showed that added salts have an effect on the Ф* (trend follows Hofmeister series). However, for the investigated systems, scission energy is independent of the nature of the added salt. These findings are in agreement with the results obtained by Oda et al.[196].

Our results in the present case show that Ф* for 10-2-10 gemini, as well as the CMC, depend strongly on the counterion (Table VI-1). For the 10-2-10 X where X = TFA, Br -and NO 3 -Ф* is 51 mM, 78 mM and 71 mM respectively, whereas for X = Cl -, C2 and PH, it is almost 5 times higher (377 mM, 425 mM and 465 mM respectively). In order to discuss these results, we will refer to the approach developed by Collins and coworkers [START_REF] Collins | Ions in water: Characterizing the forces that control chemical processes and biological structure[END_REF]. In their review, it was shown that oppositely charged ions with equal water affinity tend to come together in solution to form contact ion pairs, whereas those with different water affinity tend to stay apart. The affinity to the water is related to the surface charge density of the ion and hence polarizability.

We applied this concept to the system under investigation. First group is monoatomic anions: Cl -and Br -. For these small ions of high charge density are not polarizable and highly hydrated. Big ions of low charge density are polarizable and poorly hydrated [START_REF] Collins | Ions in water: Characterizing the forces that control chemical processes and biological structure[END_REF]. We expect self-assembly for the surfactant with polarizable anion at lower concentration due to its high affinity of the counterion to the headgroup. These results indicate that the self-assembly of the surfactants in the dilute regime (formation of spherical micelles) and semi-dilute regime (formation of entangled wormlike micelles) both depend on the degree of the association between the headgroup and counterion. Stronger headgroupcounterion association, as observed for hydrophobic counterions such as NO 3 -, Br -and TFA -with lower ionization degrees α E , leads to lower repulsion between headgroups and lower interfacial curvature, and more rapid growth of the micelles, as it favors the formation of the longer cylindrical micelles. Surfactants with more hydrophilic counterions form micelles with lower charge screening, hence, higher ionization degree. This is the case for counterions such as Cl -, C2 and PH for which the ionization degrees of the micelles α E are 0.21, 0.26 and 0.25 respectively. Due to the electrostatic interactions that increase the entropy of the system, aggregates grow only at higher concentration, leading to higher Ф*. Interestingly, Ф* value for the 10-2-10 Cl suggests that Cl -is more hydrophilic anion (the value of Ф* is closer to that of C2 than to Br -), whereas, the value of the

Conclusions

In this Chapter we investigated the behavior of micellar growth for 10-2-10 X gemini with different counterions (X = Br -, NO 3 -, TFA, PH, Cl -, C2) using rheology. It was shown that the nature of the counterion has a strong impact on the formation of the wormlike micelles, their further growth and formation of a network of entangled micelles. Hydrophobic counterions such as Br -, NO 3 -and TFA are characterized by stronger association with surfactant headgroups (low α). This provides screening of the electrostatic interactions in the micellar solution at lower concentrations, decreasing the effective headgroup area and as a result favoring formation of long entangled micellar aggregates. In contrast, hydrophilic counterions such as Cl -, PH and C2 having high α, screen micellar charge to a lower extent and exhibit micellar growth at much higher concentrations.

Results shown in Table VI-1 are consistent with the CMC and α data obtained in Chapter III.

The increase of the changes in inducing micellar growth followed the same order as CMC: TFA ~ Br -~ NO 3 -> Cl -> C2 > PH. Analyzing the increase in viscosity of the system related to Ф*, we showed that hydration free energy of the anions provides the best explanation of ion effects on the micellar solution. In addition, our results show that there are the same driving forces controlling aggregate formation at the level of spherical micelles, as well as extensive micellar growth at the level of long cylindrical micelles. Chapter VII. Impact of counterion nature on the crystal structure of cationic gemini surfactants

Introduction

Up to now our study was focused on the ion specific effects on the self-assembly properties of cationic gemini surfactants in aqueous solution. We have clearly demonstrated that the properties of cationic surfactants in the bulk are intimately linked to counterion properties such as ion size, ion polarizability, ion hydration energy, pKa, hydration and lyotropic numbers. Even though, the hydrophilic/hydrophobic properties of anions seem to predict the assembling behaviors of the system relatively well, it still is determined by the subtle interplay between the hydration of counterions and the dissociation energies (stability of crystallinity) of the ion pair. Thus, combining the study of ion effects on the self-assembly of the gemini surfactatns in the crystalline in addition to the solution state we can get better insight into the interactions controlling the ion specificity.

The counterion effects on the solubilization and melting behaviors were aready reported by our group [200]. It was found that contrary to the Kraft temperature, characterazing the solubility of the ionic surfactants, the melting temperature of the crystals was higher for the small ions and decreased as the ionic radii of the counterions increase. Another important observation is that the surfactants with different counterions exhibit different melting behavior, characterizing by liquid crystal mesophases. This strongly suggests that crystalline structure strongly depends on the counterion. In this context, the single x-ray crystallography is a powerful tool to elusidate the effect of various ions on the packing of the amphiphiles at the molecular level.

Various crystal structures of gemini-type surfactants were investigated previously[201-204].

For example, Hattori et al.[203] showed the influence of the spacer length on the gemini 4-s-4 Br, presenting two conformation of the surfactant molecule in the crystal structures. Wei et al.[201] demonstrated the crystal packing for the gemini with OH group in the spacer where hydrogen bonds have a strong impact. However, the systematic study of the impact of the counterion nature on the crystalline packing is yet to be addressed.

In this final chapter we investigate the contribution of ion specificity on the selforganization of amphiphiles in crystalline structure, studying the gemini surfactants with different halide counterions. Moreover, we demonstrate how the gemini having quaternary and tertiary ammonium headgroup (the last one is characterized by proton) crystalize differently. From the observed results, it seems that the anti-gauche conformation is energetically more favorable for the gemini with the spacer length of two and the chain length of ten or more hydrocarbons. Indeed, for the gemini with short spacer and long hydrophobic chains the steric effect and repulsive van der Waals interactions would probably favor the anti-gauche conformation. We also observed in some cases, a polymorphism for 10-2-10 I crystallized in H 2 O (no solvent in the structure). Interestingly, two conformations were found in one unit cell: gauche and anti-gauche

(Figure VII-1 C).
Although, the single-crystals X-ray measurements were not complete due to the low stability and small size of the crystal, the analysis of the anti-gauche conformation for the molecule of 10-2-10 I indicates that they are more "open" and the alkyl chains have angle more than 90° with respect to the spacer.

In order to study the influence of the headgroup properties on the packing arrangenment in the crystals, we studied the 10-2-10 gemini with tertiary ammonium headgroup denoted as 10-2-10 ter. This gemini contains a proton instead of one methyl in the headgroup. Analyzing 10-2-10 ter Br crystalized from H 2 O, we obtained the same space group (triclinic P-1) and antigauche conformation as for the quaternary analog. However, on the Z-the number of patterns in the unit cell; Zʹ-the number of patterns in the asymmetric unit; a, b, c, α, β, γ -unit cell parameters; R1 reliability factor 3 (should be < 10%); wR2 -weighting scheme applied to R1 4 In summary, two conformations are typically observed for gemini ammonium surfactant in crystals: gauche and anti-gauche. The packing arrangement of these molecules in crystal lattice depends on the subtle balance of forces such as electrostatic interactions between headgroups and counterions (nature of counterion), presents of solvent molecules in the structure, van der Waals interactions between hydrocarbons, hydrogen bonding and the solvent from which crystals were obtained.

Chapter VII. Impact of counterion nature on the crystal structure of cationic gemini surfactants 191 1.2. Distances between counterion and nitrogen of the headgroups for gemini n-2-n X, where X = I -, Br -and Cl -.

To characterize the monoatomic counterion effect on the crystal structure and to compare the characteristic distances in the crystals, we focused on the quaternary gemini having counterions I -, Br -and Cl -: 10-2-10 I, 16-2-16 I, 10-2-10 Br, 14-2-14 Br and 14-2-14 Cl, crystalized from organic solvents. Probably due to the high hydrophilicity of Cl -in comparison with Br -and I -(free energies of hydration for I -, Br -, and Cl -are -283, -321 and -347 kJ•mol -1 respectively) [START_REF] Yitzhak | Ion Properties[END_REF] we could only crystalize 14-2-14 for this counterion. We compared the distances between the ammonium atom of the headgroup and the anions (X --N + ). For all the crystallized gemini surfactants, it is observed that the effect of the aliphatic chain length is negligible, whilst the types of counterions have important effect for the relative positions of counterions with respect to N atoms in the crystals, see where q 1 and q 2 are two charges and ε 0 is the permittivity in vacuum. Since halides and headgroups of the gemini surfactants have charge equals 1 and = is a Coulomb's constant, the energy for I -, Br -and Cl -counterions can be approximated as electrostatic potential plus other contributions as follows:

= + + + ⋯ (VII-2) = + + + ⋯ (VII-3) = + + + ⋯ (VII-4)
where r 1 , r 2 and r 3 are the distances between counterion and the three closest cationic headgroups.

Since other terms contribute less to the energy of the ion, these will be neglected. Then the ratio

Chapter VII. Impact of counterion nature on the crystal structure of cationic gemini surfactants 193 between energy of I -and Br -can be shown as = 1.05, and the ratio between Cl -and Br -as = 1.03 for the population of the closer Cl -and 1.02 for the other.

Previously the free energies for the formation of ion pairs of bolaform 1-2-1 X in the gas phase were calculated as X = Cl --158.0, Br --150.8, and I --142.6 kcal mol -1 . These values give the estimated ratio between Br -and I -as = 1.06, and between Cl -and Br -as = 1.05.

Table VII-3. Distances X --N + in crystal structure for n-2-n gemini with three halide counterions: I -, Br -, Cl -.

Object1 Object2 I --N + for 10-2-10 I, Å Br --N + for 10-2-10 Br, Å Object1 Object2 Cl --N + for 14-2-14 Cl, Å The very good agreement for the estimation of the cation-anion interaction in the crystals estimated above and the ion-pair free energy obtained by the DFT calculations is remarkable and gives strong evidence that n-2-n gemini molecules have stronger crystallinity in the presence of smaller anions. The ion sizes of Cl -, Br -and I -are 1.81 Å, 1.96 Å and 2.20 Å respectively [START_REF] Yitzhak | Ion Properties[END_REF].

This observation is also in agreement with the melting temperature of 14-2-14 gemini which increases in the order, I -< Br -< Cl -as we reported previously [200].

Comparing the quaternary and tertiary ammonium gemini, we can see that for 10-2-10 ter Br each counterion strongly bounded to the respective nitrogen of the headgroup with hydrogen bond, the distance X --N + is 3.187 Å.

Conclusions

We have demonstrated the impact of the counterion nature on the crystalline structures of cationic gemini surfactant molecules, n-2-n (n = 10, 14, 16). A number of inter-and intra-molecular forces, such as Columbic forces, van der Waals interactions, hydrogen bonding and steric effect play important roles cooperatively and determines the molecular conformation of gemini surfactants and their packing. It was shown that the crystalline structures of gemini surfactants are roughly classified in two conformations: gauche and anti-gauche. In general, gemini n-2-n with halide counterions crystallize in gauche conformation when they are either crystallized from water or when water molecules are co-crystallized, for other cases, anti-gauche conformation energetically preferable even with solvent in the crystal structure.

We also characterized the strength of association between monoatomic counterions and positively charged nitrogens of the headgroups through distances between them. The distance for the halides follows the trend I -> Br -> Cl -, inversely correlated to the strength of the headgroupcounterion association. Indeed, in a crystalline ordering, more hydrophilic counterions with smaller radii have stronger electrostatic interactions with the surrounding ammonium atoms than the more hydrophobic ones. Important to note, that in aqueous solution, when the molecules self-assemble into micelles, we see the opposite trend. In previous chapters we showed that Cl -ion being the most hydrated, associate less with the interface of the micelle, where as I -make the strong ion pairing with headgroups due to its high hydrophobicity. We demonstrated that in solution the hydration free energy of the counterion has the biggest impact on the properties of surfactant, whereas in crystals the size of an ion is the determining factor.

Finally, it was shown that for the tertiary gemini surfactants formed hydrogen bonds between the headgroup and counterion are crucial for its behavior in solid as well as liquid states (as was shown in other chapters).

Chapter VII. Impact of counterion nature on the crystal structure of cationic gemini surfactants 195 3. Experimental section

Preparation of the single crystals

Single crystals of gemini 10-2-10 I, 10-2-10 Br and 14-2-14 Cl were grown from the saturated acetonitrile solution (40, 50 and 2 mg per 0.5, 0.5 and 0.25 mL respectively) by heating up (60 °C) and cooling to the room temperature and further slow evaporation until crystals would be observed. For 14-2-14 Cl evaporation was avoided in order to decrease the contact between compound and atmospheric water. Might be important to note, that for 14-2-14 Cl, a very small amount of compound was not soluble, which could favor crystals growth.

Using the same procedure single crystals were obtained from nitromethane. Note that for

X-ray single-crystal diffraction.

Crystallographic data of compounds were collected with different diffractometers with a monochromatic Cu-Kα radiation (λ = 1.54190 Å):

(i) a FR-X Rigaku diffractometer with rotating anode, Varimax optics and a Dectris Pilatus

-General conclusions -

In this work we focused on the investigation of ion specific effects on the self-assembly of cationic surfactants. We studied the effect of various counterions on the self-organization features of cationic surfactants in aqueous solution. In order to obtain a more comprehensive understanding of the effect of interfacial ionic and molecular interactions on aggregate properties, we used different approaches. Combining the investigation of physical properties of the molecular assemblies at the bulk solution level and determining interfacial water and counterion concentrations by experiment (chemical trapping) and by simulation (molecular dynamics), we demonstrated that the properties of micellar assemblies strongly depend on the type of monoatomic or polyatomic counter-anion present in solution.



In the first Chapter we present a literature review on amphiphile molecules and their selfassembly properties into micelles in aqueous solution. In order to elucidate how counterions of the ionic surfactants can affect their micellization, we introduce ion specific effects also known as Hofmeister effects. From a historical perspective we can see how all the attempts to describe the interactions involved in Hofmeister effects by simplified theories have their limitations. Introduction of new forces that might be responsible for ion specificity was done almost every decade however there is still no general agreement as to which forces and ion properties drive the ion specific effects. Summarizing all knowledge obtained so far, it is safe to say that in addition to electrostatic and dispersion forces, one should also take into account the hydration forces that play a significant role in ion specificity.



In Chapter II, we introduce the cationic gemini surfactants and counterions that are investigated in this study. We give the synthetic procedures used to obtain cationic surfactants with a variety of counterions.



In Chapter III, the physical properties of the micellar solution are studied. We demonstrate how the critical micelle concentration, CMC, ionization degree and free energy of micellization of the surfactants 1) strongly depend on the ion properties such as size, polarizability, hydration free energy, hydration number, pKa of conjugated acid and lyotropic number of the counterions; 2) follow the Hofmeister series. On the contrary, the aggregation number of the micelles did not show a strong dependence on the counterion -General conclusions -198 nature. The correlation between the ion properties and the properties of aggregation showed that for monoatomic halides, the CMC depends monotonously on all these properties. In contrast, the polyatomic anions have a more complex behavior. We found that all properties of the ion influence the CMC values; however, the hydration free energy of the ions has the most obvious impact to the micellization behavior.

Moreover, classifying anions according to their on hydrophobic/hydrophilic properties, we see that: surfactants associated with more hydrophobic counterions, having a lower ionization degree, favor surfactant micellization. Whereas, surfactants with hydrophilic counterions, having high ionization degree, disfavor the formation of aggregates.

We also demonstrated that by changing the properties of the headgroup, we can significantly affect the order of the Hofmeister series and hence the interactions responsible for ion specific effects.



The experimental investigation of the interfacial properties of the micelles was done by a probing technique called chemical trapping, as discussed in the Chapter IV.

Using chemical trapping we demonstrated that poorly hydrated counterions are strongly associated with the headgroup and are located primary at the interfacial region of the micelle. On the other hand, the interfacial concentration of highly hydrated ions is a low and indicates low degree of counterion-headgroup binding.

These results are in a good agreement with micellar physical properties in bulk solution.



In Chapter V we introduce a computational approach based on molecular dynamic (MD) simulations applied to study the structural properties of the micelles at the atomic level. First of all, the MD simulations were used as a computational approach to investigate interfacial properties of the micelles. We found that the obtained results are coherent with the experimental results obtained by chemical trapping: the hydrophobic ions tend to associate strongly with the micelle and are primarily located in the vicinity of the headgroups forming contact ion pairs. Hydrophilic counterions, having stronger association with water, interact less with the headgroups of the micelle. In addition, it was demonstrated that anion nature affects the structural properties such as compactness, roughness and sphericity of the micelles. The micelle -General conclusions -199 with hydrophobic anions was most stabilized and least with hydrophilic anions.

Moreover, we clearly showed that alkyl carboxylate counterions with increasing chain length start to penetrate the micellar hydrophobic core.

It is interesting to note that although the polarizability of the ions was not considered in the MD simulations, obtained results are in a good agreement with experimental observations. 

In Chapter VI we present a rheological study performed for wormlike micellar systems. We investigated the bulk properties of gemini having various counterions at much higher concentrations, where they form a network of entangled micelles. It was shown that the nature of the counterions strongly affects micellar growth and exhibits a similar tendency (following Hofmeister series) as was shown for spherical micelles.

The hydrophobic coutnerions, due to their association with the headgroups, screen the micellar charge, thus favoring their growth, whereas hydrophilic ions screen micellar charge to a lower extent and exhibit micellar growth at much higher concentrations.

Similarly to the CMC, the observed increase in viscosity of the system attributed to micellar growth, could be correlated with hydration free energy of anion.



In the last Chapter, we consider the impact of the counterion nature on the crystalline structures of gemini surfactants. It was found that, contrarily to the aqueous solution,

where the hydration of anions plays the crucial role, in crystals, the anions with smaller radii show a stronger association with the cationic headgroups.

Summarizing the results obtained by different approaches, we found that the ion specific effects which determine the behavior of micellar aggregates in aqueous solution strongly depend on the hydrophilic/hydrophobic properties (free energy of hydration) of the ion. It is interesting to note that the polarizability of the ions, which provides information of the dispersion forces and thus on ion specificity, seems to correlate less with the aggregation properties. Contrarily, the free energy of hydration of the ion, correlates very well with properties of surfactant aggregates. These results strongly suggest that ion hydration free energy can provide information about the ion specific effects in aqueous solution. However, note that the properties of the substrate should be taken into account not less carefully in order to predict Hofmeister effects.

-Perspectives -

The present study was focused on the fundamental century-old challenge concerning ion specificity. Our results confirmed that the properties of the cationic surfactants with soft methylated headgroups associated with different counterions follow the order in Hofmeister series. However, our preliminary observations concerning tertiary ammonium surfactants, demonstrate that slight changes in the properties of the headgroups, particularly replacement of one methyl group by a proton, resulted in the appearance new interactions that strongly changed the self-assembly of surfactants and hence the impact of conterions.

Having a comprehensive description of the ion specificity for quaternary gemini surfactants, future work can focus on a more global investigation of the systems involving the same anions but different headgroups. More work should therefore be done on tertiary ammonium surfactants in order to understand how changing the softness of the headgroup as well as introducing hydrogen bonding would influence ion specific effects. Moreover, secondary ammonium or even headgroup with another molecular structure should be investigated.

As was shown, the combination of different approaches allows to get an insight into the interactions between counterion and substrate at a molecular level. Applying different techniques, which study the system at different levels, one can obtain a better understanding of its properties and behavior. Such an approach can provide an absolutely new view on many challenges in science including ion specific effects.

202 La présente étude est une approche holistique axée sur l'étude des effets spécifiques d'ions sur les propriétés d'auto-assemblage de tensioactifs cationiques gemini. Notre objectif principal étant l'étude de l'effet de divers contre-ions sur les caractéristiques d'auto-assemblage de tensioactifs cationiques en solution aqueuse. Afin d'obtenir une vision plus complète de l'effet des interactions ioniques et moléculaires à l'interface sur les propriétés globales, nous avons utilisé des approches différentes. Nous avons combiné une étude expérimentale portant sur les propriétés en solution (concentration micellaire critique, degré d'ionisation, nombre d'agrégation, etc.), avec des approches centrées sur l'étude des propriétés micellaires interfaciales en analysant les concentrations des contre-ions et de l'eau de façon expérimentale (piégeage chimique) et informatique (simulations de dynamique moléculaire). En outre, nous avons étudié l'impact de la nature des contre-ions sur la croissance des micelles géantes par rhéologie. En plus de l'examen des propriétés de tensio-actifs en solution, les effets spécifiques d'ions sur les structures cristallines des agents tensioactifs gémini ont été étudiés.

Nous avons trouvé que les effets d'ions spécifiques qui déterminent le comportement des agrégats micellaires de gemini cationiques d'ammonium quaternaire dans des solutions aqueuses dépendent fortement de l'énergie libre d'hydratation des contre-ions, en d'autres termes, sur leur propriétés hydrophile / hydrophobe. Contrairement à la solution aqueuse, dans les cristaux, la taille de l'ion devient le facteur déterminant. La comparaison des résultats obtenus pour un même système en solution aqueuse et à l'état solide a montré l'importance des interactions ion-eau dans les effets spécifiques d'ions. Cependant, il faut noter que les propriétés du substrat (les gemini dans notre cas) doivent être prises en compte non moins soigneusement afin de prédire complétement les effets Hofmeister.

Mots-clés: effets spécifiques d'ions, auto-assemblage, tensioactifs gemini, micellisation, propriétés d'interface, piégeage chimique, simulations de dynamique moléculaire, rhéologie, micelles géantes, structure cristalline The present study is a holistic approach focused on the investigation of ion specific effects on the self-assembly properties of cationic gemini surfactants. Our main focus was on the effect of various counterions on the self-organization features of cationic surfactants in aqueous solution. In order to obtain a more comprehensive understanding of the effect of interfacial ionic and molecular interactions on aggregate properties we used different approaches. We combined an experimental study focused on the bulk solution properties (critical micelle concentration, ionization degree, aggregation number, etc.), with approaches focused on investigating the interfacial micellar properties by analyzing the interfacial counterion and water concentrations, experimentally (chemical trapping) and computationally (molecular dynamic simulations). Moreover, the impact of counterion nature was investigated by studying the growth of wormlike micelles using rheology. Besides the examination of the surfactants properties in solution, the ion specific effects on the crystalline structures of gemini surfactants were studied.

We found that ion specific effects which determine the behavior of micellar aggregates of cationic quaternary ammonium gemini in aqueous solutions strongly depend on the free energy of hydration of the counterions, in others words, on their hydrophilic/hydrophobic properties. Contrarily to aqueous solution, in crystals, the size of the ion becomes the determining factor. Comparison of the results obtained for the same system in aqueous solution and in solid state showed the importance of ion-water interactions in ion specific effects. However, one should note that the properties of substrate (the gemini in our case) should be taken into account not less carefully in order to fully predict Hofmeister effects.

Keywords: ion specific effects, self-assembly, gemini surfactants, micellization, interfacial properties, chemical trapping, molecular dynamic simulations, rheology, wormlike micelles, crystal structure Unité de recherche : Chimie et Biologie des Membranes et des Nanoobjets (CBMN) -UMR 5248

  Chapitre, nous introduisons les gemini cationiques tensioactifs C 2 H 4 -1,2-((CH 3 ) 2 N + C 10 H 21 ) 2 notée 10-2-10 gemini, et une série de contre-ions choisis pour cette étude. Pour étudier les effets spécifiques d'ions nous nous sommes concentrés sur les halogénures monoatomiques, anions inorganiques et alkyl carboxylate polyatomiques. Ici, nous donnons -Résumé -6 également les procédures synthétiques utilisées pour obtenir 10-2-10 gemini avec les différents contre-ions.

Table I- 2 . 1 . 1 . 1 . 1 . 2 . 1 . 3 . 1 . 1 . 1 . 2 . 3 . 3 . 1 .

 211112131112331 Abbreviations and structures of arenediazonium probe and the products obtained during chemical trapping experiment. 1-methylpyrene probe and cetylpyridinim chloride used for fluorescence quenching experiment. ........................................................................................................................... Results and discussions ..................................................................................................... Contribution of ion specific effects to the micelle structure ........................................ Interfacial concentrations ........................................................................................... Position of arenediazoniom probe (16-ArN 2 + ) used for chemical trapping study within the micelle ............................................................................................................................ 2. Conclusions ...................................................................................................................... 3. Experimental section ......................................................................................................... Introduction .............................................................................................................................. 1. Results and discussions ..................................................................................................... Packing of gemini surfactants in the crystalline structure. ........................................... Distances between counterion and nitrogen of the headgroups for gemini n-2-n X, where X = I -, Br -and Cl -. ............................................................................................................... 2. Conclusions ....................................................................................................................... Experimental section ......................................................................................................... Preparation of the single crystals ................................................................................ 3.2. X-ray single-crystal diffraction. .................................................................................. -General conclusions -............................................................................................................... -Perspectives -........................................................................................................................... -References -..............................................................................................................................

Figure I- 1 .

 1 Figure I-1. Types of amphiphilic molecules[3].

Figure I- 2 .

 2 Figure I-2. Relationship between structure of the amphiphilic molecule, that determines critical packing parameter (cpp), and morphology of the aggregates formed in the solution (adapted from ref.[8]).

  critical micelle concentration, cmc (Figure I-3). The aggregation properties of amphiphiles have been studied since the 1950s. In 1948 Debye proposed the theory that micellization is driven by two antagonistic forces: repulsive Coulomb forces between headroups and attractive van der Waals

Figure I- 3 .

 3 Figure I-3. Schematic representation of micellization in water upon addition of small quantities of the single chained cationic surfactant.

Figure I- 4 .

 4 Figure I-4. Typical ordering of anions and cations in Hofmeister series (adapted from ref.[23])

Figure I- 5 .

 5 Figure I-5. Ion mobility, μ i , as a function of ion radius, r, for some anions (left) and cations (right). Values for μ i and r were taken from ref.[29] and [30]respectively.

  Figure I-6 (a). Later this model was modified by Gouy in 1910 and Chapman in 1913. They took into account the fact that ions in the electrolyte solution are mobile and introduced the diffuse layer model, where ions of opposite sign to that of the electrode are distributed in a region of thickness much larger than H, as describes the Figure I-6 (b). Differently from Helmholtz's model, the electric potential within the diffuse layer decreases exponentially. In 1924 Stern proposed to combine Helmholtz and Gouy-Chapman models that describe two different regions for the charge density called Stern layer and the diffuse layer, Figure I-6 (c).The last model is consider to be the basis of the EDLs, even though different modifications were performed in order to improve it[START_REF] Grahame | The Electrical Double Layer and the Theory of Electrocapillarity[END_REF][START_REF] Bockris | On the Structure of Charged Interfaces[END_REF].

Figure I- 6 .

 6 Figure I-6. Schematic representations of EDL structures according to the Helmholtz model (a), the Gouy-Chapman model (b), and the Gouy-Chapman-Stern model (c). H is the double layer distance described by the Helmholtz model. Ψ s is the potential across the EDL[51].

Figure I- 7 .

 7 Figure I-7. Schematic representation of the free energy profile of interaction between two particles according to the DLVO theory.

  Figure I-8.

Figure I- 8 .

 8 Figure I-8. Division of the group IA cations and the VIIA halide anions into strongly hydrated kosmotropes and weakly hydrated chaotropes. The ions are drawn approximately to scale. A virtual water molecule is represented by a zwitterion of radius 1.78 Å for the anionic portion and 1.06 Å for the cationic portion. In aqueous solution, Li + has 0.6 tightly attached water molecules, Na + has 0.25 tightly attached water molecules, F -has 5.0 tightly attached water molecules, and the remaining ions have no tightly attached water[77].

  Collins deduced his rules of cation-anion pairing by the observation of the "volcano plot" (Figure I-9 (A)). The "volcano plot" is a relationship between the standard enthalpy of solution of a crystalline alkali halide (at infinite dilution) ΔH°s ol , and the difference between the absolute enthalpies of hydration of the corresponding gaseous anion ΔH°h yd (anion) and cation ΔH°h yd (cation).

Figure I- 9 .

 9 Figure I-9. The law of matching water affinities. (A) Volcano plot. (B) Schematics of law of matching affinities: a kosmotropic (chaotropic) cation would form a contact ion pair with a kosmotropic (chaotropic) anion, whereas a kosmotropic (chaotropic) cation would not form a contact ion pair with a chaotropic (kosmotropic) anion[23].

Figure I- 9 (

 9 B).

Figure I- 10 .

 10 Figure I-10. "Like seeks like": hard headgroups preferentially interact with hard counterions; soft headgroups preferentially interact with soft counterions. (a) Cations with negatively charged headgroups, and (b) anions with positively charged headgroups[24].

  + > Na + > K + > Li + > H 3 O + , see Figure I-11.

Figure I- 11 .

 11 Figure I-11. Repulsive force between two plates at various surface charge densities for various anions and a cations. Salt concentration c = 0.1 M. Surface charge density 0.02 C/m 2 [81].

Figure I- 12 .

 12 Figure I-12. Equilibrium QCM-D frequency response and dissipation changes of poly(DEA) brush as a function of the solution concentration of potassium acetate (■), nitrate (•), and thiocyanate (▲).The pH was controlled at 5.5 ± 0.1. On the right there is a schematic illustration of the ionic-strength-dependent conformational brush behavior[83].

Figure II- 1 .

 1 Figure II-1. Structure of quaternary (A) and tertiary (B) ammonium 10-2-10 gemini surfactants.

  approaches for anion exchange, depending on the pKa of the acid as summarized in FigureII-2.When the pKa of the acid (HX) is above 3, bromide counterions of amphiphiles are exchanged by mixing the corresponding HX and gemini bromide, in the presence of Ag 2 CO 3 (Figure II-2, synthetic route A). In this case, the silver salt of the acid and the 10-2-10 with the carbonate anion

Figure II- 2 . 1 . 1 . 1 .

 2111 Figure II-2. Scheme of the synthetic routes for counterion exchange for 10-2-10 X gemini. When the pKa of the acid is below 3 (Figure II-2, synthetic route C), gemini acetate is used for anion exchange with the corresponding acid, leading to the formation of amphiphiles with various counterions. For all these ion exchange reactions, methanol was used as solvent. The obtained salts were purified by washing the solid with adequate solvent or by crystallization.

Figure II- 3 . 1 H

 31 Figure II-3. 1 H NMR spectrum of 10-2-10 Br in MeOD. The colors indicate different protons detected by 1 H NMR in the molecule. The peaks with the star are attributed to the solvent.

Figure II- 4 .

 4 Figure II-4. XPS high resolution spectrum of the N1s region for 10-2-10 Br.

  completion, the solvent was concentrated under vacuum and the silver salts were removed by filtration through celite. This step was performed 2 to 3 times. Note that a minimum amount of MeOH was used to avoid the solubilization of silver salts. After filtration, the methanol was evaporated under vacuum. The residue in the case of acids with three and four carbon atoms (C3-C4) was dissolved in hot (60 °C) CH 3 CN (~ 10 mL) and cooled to room temperature to give crystals of 10-2-10 C3/10-2-10 C4. For the compound bearing counterions of five carbon atoms (C5), a mixture of CH 3 CN/MeOH (200/0.5mL, v/v) was used. Gemini with C6-C8 counterions were solubilized in hot methanol (60 °C) and precipitated with acetone due to their much higher hydrophobicity. All obtained products (crystals or powder) had a grey color due to the impurity of silver salts. These compounds were again solubilized in hot CH 3 CN for C3-C4 and CH 3 CN/MeOH for C5 and these hot mixtures were filtered through celite. After cooling down the solvent, a white precipitate was formed. It was collected and dried under vacuum. The compounds with counterions 60 of C6-C8 chain length were filtered through Celite from MeOH (r.t.). The MeOH was evaporated under vacuum, until the residue on the bottom becomes totally dry, otherwise the traces of water will influence the solubility of the compound during precipitation. The residue was dissolved with a minimal amount of MeOH at 60 °C and precipitated by adding acetone.The purity of the obtained compounds (C3-C8) was verified by 1 H NMR in MeOD with increased recycling delay (D 1 ): C3 -20s, C4-C5 -15s, C6 -10s instead of 2s, usually used.

Scheme II- 3 :

 3 Scheme II-3: Synthesis of 10-2-10 Ac.

Stability of 10 - 2 - 10 F 10 - 2 - 10 F

 1021010210 prepare 10-2-10 F we used 10-2-10 I instead of 10-2-10 Br for the exchange reaction, since for the iodide salt reaction completion can be easily assessed, by adding hydrogen peroxide to the mixture. If the reaction is incomplete, the colorless solution of 10-2-10 I becomes yellow due to the oxidation of iodide to molecular iodine, indicating that there is still 10-2-10 I present in the reaction mixture.In this case the photoreduction experiment was inappropriate to follow the reaction, since it will oxidize both the 10-2-10 Br and AgF, consequently the disappearance of 10-2-10 Br cannot be quantified precisely. To check if the reaction is complete, the test experiment has been done as follow: we stopped the stirring and took an aliquot of the transparent solution and then add H 2 O 2 to this solution. If the solution turns yellow, there is still 10-2-10 I and the exchange reaction is not finished.The purification of 10-2-10 F was performed by filtering the reaction mixture through celite and evaporation of methanol solution under vacuum. This step was repeated several times until the residue of 10-2-10 F will become white. Crystals of 10-2-10 F were obtained by crystallization the compound from acetone, filtered and dried under vacuum. It is important to note that, this compound cannot be dried by lyophilization, due to its decomposition under strong vacuum. is stable for a few days at room temperature, and after it starts to decompose slowly. The use of strong vacuum facilitates its decomposition. The compound changes its color from white to brownish and acquires unpleasant odor. The decomposition of 10-2-10 F can be followed by proton NMR as shown on the Figure II-5. This figure presents the 1 H NMR spectra of 10-2-10 F just after the synthesis (A) and after several days at room temperature (C).

Figure II- 5 . 1 H 4 .

 514 Figure II-5. 1 H NMR spectra of 10-2-10 F just after synthesis (A) and 1 H NMR of 10-2-10 F after the decomposition (C). Each signal at 1 H NMR spectrum (A) represents a group of protons in the structure of gemini (B). On the Figure (C), label N represents new peaks that appeared during decomposition, whereas label G represents the remaining peaks of 10-2-10 F. Peaks for solvent are noted with the star. The decomposition of 10-2-10 F is confirmed by the appearance of new peaks in the range of 5-7 ppm, indicating the formation of unsaturated bonds. The integration of the peak attributed to the spacer decrease from 4 to 1.5. The intensity of the signals attributed to methyl groups in α and β position to the nitrogen atom decreases, when the new peaks appear (Figure II-5 C), indicating the transformation of 10-2-10 F gemini molecule to decyl(dimethyl)amine and decyl(dimethyl)vinylammonium fluoride as shown in Scheme II-4. Due to the low stability of the

Figure II- 6 .

 6 Figure II-6. XPS high resolution spectrum of the N1s region for 10-2-10 F. Peaks A and B correspond to amine (~ 399 eV) and ammonium (~ 403 eV) respectively.

  P NMR (D 2 O) respectively as reference. The signal of 10-2-10 H 2 PO 4 in 31 P NMR appears at 3.2 ppm, indicating that the phosphate was monoanionic, see Figure II-7.

Figure II- 7 . 31 P

 731 Figure II-7. 31 P NMR spectrum of Na 2 HPO 4 (A), NaH 2 PO 4 (B) and 10-2-10 H 2 PO 4 (C) in D 2 O.

1. 2 .

 2 Synthesis of tertiary ammonium 10-2-10 gemini surfactants In the tertiary 10-2-10 gemini surfactants, one methyl group of the headgroup was substituted by a proton as shown in Figure II-8.

Figure II- 8 .

 8 Figure II-8. Structure of quaternary (A) and tertiary (B) ammonium 10-2-10 gemini surfactants.

  Scheme II-6. Synthesis of 10-2-10 ter I.

Figure II- 9

 9 Figure II-9 shows the 1 H NMR spectrum of tertiary (A) and quaternary (B) 10-2-10 I in MeOD. The integration for the signals attributed to the methyls of headgroups (in green) changed from 12 (Figure II-9 B) to 6 protons (Figure II-9 A). Moreover the singlet, multiplet and singlet that

Figure II- 9 . 1 H

 91 Figure II-9. 1 H NMR spectrum of tertiary (A) and quaternary (B) ammonium 10-2-10 I in MeOD. Color labeling of certain proton groups on the molecular structure and NMR spectrum highlight the shift of the peaks corresponding to the protons in α position regardless ammonium for tertiary gemini in comparison with quaternary ammonium. Peaks for solvent were noted with the star. 10-2-10 ter I was also characterised by XPS. Taking into account a standard error due to the fitting and the sensitivity of the machine, obtained stoichiometry of iodide correlates well with theoretical values (Table II-2). On the Figure II-10 we can see that position of N1s peak (~ 401 eV)

2 .Figure II- 10 .

 210 Figure II-10. XPS high resolution spectrum of the N1s region for 10-2-10 ter I.

of 10- 2 -10 amine 10 - 2 -

 2102 10 amine was prepared by dialkylation of DMEDA with 1-bromodecane in the presence of Hünig's base N,N-diisopropylethylamine (DIPEA) in acetonitrile, at 40 °C (Scheme II-7).

  Scheme II-7. Synthesis of 10-2-10 amine.

Figure II- 11 .

 11 Figure II-11. XPS high resolution spectrum of the N1s region for 10-2-10 ter Br.

10- 2 -

 2 10 with carboxylate counterions. Here we present NMR spectrum for 10-2-10 with counterions: C3-C8. 10-2-10 C3: 1 H NMR (300 MHz, MeOD, 25˚C, D 2 =20 s): δ = 3.93 (s, 4H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 3.43 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.21 (s, 12H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 2.16 (q, 4H, CH 3 -CH 2 -COO -), 1.83 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + ), 1.41-1.31 (m, 28H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + ), 1.10 (t, 6H, CH 3 -CH 2 -COO -), 0.90 (t, 6H, CH 3 -(CH 2 ) 9 -N + ). 10-2-10 C4: 1 H NMR (300 MHz, MeOD, 25˚C, D 2 =15 s):

10- 2 -

 2 10 C5: 1 H NMR (300 MHz, MeOD, 25˚C, D 2 =15 s): δ = 3.94 (s, 4H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 3.43 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.21 (s, 12H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 2.15 (t, 4H, CH 3 -(CH 2 ) 2 -CH 2 -COO -), 1.83 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + ), 1.63-1.55 (m, 4H, CH 3 -CH 2 -CH 2 -CH 2 -COO -), 1.41-1.31 (m, 28H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + and 4H, CH 3 -CH 2 -(CH 2 ) 2 -COO -), 0.92 (m, 6H, CH 3 -(CH 2 ) 9 -N + and 6H, CH 3 -(CH 2 ) 3 -COO -). 10-2-10 C6: 1 H NMR (300 MHz, MeOD, 25˚C, D 2 =15 s): δ = 3.93 (s, 4H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 3.43 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.21 (s, 12H, (CH 3 ) 2 N + -(CH 2 ) 2 -N + (CH 3 ) 2 ), 2.14 (t, 4H, CH 3 -(CH 2 ) 3 -CH 2 -COO -), 1.82 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + ), 1.65-1.55 (m, 4H, CH 3 -(CH 2 ) 2 -CH 2 -CH 2 -COO -), 1.41-1.31 (m, 28H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + and 8H, CH 3 -(CH 2 ) 2 -(CH 2 ) 2 -COO -), 0.90 (m, 6H, CH 3 -(CH 2 ) 9 -N + and 6H, CH 3 -(CH 2 ) 4 -COO -). 10-2-10 C8: 1 H NMR (300 MHz, MeOD, 25˚C):

  2.15 (t, 4H, CH 3 -(CH 2 ) 5 -CH 2 -COO -), 1.82 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + ), 1.60 (m, 4H, CH 3 -(CH 2 ) 4 -CH 2 -CH 2 -COO -), 1.41-1.31 (m, 28H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + and 16H, CH 3 -(CH 2 ) 4 -(CH 2 ) 2 -COO -), 0.90 (m, 6H, CH 3 -(CH 2 ) 9 -N + and 6H, CH 3 -(CH 2 ) 6 -COO -). 10-2-10 Ac. To a solution of 10-2-10 Br (30g, 53.7 mmol) in 200 mL MeOH was added silver acetate (22.41 g, 134.

10- 2 -

 2 10 with ions of strong acids. 10-2-10 gemini with counterions of strong acids was synthesized by reaction of 10-2-10 Ac with HX in molar ratio 1:2 (1:3 in case of HCl). The reaction mixture was stirred at 40 °C in MeOH for 2 h. The reaction was monitored using 1 H NMR by the disappearance of the signal of Ac peak at 1.90 ppm. The resulting mixture was evaporated under vacuum and dry residue was dissolved in a minimal volume of MeOH (Table

Figure III- 1 .

 1 Figure III-1. Relationship between salt heat of dissolution and difference in enthalpies of hydration for cation and anion for salts composed from soft/hard ions[START_REF] Collins | Ions in water: Characterizing the forces that control chemical processes and biological structure[END_REF]

  conductivity (κ) versus concentration (C) values. The first line (with slope S 1 ) and second line (with slope S 2 ) correspond to the conductivity set of values below and beyond the CMC respectivly. This commonly used approach gives easy access to the CMC when the break between the two slopes of the premicellar and micellar transition is abrupt (Figure III-2 left). However in the case when micellar aggregates are formed very gradually it becomes more difficult to define the break in the slopes (Figure III-2 right

Figure III- 2 .

 2 Figure III-2. Example of conductivity dependence versus surfactant concentration for 10-2-10 Br (left) and 10-2-10 HP (right) 1.1.2.Ionization degree

  1956 Evans proposed a more correct method to estimate the ionization degree, which elucidates the contribution of the micelles to the conductivity of the solution by taking into account the screening of the micellar charge by the counterions[101]. To describe this screening, parameters need to be adjusted. The molecule of the 10-2-10 X gemini, composed of amphiphilic cation with CMC Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 85 charge +2 (A 2+ ) and 2 counterions of the charge -1 (2X -), is written as A 2X for simplification.

13 )

 13 Substituting Equations (III-5), (III-6) and (III-7) into Equation (III-12) we get κ C>CMC :Evans proposed the way to estimate the conductivity of micellar ions that gives the most Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 86 adequate results for the determination of ionization degree[101, 102]: = (III-14) Substitution of the Evans model of micellar conductivity, Equation (III-14), in Equation (III-12) and expression of λ A from Equation (III-11) will give us the conductivity of the surfactant solution above the CMC:

a

  -CMC was calculated by intersection point of two straight lines before and after the CMC, S 1 and S 2 respectively b -CMC by fitting based on the integration of a Boltzmann function[START_REF] Carpena | Problems Associated with the Treatment of Conductivity-Concentration Data in Surfactant Solutions: Simulations and Experiments[END_REF]. All other parameters were calculated accounting these values of CMC.

89 * -experiment was done at 50°C Figure III- 3 .

 8950°C3 Figure III-3. CMC values of 10-2-10 X (at 30 °C and 50 °C in the case of 10-2-10 I) as a function of ion properties. (A) Hydration free energy -ΔG hyd , kJ·mol -1 ; (B) hydration number n H ; (C) partial molar volume ν s , cm 3 ·mol -1 ; (D) polarizablility α, Å 3 [30]; (E) pKa [commom values]; (F) lyotropic number N lyo [104, 105].Considering halide counterions only, it is clear that CMCs versus all ion properties exhibit a quasi-linear dependence, despite the fact that CMC for 10-2-10 I was measured at a higher temperature, due to its low solubility. The CMC values increase with increasing -ΔG hyd , n H , and pKa, (A, B, and E), and decreasing counterion size, polarizability, and N lyo (C, D and F). The good

  Figure III-3 (A)

2 equals 1 ,

 21 It is interesting to compare counterions such as monoatomic Br -and polyatomic NO 3 -, as their CMC (for 10-2-10 Br and 10-2-10 NO 3 ) are almost the same (6.4 and 6.3 mM respectively), but, the properties of the anions differ slightly. The free energy of hydration -ΔG hyd and the partial molar volume ν s (Figure III-3 (A) and (B) respectively) suggest that the aggregate formation for the surfactant with NO 3 -counterions is expected to be at lower concentration (lower CMC) than for Br -. However, the hydration number n H , polarizability and pKa (Figure III-3 B, D and E respectively)suggest that CMC for NO 3 -to be higher than for Br -. Moreover, the planar morphology of the NO 3 anion could affect the self-assembly process. Being planar, NO 3 -can intercalate between the polar headgroups and dehydrate the micellar surface in comparison with spherical Br -. All these parameters influence the balance of forces that drives the micellization resulting in similar CMC values for gemini with Br -and NO 3 -. How much each property contributes to the micellization is difficult to estimate at this point.Although PH and F -have almost the same high hydration free energy, -ΔG hyd , (473 and 472 kJ·mol -1 correspondingly, Figure III-3 A) there is a strong difference in their hydration number, n H :1.8 for PH, compared to 2.7 for F -, (Figure III-3 B) which may have some effect on the CMC. In a previous study[START_REF] Manet | Counteranion Effect on Micellization of Cationic Gemini Surfactants 14-2-14: Hofmeister and Other Counterions[END_REF] on 14-2-14 gemini, comparing PH and F -counterions, a higher CMC was observed for PH and attributed to entropic effects. High n H of F -would induce entropy of the system due to the liberation of water during micellization. Whereas, polarizability and low hydration number of PH would not favor micellization. However, in the present case, the opposite tendencyChapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 91 was observed for the related 10-2-10 gemini. The CMC obtained for PH (26.0 mM) is lower than that for F -(31.7 mM). These results are puzzling and additional investigation of these systems is required in order to understand the mechanism that drives their aggregation.Comparing the tree large polyatomic anions, PH, C2 and MeSO 3 -(ν s is 34.6, 46.2 and 61.3 cm 3 ·mol -1 ) with monoatomic iodide (ν s is 41.7 cm 3 ·mol -1 ), we can see that even though they are highly polarizable, their CMCs are much higher than that of I -(Figure III-3 C and D). These resultssuggest that hydrophilic properties of PH, C2 and MeSO 3 -have a higher contribution than their size and polarizability.For halides, the electronic nature of the ions, reflected by their pKa, correlates very well with the values of the CMC. This is not the case for the polyatomic PH, C2, NO 3 -and TFA, which do not have a homogeneous electron density (Figure III-3 E). Finally, we observed that obtained CMC values correlate well with the lyotropic number of the anions (Figure III-3 F). This number gives information on the ability of salts to induce the flocculation of colloidal systems[106]. Furthermore, there is a linear dependence between the lyotropic number and the electric field strength of the ion[105, 106]. Since we observe a direct impact of the lyotropic number on the assembly behavior of micelles in solution, we can conclude that there is a close correlation between the electric field strength and micellar assembly. The correlation coefficient R 2 for different graphics (Figure III-3) was determined. When R the dependence of Y on X is linear. In our case, all values of R 2 are small, indicating that the CMCs have no linear dependence on ion properties. However, analyzing obtained values of R 2 we can see which property of the ion can better describe the CMC. Obtained results are presented in

Figure III- 4 . 93 Figure III- 5 .

 4935 Figure III-4. Ionization degree α E , as a function of CMC for 10-2-10 X.The free energy of micellization (TableIII-1) follows the same trend as the CMC. -ΔG°M is higher for more hydrophobic counterions (15.69 kJ•mol -1 for 10-2-10 Br) and decreasing in order Br -~ NO 3 -> Cl -> MeSO 3 -> C2 > PH > F -(9.28 kJ•mol -1 ), see Figure III-5.

94 Figure III- 6 .

 946 Figure III-6. CMC (A) and ionization degree α (B) as a function of counterion chain length.

B 95 Figure III- 7 .

 957 Figure III-7. Structure of quaternary (A) and tertiary (B) ammonium 10-2-10 gemini surfactants.To obtain CMC values for 10-2-10 ter X conductivity measurements were performed. Two types of counterions: Br -and Cl -were analyzed. The results are presented in the Table III-4 along with the quaternary analogs.

  While the transition of the conductivity slope is abrupt at the CMC for 10-2-10 Br (Figure III-8 right), the break in the slope for 10-2-10 ter Br is very subtle (Figure III-8 left) and it is difficult to define the CMC from the conductivity measurements only.

Figure III- 8 . 97 Figure III- 9 .

 8979 Figure III-8. Conductivity as a function of the surfactant concentration for 10-2-10 ter Br (right) and 10-2-10 Br (left). Considering the issue cited in the previous paragraph, we therefore used a complementary technique, 1 H NMR[107] in order to confirm the CMC values. Figure III-9 presents the NMR spectrum for 10-2-10 ter Br in D 2 O over the concentration range 0.3 -2 mM. Chemical shifts for the signals of the singlet at 3.61 ppm and the multiplets at 3.22 ppm and 2.78 ppm at a surfactant concentration of 0.5 mM indicate the aggregation formation and the appearance of the micelles. The most efficient method to monitor micelle formation by NMR is to follow the shift in the resonance attributed to the spacer protons that are expected at 3.61 ppm in the dissociated form. As a result, values of the CMC for 10-2-10 ter Br obtained by proton NMR (~ 0.5 mM) correlate with values obtained by conductivity measurements within experimental error.

  Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level[START_REF] Galgano | Micellar properties of surface active ionic liquids: A comparison of 1-hexadecyl-3-methylimidazolium chloride with structurally related cationic surfactants[END_REF] In order to estimate the pKa for both protons we did a titration of 10-2-10 ter Br with added HBr by a 25 mM solution of NaOH. On Figure III-10 one observe three equivalence points indicated by dash lines. The first (1.24 mL), second (1.76 mL) and third (2.47 mL) equivalence points correspond to the volume of NaOH required for the titration of HBr, the deprotonation of one headgroup and of the second headgroup of the gemini respectively. The pKa 1 and pKa 2 for the gemini are defined as the pH at the half-way of each titration and are equal to 4.40 and 5.35 respectively, see Figure III-10.

Figure III- 10 . 2 . 2

 1022 Figure III-10. Titration curve for 10-2-10 ter Br at a concentration 1.5 mM in presence of 3 mM HBr with NaOH (25mM). Dash lines indicate the equivalence points, blue solid lines correspond to pKa. The HBr solution was added progressively (the variation of pH 4 down to 2) to the 1.5 mM aqueous solution of 10-2-10 ter Br (pH ~ 4) and followed by 1 H NMR. No change in the chemical shifts was observed. This indicates that no change in protonation occurs over this range of pH, that is, 10-2-10 ter Br is dicationic in the studied concentration range. This result was further confirmed by the titration done on tetramethylethylenediamine (TMEDA) with HBr, Scheme III-2 (performed by our collaborator Changyao Liu).

Figure III- 11 .

 11 Figure III-11. Position of the 1 H NMR peaks for the TMEDA as a function of added HBr.Since pH and NMR measurements do not show evidence of the monocationic gemini molecules, we can suppose that the stronger tendency for 10-2-10 ter Br to form aggregates should result from other reasons. In order to investigate the nature of cation-anion interactions, density functional theory (DFT) calculations were performed to probe the binding energies in continuum water for 1,2-bis-trimethylammonium (1-2-1) and 1,2-bis-dimethylammonium (1-2-1 ter) complexes

Figure III- 12 . 1 - 2 - 1

 12121 Figure III-12. Schematic representation of the free energy of binding on the example of 1-2-1 Cl complex formation.

2 - 1

 21 in comparison to the quaternary ones can be attributed to the formation of hydrogen bonds between counterions and the proton of the gemini headgroups as shown in Figure III-13. Hydrogen bonding is clearly a major factor in stabilizing each of the complexes with the halogen series. Formation of hydrogen bonds between X -and ammonium proton was shown as well by crystallography and + - ΔG B = -3.17 kcal/mol Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 101 discussed in Chapter VII. Although in solution the strength of this hydrogen bond is weaker than in the crystal structure, the contribution to the properties of the surfactant in solution is clearly observable experimentally, as seen by the low solubility and computationally by noting the lengthened N-H bonds and the shortened X-H bonds.

Figure III- 13 .

 13 Figure III-13. Representation of the hydrogen bonds between protons of the 1-2-1 ter and halides. Color code: grey, white, blue, green, red and purple corresponds to carbon, hydrogen, nitrogen, chloride, bromide and iodide respectively.

Figure III- 1 ,

 1 Figure III-1, the highest association are expected for soft-soft ions due to their high polarizability and hydrophobicity. The next more stable ion pairs are formed by hard-hard ions. Hard ions are often highly hydrated, and ion pair formation is favored due to the liberation of the water to the bulk, and hence the increase in the entropy of the system. Ion pairing between soft-hard or hard-soft ions is less preferable. For example, quaternary ammonium gemini have fully methylated headroups and can be considered to be soft cations. They tend to form strong ion pairs with soft anions such as I -. Hard counterions such as F -do not form tight ion pairs with soft quaternary headroups, as was shown previously. Full protonation of the headgroup (for secondary ammonium ions) would in theory convert these to harder cations, and the interactions between secondary ammonium

  They are generally based on determining the size of aggregates using techniques such as lightscattering[109, 110], fluorescence correlation spectroscopy[111], small-angle neutron scattering[112], NMR self-diffusion coefficient[113], electron-paramagnetic resonance[114], etc. However most of these techniques have restrictions to the properties of colloid systems or depend on the intermicellar interactions, or are not easily accessible because of the necessary facilities to house such experimental setups[115, 116]. Fluorescence quenching (FQ) techniques are amongst the most widespread methods for investigating the structure of microheterogeneous media. In contrast to the techniques mentioned previously, fluorescence probing methods allow to determine the aggregation number independently of intermicellar interactions and micelle shape, by measuring the concentration of aggregates[86, 115]. Fluorescence quenching techniques are generally based on using a luminescent hydrophobic chromophore to probe the interior of micelles. Moreover, this technique can also provide information about the probe's microenvironment[117] and polydispersity of the micelles[118].

quenching can be described by the following 22 )

 22 where τ 0 =1/k is a probe fluorescence lifetime in the absence of added quencher, k is the sum of the unimolecular decay constants and k Q is the second-ordered quenching rate constant. In the presence of Q at concentration [Q] the probe lifetime τ equals: Equation III-22, known as the Stern-Volmer equation and is widely used to determine bimolecular rate constants for excited-state reactions involving an emissive species:

[ 27 )

 27 Q]/[M], the ratio of the fluorescence emission intensities in the absence and in the presence of Q is given by[115, 119]: Since the P and Q are immobile during the photochemical reaction we can compare this system with quenching in homogeneous solutions. The difference between Equation (III-24) and Equation (III-27) comes from the compartmentalization of the P and Q in the micellar microenvironment.

  Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level106Obtaining [M] from the semi-logarithmical dependence of I 0 /I Q against [Q] one can determine N:

III- 29 )

 29 Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 107 where A 1 is the fluorescence intensity at time equals zero (I 0 ); A 2 is the fluorescence decay constant in the absence of Q (A 2 = k 0 = 1/τ 0 ); A 3 is the average occupation number of quenchers per micelle (<n> = [Q]/[M]); and A 4 is the quenching rate constant (k Q ) of P* by Q inside a micelle. The fluorescence decay curves are fitted to Equation (III-29) using a non-linear least-squares algorithm contained in the commercial software from the instrument or the Decan 1.0 software[123],

  yields the true or weight averaged aggregation number, N W , and the slope yields the variance of the aggregation number distribution[124]. Note, that at very low quencher concentrations, the fits to the decay curves are subject to a higher error and have been found to overestimate the aggregation numbers, sometimes by as much as 15%. Recommended values of <n> for single chain amphiphiles are around 1[124].

Figure III- 14 .

 14 Figure III-14. Molecular structure of probe, 1-methylpyrene, and quencher, cetylpyridinium chloride.

Figure III- 15 .

 15 Figure III-15. N av as a function of [Q]/(2(C-CMC)) for 10-2-10 X at 3xCMC, where X= F -, Br -, Cl -, NO 3 -, PH, TFA, C2 and MeSO 3 -. A diagram presenting N av for 2xCMC and 3xCMC is shown in Figure III-16. For all cases,

Figure III- 16 . 3 -,

 163 Figure III-16. Diagram presenting N av for 10-2-10 X, where X = F -, PH, C2, MeSO 3 -, Cl -, Br -, TFA at twice and trice the CMC and for 10-2-10 ter Br at twice the CMC. It is interesting to compare the N of 10-2-10 gemini with that of the monomeric analogs, decyltrimetylammonium bromide (DeTAB). Different values of the aggregation number are reported for the latter, depending on the technique that was used: 36[125], 39[126], 48[127] at the

Figure III- 17 .

 17 Figure III-17. N av as a function of α E for 10-2-10 X gemini, where X = F -, PH, C2, MeSO 3 -, Cl -, NO 3 -, Br -. Red color indicates hydrophobic counterions and green corresponds to the hydrophilic counterions. On the right there is the schematic representation of aggregation behavior according to Hofmeister series.

F

  Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 112

Figure III- 18 ,

 18 the dependence of N av versus [Q]/(2(C-CMC) is

Figure III- 18 .

 18 Figure III-18. N av as a function of [Q]/(2(C-CMC) for 10-2-10 X at 3xCMC, where X = C1, C2, C3, C4, C5 and C6.It is expected that the aggregation number for gemini would be larger at 3xCMC than at 2xCMC due to the growth of the aggregates with surfactant concentrations. The difference in N av between 2xCMC and 3xCMC for 10-2-10 gemini with carboxylates is relatively small, except for C1, the most hydrophilic counterion (Figure.

Figure III- 19 .

 19 Figure III-19. Diagram presenting N av for 10-2-10 X, where X = C1-C6 at twice and trice the CMC.

Figure III- 20 .

 20 Figure III-20. N av as a function of α Z for 10-2-10 X gemini at 2xCMC, where X = from C1 to C6.

Figure III- 21

 21 Figure III-21 compares the number of carbons in the liquid hydrocarbon core of the micelle without the contribution of counterions (black rounds, Carb S ) and counting the contribution of the

Figure III- 21 .

 21 Figure III-21. Carbone content of the micelle as a function of α Z for 10-2-10 X gemini, where X = from C1 to C6. Black solid rounds represent the carbon content of the micelle without accounting the counterions (Carb S ), and the red squares represent carbon content of the micelle accounting the counterions (Carb M ).

  Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level116

~ 4

 4 times higher than CMC) with extra pure water. The solution was continuously stirred to keep the equilibrium in the system. Indicated conductivity values were collected at each concentration after stabilization of the system (5-10 min). Treatment of the results. As it was already mentioned in subchapter 1.1. of this chapter, we used two methods for the data treatment to determine CMC's and α Z values: (a) intersection point of two straight lines before and after CMC, S 1 and S 2 respectively; (b) fitting based on the integration of Boltzmann function. The data for the both methods are given in the Table III-8.

.

  Stock solutions: amphiphile at 2xCMC and 3xCMC in ultrapure water (18.2 MΩ•cm), probe at concentration ~ 5•10 -4 M (absolute ethanol) and quencher (~ 4•10 -3 ) in MilliQ water were prepared the day before to make the systems stabilize. To a solution of gemini (2.5 mL) in quartz cell (10 mm) was added 15μL (10 μL or less for surfactants with low CMC) of 1-methylpyrene. Final probe concentration in the cell is ~ 3•10 -6 M. The decay of emission was first recorded in the absence of the quencher, which gave us a lifetime of 1-methylpyrene equal to ~ 110ns. For the determination of N we added quencher aliquots such that the average number of quencher molecules per micelle varied from 0.2 to 2. The fluorescence decay curves were recorded using a FL3-22 SPEX spectrofluorometer (Figure. The excitation wavelength was 310 nm, and the emission was monitored at 370 nm. All measurements were performed at room temperature.

Figure III- 22 .

 22 Figure III-22. Time-resolved fluorescence emission of 1-methylpyrene in presence of 10-2-10 C3 (60 mM 3xCMC) and cetylpyridinium chloride (0.6 mM) Data analysis. Data processing was done by fitting the fluorescence decay curves to Equation (III-29) using a non-linear least-squares fitting program provided with the instrument or with the program developed by Boens et al.[123].

1 . 1 . 1 .

 111 Scheme IV-1. Heterolytic dediazoniation by release of N 2 in the presence of weakly basic nucleophiles for 4-alkyl-2,6-dimethylbenzenediazonium ions[134]. Compared to other spontaneous reactions, dediazoniation of arenediazonium ion has several advantages that make it suitable for CT. The rate constant for heterolytic dedizoniation is almost totally insensitive to the solvent polarity[138, 139] and hence changes in the reaction medium have

Figure IV- 1 .

 1 Figure IV-1. Representation of cationic micellar solution where arenediazonium ion traps H 2 O (not shown) and counterions in the interfacial region of the aggregates (adapted from image by C. Liu and ref[141]).

  shown in Figure IV-2, where on the left, the reaction at the interfacial region of aggregates is presented, and on the right the reaction in aqueous solution for short chain analogs is shown. When the product yields for the short chain dediazoniation reaction, 1-ArX and 1-ArOH, in the reference aqueous solution (Figure IV-2 right) are the same as product yields for the long chain dediazoniation products, 16-ArX and 16-ArOH, in the interfacial region (Figure IV-2 left), i.e., when the selectivity of the arenediazonium ion is the same in both cases, the concentration of the reactive components in the water and interface are the same. When the yield is the same, the concentration is the same.

Figure IV- 2 .

 2 Figure IV-2. Left picture represents dediazoniation reaction of the probe 16-ArN 2 + within the interface of an amphiphile aggregagtes with subsequent formation of products 16-ArX and 16-ArOH (H 2 O molecules not shown). Right picture represents the same dediazoniation reaction of the short chain analog of the probe -1-ArN 2 + in aqueous solution with formation of 1-ArX and 1-ArOH (H 2 O molecules not shown), (adapted from image by C. Liu and ref[141]). Put differently, the product yields from the reference reaction of the short chain analog (1-ArN 2 + ) are proportional to the stoichiometric concentration of reactants (water and nucleophiles) in the same way as product yields for probe (16-ArN 2 + ) reaction are proportional to the

  Total yields of the desired products vary between 83 and 100%. Variation of this yield is caused by side products produced because of the rich chemistry of arenediazonium ion. One of them, for example, is reaction between z-ArN 2 + and CH 3 CN, solvent in which stock solution was prepared[134, 146]. Meanwhile, the formation of the side products generally does not affect the relative yields of 16-ArX and 16-ArOH significantly[145]. Normalized product yields for 1-ArX (left) and 1-ArOH (right) from the reference dediazoniation reaction in aqueous solution are on the Figure IV-3. We can see that values of 1-ArX(%) as a function of the counterion concentration ([X t ] = 2[1-2-1 X]) are the highest for I -, the most hydrophobic counterion, and lowest for the most hydrophilic acetate. The opposite trend is observed for 1-ArOH(%), that is, the more hydrophobic counterion, the lower the product yield of 1-ArOH(%). Using this data, selectivities of arenediazonium ion towards counterions compared to H 2 O, S W X , were estimated (Equation (IV-1)).

Figure IV- 3 .

 3 Figure IV-3. Normalized product yields (%) for 1-ArX and 1-ArOH as a function from counterion concentration [X t ] = 2[1-2-1 X].The same tendency as for normalized product yields was observed for S W X : dediazoniation reaction is more selective towards hydrophobic counterions (I -, ΔG hyd = -283 kJ•mol -1 ) and least selective towards the anions with high hydrophilicity (C2, ΔG hyd = -373 kJ•mol -1 )[START_REF] Yitzhak | Ion Properties[END_REF]. It is noteworthy that the arenediazonium ions are much more selective towards ions than water at low counterion concentrations, however with increasing [X t ] the selectivity decreases (FigureIV-4).

Figure IV- 4 .

 4 Figure IV-4. Selectivities, S W X , from the reaction of 1-ArN 2 + with X -in 1-2-1 X aqueous solution. [X t ] = 2[1-2-1 X] As mentioned in subchapter 1.3, CT uses the assumption that the selectivity of 1-ArN 2 + in the aqueous solution and selectivity of 16-ArN 2 + in the micellar solution are the same: if concentration is the same, the product yield is the same. Using the yields of the reaction with the short chain analog as a function of [X t ] in which the stoichiometric solution concentrations were known, we can transfer normalized product yield of 16-ArX to the interfacial concentrations (X m ,

  Chapter IV. Interfacial properties of micellar aggregates via the chemical trapping experimental method 134 mol/L) using graphs on Figure IV-3. At the same time estimation of H 2 O m within the interfacial layer of micelles can be done using the second part of Equation (IV-1):H 2 O m = [(%16-ArOH)•S W X •X m ]/[100 -(%16-ArOH)].On the Figure IV-5 we represent estimated interfacial molarities of counterions, X m , (left) and interfacial water molarities, H 2 O m , (right) for the micellar solution of 10-2-10 gemini with I -, Br -, Cl -, C2 and MeSO 3 -as a function of stoichiometric surfactant concentration in solution. Note that even though stoichiometric surfactant concentration is in mmol/L, the interfacial molarities of counterions are 100 to 1000 fold higher.

Figure IV- 5 .

 5 Figure IV-5. Interfacial counterion (X m ) and water (H 2 O m ) molarities in 10-2-10 X micellar solution at 25 and 50 °C in case of 10-2-10 I.

  1.), because of the low selectivity of this ion and because of the presence of the same carboxylate reactive group. Interfacial counterion (X m ) and water (H 2 O m ) molarities for micellar solutions of 10-2-10 gemini with carboxylate counterions are show in Figure IV-6 (left and right respectively). Although X m and H 2 O m values for all three counterions are different, C2, being the smallest carboxylate anion has the lowest X m (at 2xCMC 1.0 M) and higher for C3 and C4 (at 2xCMC 1.3 and 1.5 M respectively) respecting the hydrophobicity order. Interfacial water concentrations for C2, C3 and C4 show the oposite trends as the anions, but there is a smaller spread in the H 2 O m values, especially for C2 and C3 (Figure IV-6 right).

Figure IV- 6 .

 6 Figure IV-6. Interfacial counterion (X m ) and water (H 2 O m ) molarities in 10-2-10 X micellar solution at 25 °C.

3 O

 3 Chapter IV. Interfacial properties of micellar aggregates via the chemical trapping experimental method 138

  Chapter IV. Interfacial properties of micellar aggregates via the chemical trapping experimental method 139

4

 4 Chapter IV. Interfacial properties of micellar aggregates via the chemical trapping experimental method 140 gemini with different counterions were synthesized as described in Chapter II. Values of CMCs were measured by conductivity (Chapter III). The arenediazonium salt 4-n-hexadecyl-2,6-dimethylbenzenediazonium tetrafluoroborate (16-ArN 2 BF 4 ) and its short chain analog 2,4,6-trimethylbenzenediazonium tetrafluoroborate (1-ArN 2 BF 4 ) were prepared by C. Liu as previously reported[139]. The products of dediazoniation reaction that were used as HPLC standards in reference aqueous solution such as 1-ArOH, 1-ArBr, 1-ArCl, 1-ArI were purchased from Sigma-Aldrich, and others (1-ArC2, 1-ArO 3 SMe) were synthesized by C. Liu (publication is in preparation). For dediazoniation products of the probe (HPLC reference for long chain compounds), such as 16-ArOH, 16-ArH, 16-ArBr and 16-ArCl, the synthesis procedure was established previously[139, 146]. Other long chain stable products were synthesized using a protocol created by C. Liu and X. Gao. The short chain analogs of gemini 1-2-1 X were prepared by C. Liu following the same procedure as the one was used for gemini 10-2-10 X (Chapter II). All solutions were prepared with distilled deionized water.

2 . 2

 22 Conditions for product separation on the HPLC for 10-Each product yield was analyzed thrice by HPLC.

  [162]. (ii) Solvent accessible surface area is the surface area of a macromolecule that is accessible to a solvent molecule. It can be schematically presented as shown on Figure V-1. SASA was estimated using VMD software[163].

Figure V- 1 .

 1 Figure V-1. Representation of SASA. Color code: in red is VdW surface of a molecule, in green is the accessible surface of the same molecule, in magenta is molecular surface[164]. (iii) Radial distribution function describes how density varies as a function of distance from a reference particle (r), see Figure V-2. RDF can be described by equation:

Figure V- 2 .

 2 Figure V-2. Schematic representation for the evaluation of the radial distribution function[166].

Figure V- 3 A

 3 Figure V-3 A, B and C we report the distribution of the three structural parameters that were measured via MDs: R g , SASA and ε for the 10-2-10 gemini micelle with I -, Br -, Cl -and F -as counterions.

Figure V- 3 .

 3 Figure V-3. Distributions of the (A) Rg, (B) hydrophobic tails SASA, (C) ε and (D) RDF of counterions with respect to nitrogen atoms of the micelle. Black, red, green and blue refer to the solution with fluoride, chloride, bromide and iodide ions respectively.

Figure V- 3 A

 3 Figure V-3 A demonstrates that the lowest R g , that indicates the compactness of the micelle (i.e. the lower the R g , the more compact the micelle), corresponds to 10-2-10 I micellar system, and the average R g increases for 10-2-10 micelle along with Br -, Cl -and F -. 10-2-10 X micellar systems are referred to by their respective counterions (X) throughout this chapter. Figure V-3 B shows the

  parameter, as was discussed in Chapter III. To analyze the micellar properties of the 10-2-10 gemini with polyatomic counterions (TFA, NO 3 -, C2 and PH) by MD, we investigated their R g , SASA, ε and RDF, see Figure V-4 A, B, C and D respectively.

Figure V- 4 .

 4 Figure V-4. Distributions of the (A) Rg, (B) hydrophobic tails SASA, (C) ε and (D) RDF of counterions with respect to nitrogen atoms of the micelle. Central atom for each anion chosen as X. Black, red, green and blue refer to the solution with acetate, trifluoroacetate, phosphate and nitrate ions respectively.

Figure V- 4 A

 4 Figure V-4 A shows that NO 3 -leads to the lowest R g by far (~ 14.4 Å). Compared to the micelles with 10-2-10 Br (~ 14.7 Å), which has the similar CMC (6.4 mM (Br -) and 6.3 mM (NO 3 -)), the micelles of 10-2-10 NO 3 are slightly more compact. Such behavior could be explained by the flat morphology of NO 3 -anion that allows it to embed itself between the headgroups and shrinks the micelle due to the electrostatic interactions. Hydrophilic PH and C2 have R g values similar to that of F -, where the micelles are characterized by a lower compactness. On the other hand, TFA, being the most hydrophobic counterion, with properties close to the Br -and NO 3 -, exhibits the highest R g (~ 15 Å). The same trend was observed for the roughness of the micelles characterized by SASA (see Figure V-4 B). The packing of the micellar surface is the tightest for the NO 3 -anion and lowest for the TFA. PH and C2 take the intermediate positions. The observed behavior for TFA does not correlate with our expectations in view of the experimental data. We

Figure V- 4 C

 4 Figure V-4 C represents the eccentricity for the micelles with different counterions. One can see that micelles with NO 3 -are more spherical (~ 0.15) than with PH, C2 and TFA (~ 0.2), and close in value to the monoatomic I -and Br -. Whereas, there is no significant difference between the hydrophilic PH, C2 and TFA. Similarly to ε for F -, we found two populations of micellar shape for PH (first and second peaks are at 0.15 and 0.3 respectively). These results again reflect very well the polydispersity for 10-2-10 PH micelles, obtained experimentally by fluorescence quenching technique.

  Figure V-4 D shows that similarly to monoatomic counterions, RDF is characterized by three peaks that correspond to contact, solvent-shared and fully-solvated ion pairs. The first peak (contact ion pairs) appears at around 4.5 Å for all the counterions and corresponds to that of halides. The NO 3 -peak is almost 4 times higher when compared to PH, C2 and TFA and has g(r) close to the ones of I -and Br -. It indicates that NO 3 -has much stronger interactions with positive headgroups than others. This fits wells with the behavior observed experimentally (CMC, α). However, the TFA shows RDF g(r) very close to that of C2, whereas from the experimental data, we would expect g(r) close to NO 3 -. Such discrepancy between experimental observation and obtained calculations can be explained by the choice of parameters for the counterion. TFA parameters are missing in CHARMM36 force field and to derive them we used CGenFF program[167]. However, the penalty scores 2 were large (some of them were ~ 80) and unfortunately we did not reoptimize them due to the lack of time. This can be the reason why the properties of the simulated 10-2-10 TFA do not reflect the experimental observation. Second peaks attributed to the solvent-shared ion pairs is around 7 Å for C2 and TFA, 7.5 Å for PH and 8 Å for NO 3 -. The third peaks, indicating fully-solvated ion pairs, are very weak for all counterions except NO 3 -(12 Å)which shows very distinctive three peaks. For C2, TFA and PH, the distribution around the polar heads is less structured, and the third peaks are not observable. also performed for the 10-2-10 gemini with carboxylate counterions in order to investigate the effect of hydrophobicity on the micellar properties such as compactness, roughness, sphericity and counterions density described by R g , SASA, ε and RDF respectively(Figure V-6).Surfactants with carboxylate counterions are of interest because increasing the chain length of carboxylates, and thus increasing their hydrophobicity, induces a change in their behavior regardless of the micelle. Short chain carboxylates show "ordinary" counterion like properties, form "classical" micelles. Whereas, alkyl carboxylates with chain lengh ≥ 6 make more complex structures: catanionic (mixed) micelles. On Figure V-5, snapshots of the 10-2-10 micelles with the C1, C2, C3 and C6 counterions are represented. One can see that the small anions C1, C2 and C3 are located at the interface of the micelle. In sharp contrat, for the C6 molecule, all the counterions are integrated with their chains residing into the aliphatic micellar core.

Figure V- 5 .

 5 Figure V-5. Images from MD simulations of 10-2-10 micelle with formate (C1), acetate (C2), propionate (C3) and hexanoate (C6) as counterions.

Figure V- 6 .

 6 Figure V-6. Distributions of the (A) Rg, (B) hydrophobic tails SASA, (C) ε and (D) RDF of counterions (oxygen) with respect to nitrogen atoms of the micelle. Black, red, green, blue, yellow and brown refer to the solution with C1, C2 , C3, C4, C5 and C6 ions respectively.

  Figure V-6 D shows that tree peaks corresponding to the contact, solvent-shared and fully-solvated ion pairs have the same position for all carboxylate counterions. The increase in the peak height from C1 to C6indicates that increasing the length of the hydrophobic chain of the counterions results in an increase of the association between the nitrogen atom of the headgroup and its counterion and thus, in an increase in the density of the ion pair.

Figure V- 7 .

 7 Figure V-7. Radial distribution function g(r) as a function of the distance from the center of mass (COM) of the micelle for 10-2-10 gemini with carboxylate counterions. Color code: green, red, white, silver and black refer to carbonyl oxygen, carbonyl carbon, alkyl carbon, CH 2 hydrogen and terminal methyl hydrogen atoms respectively. The Figure V-8 summarizes the parameters of the micelle for all types of coutnerions (halides, polyatomic and carboxylates) obtained by MD simulations as a function of CMCs. The radius of gyration R g (A) for the halides increases for the halides and polyatomic counterions going from more hydrophobic I -, Br -and NO 3 -, having lower CMCs, towards hydrophilic F -and PH, having relatively high CMCs. In contrast, the micelles associated with carboxylate counterions have the opposite behavior. The R g of more hydrophobic counterions such as C6, C5, which CMC values are close of those for I -, Br -and NO 3 -, is much higher than for hydrophilic C1 and C2. The similar tendency was obtained for SASA (B) and ε (C). The dependence of the roughness of the micelle on the type of counterion is less pronounced for halides and polyatomic ions, whereas for carboxylate counterions it increases significantly from C1 to C6 (B). Eccentricity of the micelle does not vary much with counterions, from 0.16 for the most spherical micelle with iodide to 0.26 for C6counterion. Nevertheless, the tendency to form more spherical micelles for hydrophobic "ordinary" counterions and hydrophilic counterions is observed.

Figure V- 8 .

 8 Figure V-8. Structural parameters of the micelle: Rg (A), SASA (B) and ε (C) as a function of CMC for 10-2-10 gemini with different counterions.

Cl

  The interfacial molarities obtained from chemical trapping experiments for the micellar solutions of 10-2-10 gemini in the following millimolar concentration ranges 50 < [10-2-10 C2] < 250, 25 < [10-2-10 Cl] < 250 and 10 < [10-2-10 Br] < 250 are presented in Chapter IV. The concentrations distributions of counterions and water along the radial distance from the micelle centre of mass (COM) were analyzed. Starting from the COM of the micelle, we considered concentric spherical layers of 1.0 Å in width and then evaluated the cumulative number of counterions and water molecules therein. In the case of acetate, we considered in the calculation the two oxygen atoms, which represent the reactive (nucleophilic) moiety of the molecule. Note that for the 10-2-10 gemini surfactant, micelles were formed with the aggregation number measured at 2xCMC, where the stoichiometric concentration was much lower than those of the simulated systems (ca. 82 mM). At the concentration 82 mM, which means ~ 4, ~ 6 and ~ 14 times the critical micelle concentration (CMC) of the surfactant in presence of C2, Cl -and Br - respectively, there might occur a sphere-to-rod transition[133, 145], however such a transformation cannot occur in the simulation box.

Figure V- 9 .

 9 Figure V-9. Dimension of the micelle R M was considered as being the distance from the center of mass of the micelle until the first hydration layer (methyl groups were included). The RDF of the water hydrogen atom with respect to the nitrogen of the headgroup shows the distance 4.5 Å. R M is calculated by adding the distance of the nitrogen atoms from the centre of mass (R COM ) to this Nitrogen-Hydrogen distance and substracting the radius of water (1.4 Å) which determined the dimension of the micelle[157]. The Figure V-10 presents the local concentrations of water [H 2 O] L and counterions [X] L as a

Figure V- 10 .

 10 Figure V-10. The x axis defines the radial distance of the spherical layers from the micelle COM. The y axis of the top and bottom graphs shows the local concentrations (within each layer of 1 Å) molarities of water and counterons, respectively. Black, red and green lines refer to C2, Cl -and Br -respectively. Vertical dotted lines delimit the average radius of the micelle and vertical dash-dotted lines are placed at the location of the maximum of counterions concentrations. In Table V-1 the comparison of interfacial concentrations (in M) between chemical trapping measurements and MD calculations is made. Only the molarity at the peak of distributions of the counterion are reported (see Figure V-10), the electrophilic moiety of the arenediazonium probe samples the region around peak of the counterion distribution in chemical trapping experiments (this assumption was validated from the MD runs in the presence of the probe, see below). The notable agreement between measured and computed interface molarities of counterions validate mutually the MD simulation approach (the choice of the force field, i.e. the use of the set of mixed parameters within CHARMM force field) and chemical trapping technique to characterize quantitatively the interfacial properties of cationic association colloids.

  Figure V-11 we show the curves for the fraction of the number of counterions (left axis) as a function of r COM-X . Beyond the interfacial region as defined above 18 Å (Figure V-11), the tendency of counterions dissociation is consistent with the experimental values from conductivity measurements: bromide ions are the most associated and acetate ions are the most dissociated. At the same time to make a more quantitative analysis from MD results, RDFs profiles of the counterions with respect to nitrogen atoms of surfactants polar heads, r n-x were shifted along the x axis by (added to) a length equal to the average distances of nitrogen atoms from the micelles COM, (r COM ) and then superposed to the trends of the ion dissociation[84] r n-X + r COM = r COM-X . The resulting graph is displayed in Figure V-11.

Figure V- 11 .

 11 Figure V-11. The fraction of the number of counterions (left y axis) as a function of r COM-X with the steps of 1.0 Å and N-X RDFs profiles (right y axis) for 10-2-10 micelles. Black, red and green lines refer to C2, Cl -and Br -respectively. The RDF profiles have been translated along the x axis by the average distance of the nitrogen atoms from the micelle COM (17.5, 17.6 and 17.9 Å for C2, Cl -and Br -respectively). The inset shows a magnification of the RDF profiles within the range 23 Å ≤ r ≤ 40 Å.

( 10 - 2 - 1 . 3 .

 10213 10) from the micelles COMs are quite broad. They can be regarded as the delimitation of the so called double layer (that is Stern layer plus diffuse layer) defined in the field of electrochemistry[171, 172]. Beyond the diffuse layer there is the bulk solution, where the counterions are completely dissociated from the micelle. The charge layer thickness of the micelles (that is the surface delimited by the average distance of the nitrogen atoms from the micelle COM ~ 17.7 Å) is ≳ 13 Å. These results are in good agreement with the theoretical value of the Debye length, κ -1 = 3.04/I 1/2 , where I is the ionic strength of the solution in mol/L. For the gemini 10-2-10 micelles I ≈ 0.082 M, which give a Debye length of about 11 Å. It is worth noting that the Debye length in the Debye-Hückel theory quantifies the length range of the decay of the micelle electrostatic interactions. In conductivity measurements of this type of cationic gemini surfactants solutions, the ions residing within the diffuse layer can be considered associated with the micelle and the micelle itself is able to drag them to the electrode. Position of arenediazoniom probe (16-ArN2 + ) used for chemical trapping study within the micelle In order to evaluate the position of the arenediazonium probe within the micelle interface we simulated a micelle (27 gemini) in the presence of the 16-ArN 2 + probe. Originally in the chemical trapping experiments the probe concentration is kept 100 times lower than surfactant concentration, so that in the average only one probe molecule is incorporated per micelle. In Figure V-12 A we show the RDFs calculated from the micelle's COM of headgroups nitrogen atoms, counterions (Cl -), nitrogen atoms bound to the benzyl ring and aromatic carbon from which molecular nitrogen Chapter V. Structural and interfacial properties of micellar aggregates via computer simulations 163 dissociates and to which the competing nucleophiles eventually bind. These results show that although the aliphatic 16-carbon tail of the probe is longer than the 10-carbon tail of the 10-2-10 surfactants, the arenediazonium ion can accommodate totally within the micelle, with its reactive moiety exactly at the interface: the position of the probe nitrogen atoms overlap both with the nitrogen atoms of the aggregate polar heads and the counterions (Figure V-12 (A)). These results also demonstrate well the "liquid" nature of both micelles and probe, as it is well visualized from a simulation snapshot shown in Figure V-12 (B).

Figure V- 12 . 2 +(

 122 Figure V-12. (A) RDFs evaluated from the micelle COM. In blue, ice-blue, orange, red and green are RDFs for arenediazonium nitrogen (N2), nitrogen correspondent to gemini (N gemini surfactant), carbon from the last methyl group of the probe's tail (C16), chloride counterions (Cl -) and water respectively. (B) Depiction of the 10-2-10 micelle (yellow surface for the aliphatic tails and blue beads for the polar heads), together with the probe 16-ArN 2 + (shown with VDW representation: white, cyan and ice-blue balls for hydrogen, carbon and nitrogen atoms respectively) and with Cl -anions (red balls).

(

  300 K) (NPT) ensemble for 200 ps, implementing a modified Nosé-Hoover method in which Langevin dynamics is used to control fluctuations in the barostat[179]. In both thermalisation and density equilibration, the micelle was weakly restrained using a positional harmonic force constant of 2.0 kcal/mol/Å 2 . A further NPT equilibration was run again up to 200 ps with removing all the restraints from the micelle. The production runs were performed in NVT ensemble, using Langevin dynamics with a damping coefficient of 3 ps -1 and a simulation time of 50 ns.

Figure V- 13 .

 13 Figure V-13. Box of water with volume of ~ 83x80x82 Å 3 and ~ 17900 water molecules, where micelle made up of 27 gemini surfactant neutralized by 54 counterions was immersed. In all the simulations a radial cutoff of 12.0 Å for the non-bonded interactions and a timestep of 2.0 fs for integrating the equations of motion were utilized; all the bonds involving hydrogen atoms were kept fixed at their equilibrium value. The trajectories were propagated with NAMD software and subsequently analyzed with the module cpptraj[180] of AmberTools13[162], VMD[163] software and GROMACS analysis tools[165].

Figure V- 14 .

 14 Figure V-14. Equilibration assessment on the example of C2, Cl -and Br -counterions. To assess the convergence we run one more simulation up to 100 ns; after splitting this trajectory into two 50 ns sub-trajectories, we evaluated Rg, SASA and ε for the two halves and saw that the values were practically the same (Figure V-15).

Figure V- 15 .

 15 Figure V-15. Rg, SASA and ε evaluated for 100 ns trajectory as well as two 50 ns subtrajectories after splitting the first one.

  higher concentration. Upon increasing the surfactant concentration or adding salts, small spherical micelles grow in size and form rodlike then wormlike micelles[110, 181-184]. At high enough concentration these long and flexible wormlike micelles get entangled and behave as polymers[185, 186]. The formation of such entangled supramolecular structures increases the viscosity and elasticity of the system. Transition from dilute (growth of the micellar size is slow as a function of surfactant concentration) to semi-dilute (rapid growth of micelles) regime is characterized by the overlap concentration (Ф*)[186-188]. Because of the significant change in the viscosity at this concentration, rheological study can be used to examine the mechanical properties of the wormlike micellar solutions.

For, p = 1 / 3

 13 spherical micelles are formed; for 1/3 < p < 1/2, cylindrical (wormlike) micelles are formed; for 1/2 < p < 1, vesicles are formed; and for p ~ 1, bilayers are formed (Figure VI-1).

Figure VI- 1 .

 1 Figure VI-1. Contribution of the packing parameter to the morphology of the surfactant aggregates[189].

  Chapter VI. Rheological study of the Hofmeister effects at semi-dilute regime 174In dilute regime the micelle length is shorter than Debye screening length and the system is characterized by a slow increase in aggregates size with concentration[183]. The transition from dilute to semi-dilute regime occurs when the micellar length becomes comparable with the Debye length of electrostatic interactions. With increasing the concentration, the contribution of the energy promoting the breaking of the micelle (formation of end-caps) decreases due to the screening of the electrostatic interactions[START_REF] Zana | Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications[END_REF] ( (Ф) ~ -1 √Ф ⁄ , where ΔE is a contribution from electrostatic interactions). The deviation from the growth law ~ Ф ⁄ exp[ 2 ⁄ ] to ~ Ф ⁄ exp [ (Ф)],

Figure VI- 2 .

 2 Figure VI-2. Zero shear rate viscosity as a function of surfactant concentration for 10-2-10 gemini with different counterions.

  Figure VI-3 shows the CMC and the Ф* as function of polarizability for the 10-2-10 gemini with different counterions. All the data on this Figure could be divided into two groups.

  Figure VI-3 shows that for more polarizable anion Br -(4.85 Å 3 ), the CMC as well as Ф* are lower than for the less polarizable Cl -(3.42 Å 3 ). The second group is polyatomic counterions: NO 3 -, C2 and PH. The situation for them is more complex. For example, polarizable and big C2 and PH have the highest values of the CMC and Ф*. It is opposite of what we would expect. Such behavior could be explained by change in the surface charge density of the anions. It is no longer homogeneous as for the monoatomic counterions. The size and polarizability of the ion are not any more directly related to hydration of the anion. Hence, dispersion forces alone cannot describe the interactions that ensure the aggregate formation and growth; thus, other properties of the anions should be taken into account.

Figure VI- 3 .Figure VI- 4 .

 34 Figure VI-3. CMC and Ф* as a function of polarizability for the 10-2-10 gemini with different counterions.In the Chapter III we discussed that for the 10-2-10 X the CMC values correlate well with the hydration free energy and the lyotropic number. Same trend was observed for Ф*as it can be seen in Figure

CMC for the 10 - 2 -

 102 10 Cl indicates an intermediate behavior for the Cl -anion (Figure VI-4).

Figure VI- 5 .

 5 Figure VI-5. Dependence of viscosity ( ) on shear rate ( ̇) for 10-2-10 Cl at the concentration 768 mM obtained using Rheo win Data Manager software. Horizontal green solid line indicates the extrapolation of the viscosity to zero shear rate. Estimated 0 values were plotted as a function of concentration in log-log scale to obtain the overlap concentration (Ф*). The intersection of two straight lines, at the concentrations before and after Ф* gave as the values of overlap concentration (Figure VI-2).

1. Results and discussions 1 . 1 .

 11 Packing of gemini surfactants in the crystalline structure. In Figure VII-1and VII-2, single-crystal structures and the packing arrangement of n-2-n gemini (n = 10, 14, 16) with various counterions such as: I -, Br -and Cl -as well as 10-2-10 gemini with tertiary ammonium headgroup are shown. Quaternary gemini with two halide counterions Br - and I -crystallized from different organic solvents have the same triclinic space group P-1 (Table VII-1, Figure VII-1)). The packing of the molecules driven by electrostatic and van der Waals interactions represents anti-gauche conformation, where two hydrocarbon chains lie perpendicular to the spacer and extend to opposite sides of the headgroups (for clarity see Figure VII-1 A and B). The plane defined by the C-C bonds of the two hydrocarbon chains are parallel and shifted with each other, and are perpendicular to the plane defined by the C-C bonds of the spacer. No solvent molecules are co-crystallized. Such type of molecular configuration in crystal structure for cationic gemini with halide counterions was reported previously[202, 203, 205]: Berthier et al.[202] and Svensson et al.[205] have reported the same space group (P-1) and ordering of hydrocarbon chains in anti-gauche conformation for 12-2-12 Br and 12-2-12 I respectively, crystallized from organic solvents. The Figure VII-2 shows that the packing arrangement into the crystal for the n-2-n quaternary gemini with I -and Br -as counterions are almost the same and do not depend on the chain length for 10 < n < 16 (not presented here). In the crystal lattice, molecules are organized in parallel with interdigitated tails and headgroups are aligned with axis b, see Figure VII-2 A for clarity.

  Figure VII-1 clearly seen thatthe carbons in α and β lie down at the same plane as spacer and the angle between alkyl chains and spacer plane is more than 90°. Moreover, the position of counterions differs from the one typical for the quaternary 10-2-10 Br, why we have such changes would be discussed later. In Chapter III we mentioned that the low solubility of the tertiary gemini surfactants in water was attributed to hydrogen bonding between the hydrogens of the headgroups and the counterions. In crystal structure of 10-2-10 ter Br, crystallized from water we can clearly see that the hydrogen bonding is the main interaction that determines the location of the counterions Figure VII-2 D. Moreover, the antigauche conformation is round edged or wave-like compared to the one of quaternary gemini surfactants, see Figure VII-2 A. The interdigitation for the alkyl tails is not observed. So one can see that decrease in the size of the headgroups and formation of the hydrogen bonds, significantly change the shape and packing of the molecule within the crystal.Meanwhile, orthorhombic crystals with Pbca space group (Table VII-1) were observed with 14-2-14 Cl co-crystallized with one water molecules per gemini. In contrast to n-2-n with Br -and I - counterions, 14-2-14 Cl molecules in the crystals have gauche conformation (FigureVII-1 D). The aliphatic chains lie down perpendicular to the spacer and extend in one side of the headgroups (in the same plane with respect to the spacer). The presence of water molecule in the crystals is attributed to the hygroscopic properties of the Cl -anion. Being hydrophilic, probably chloride "dragged in" the water molecule from the solvent in the crystal structure. Intriguingly, the presence of water molecules changes the balance of forces controlling crystal organization from anti-gauche, as we observed for the bromide and iodide, to gauche. The water molecules favorably settle in the hydrophilic region, in the vicinity of the headgroups, and hence the conformation which has well defined hydrophilic and hydrophobic areas is more preferable. Crystal lattice for the 14-2-14 Cl is shown on the Figure VII-2 B. Since 14-2-14 Cl have gauche conformation, the way molecules arrange into the crystal is very different from those with Br -or I -counterions as described above. This structure is very close to lamellar phases with repeated bilayer structures and the two hydrocarbon chains are facing one another on the same side of the spacer, and they are interdigitated and form bilayers with well-defined polar and non-polar regions, see Figure VII-2 B. The water molecules are located within the hydrophilic regions close to the headgroups.

3 1 =

 1 ∑ | | -| | / ∑| | where Fo could be assimilated to experimental data (or electron density) and Fc to the calculated data (or purposed model 4 2 = [∑[ ( -) ]/ ∑[ ( ) ]] where = 1/[ ( ) + ( ) + ] where = ( + 2 )/3where a and b are the first and the second parameter in the weighting scheme instruction

Figure VII- 1 .

 1 Figure VII-1. Stick and ball representation of n-2-n (n = 10, 14, 16) gemini found in the singlecrystal structure with counterions such as: I -, Br -and Cl -. Color code: N, C, I, Br, Cl and O atoms are in blue, grey, purple, brown, green and red respectively. Hydrogens are not presented for clarity. The polymorphism obtained for the 10-2-10 I, when the molecule was crystalized from water is very interesting. As we mentioned previously gemini surfactants have two conformations in one crystals. Probably the way molecules aggregate in water (the hydrophobic chains hidden within the core of the micelle) favored the formation of gauche conformation in the crystalline structure even without co-localized solvent molecules. On the Figure VII-2 C it is shown how two molecules in gauche conformation are packed between layers of molecules with more "open" anti-gauche conformation, making kind of "sandwich" structure parallel axis c. Considering the bilayer motifformed with the molecules in gauche conformation, the angle of the tilt of the hydrophobic chains is higher than gauche conformation in the crystals obtained with the 14-2-14 Cl.

Figure VII- 2 .

 2 Figure VII-2. Packing arrangement of 10-2-10 I (A), 14-2-14 Cl (B) and 10-2-10 I polymorph (C) and 10-2-10 ter Br (D). View alone the a axis. (A), (B): crystals were grown from organic solvent, CH 3 CN; (C) and (D) crystal was grown from H 2 O. Color code: N, C, O, I and Cl atoms are in blue, grey, red, purple and green, respectively. Hydrogen atoms were not shown for clarity. Red dash lines indicate the hydrogen bonds.

Now we will focus on 10 - 2 -

 102 10 I, 10-2-10 Br and 14-2-14 Cl crystalized from the same solvent (CH 3 CN). Using the Mercury software, we defined the positions of the halide of interest relative to its three nearest nitrogen neighbors, as shown in Figure VII-3 and summarized in Table VII-3. For 14-2-14 Cl in gauche conformation, there are two populations of Cl -, one located at the interface of hydrophilic part of the molecule and second one more buried in the hydrophobic layer Chapter VII. Impact of counterion nature on the crystal structure of cationic gemini surfactants 192 as you can see on the Figure VII-2 B, where the packing arrangement shown. Hence distance Cl --N + would be different for two Cl -populations.

Figure VII- 3 .

 3 Figure VII-3. Distances between the counterion and the ammonium atom of the headgroup in the crystal structure for (A) 10-2-10 I, (B) 10-2-10 Br and (C) 14-2-14 Cl. Color code: N, C, O, I, Br and Cl atoms are in blue, grey, red, purple, brown and green respectively. Hydrogen atoms were not shown for clarity.The summary in Table VII-3 clearly shows that Cl -are closest to ammonium ion whereas I - are the farthest. To get better insight into association energy between counterions and positively charged headgroups, we can look at the Coulomb interactions using rough approximations. The energy between two point charges separated by a distance r can be expressed by the following equation:

14- 2 -

 2 14 Cl stepwise cooling from 40 to 20 °C was performed by 1 °C per day. Temperature control was done by thermostat. Single crystals of 10-2-10 I also were obtained from H 2 O by heating a C = 5 mM solution up to 60 °C and cooling it down to r. t. After few days thin needlelike crystals were formed. Crystals of 14-2-14 Br and 16-2-16 I were obtained from vapor diffusion of acetone in saturated MeOH solution.

  a R-Axis Rapid Rigaku MSC diffractometer with rotating anode, Varimax optics and a curved image plate detector (iii) a Proteum Bruker diffractometer with rotating anode, Helios optics and a Platinum CCD camera The unit cell determination and data reduction were performed using the Crystal Clear program suite[206] or Proteum2 program suite on the full set of data. The structure was solved by direct methods and refined using Shelx 97 suite of programs[207] in the integrated WinGX system[208]. The positions of the H atoms were deduced from coordinates of the non-H atoms and Chapter VII. Impact of counterion nature on the crystal structure of cationic gemini surfactants 196confirmed by Fourier synthesis. The non-H atoms were refined with anisotropic temperature parameters. H atoms were included for structure factor calculations but not refined.

Table I -1. Abbreviations and molecular structures of cationic gemini and counterions used in this

 I

Table II -1. Quantification of N, C and Br for 10-2-10 Br molecule via XPS.

 II

	Element	N1s (300 eV)	C1s (285 eV)	Br3d (68 eV)
	Theoretical	2	26	2
	Experimental	2	27.1	2.1

Table II -3. Quantification of N, C, Br and O for tertiary ammonium 10-2-10 ter Br molecule via XPS.

 II

	Element	N1s (300 eV)	C1s (285 eV)	Br3d (68 eV)	O1s (68eV)
	Theoretical	2	24	2	0
	Experimental	2	26.6	2.1	0.2

Table II -4. Quantification of N, C, Cl, O, Si and Br for tertiary ammonium 10-2-10 ter Cl molecule via XPS.

 II

	Element	N1s (300 eV) C1s (285 eV) Cl2p (198 eV) O1s (68eV)	Si2p (99 eV) Br3d (68 eV)
	Theoretical	2	24	2	0	0	0
	Experimental	2	22.4	1.9	0.6	0.3	0.04

Figure II-12. XPS high resolution spectrum of the N1s region for 10-2-10 ter Cl.

	Counts / s
	410	408	406	404	402	400	398	396	394
				Binding Energy (eV)

NMR (300 MHz, MeOD, 25˚C): 65

  0.90 (t, 6H, CH 3 -(CH 2 ) 9 -N + ). .6 ( + NCH 2 ), 55.8 ( + NCH 2 ), 50.5 ( + NCH 3 ), 31.7 (CH 2 ), 29.3 (CH 2 ), 29.2 (CH 2 ), 29.1 (CH 2 ), 28.9 (CH 2 ), 25.9 (CH 2 ), 22.4 (CH 2 ), 22.3 (CH 2 ), 13.1 (CH 3 ).

	Chapte II. Synthesis and characterization of cationic gemini surfactants
	13 C 10-2-10 I. 3 mL (19.87 mmol) of TMEDA was mixed with 9.33 mL (43.7 mmol) of 1-
	iododecane in acetonitrile
	73

  13 C NMR (300 MHz, MeOD, 25˚C): 178.7 (C=O), 65.6 ( + NCH 2 ), 55.7 ( + NCH 2 ), 50.3 ( + NCH 3 ), 31.7 (CH 2 ), 29.3 (CH 2 ), 29.2 (CH 2 ), 29.0 (CH 2 ), 28.9 (CH 2 ), 25.9 (CH 2 ), 23.2 (CH 3 CO 2 ), 22.4 (CH 2 ), 22.3 (CH 2 ), 13.1 (CH 3 ).

-10 H 2 PO 4 .

  The solution of 10-2-10 Ac (7 g, 13.54 mmol) in MeOH (80 mL) was prepared and added dropwise to the solution of H 3 PO 4 85 wt% (1.82 mL, 31.87 mmol) in MeOH (80 mL) under continuous stirring at room temperature. The obtained white precipitate was filtered and dried under vacuum and by lyophilization. The yield was 68%.

	10-21 H NMR (300 MHz, D 2 O, 25˚C):
	δ = 3.90 (s, 4H, (CH 3 ) 2 -N + -(CH 2 ) 2 -N + -(CH 3 ) 2 ), 3.44 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.25 (s,
	12H, (CH 3 ) 2 -N + -(CH 2 ) 2 -N + -(CH 3 ) 2 ), 1.76 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 1.38-1.30 (m, 28H,
	CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 0.88 (t, 6H, CH 3 -(CH 2 ) 9 -N + -)
	13 C NMR (300 MHz, D 2 O, 25˚C): 63.6 ( + NCH 2 ), 53.9 ( + NCH 2 ), 52.5 ( + NCH 3 ), 31.7 (CH 2 ),
	29.3 (CH 2 ), 29.2 (CH 2 ), 29.1 (CH 2 ), 28.9 (CH 2 ), 25.6 (CH 2 ), 22.5 (CH 2 ), 13.8 (CH 3 ).
	10-2-10 gemini	mmol of 10-2-10 Ac	mmol of HX (per 100 wt%)	V of MeOH for solubilization (mL)	Solvent for precipitation and washing
					Diethyl ether (was added
	NO 3	11.42	22.83	12	dropwisely under
					continuous stirring)
					Diethyl ether (was added
	Cl	29.02	87.07	15	dropwisely under
					continuous stirring)
	TFA	25.15	50.31	4	Diethyl ether
	MeSO 3	11.61	23.22	1	Acetone

31 P NMR (300 MHz, D 2 O, 25˚C): 3.2. 10-2-10 C1. To a MeOH solution (150 mL) of 10-2-10 Ac (13 g, 25.15 mmol) was added HCOOH (17.75 mL, 503.06 mmol) in molar ratio 1:20. The yield was 87%.

1 H NMR (300 MHz, MeOD, 25˚C, D 2 =25 s): δ = 8.57 (s, 2H, H-COO -), 3.93 (s, 4H, (CH 3 ) 2 -N + -(CH 2 ) 2 -N + -(CH 3 ) 2 ), 3.42 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 3.21 (s, 12H, (CH 3 ) 2 -N + -(CH 2 ) 2 -N + -(CH 3 ) 2 ), 1.83 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N + -), 1.42-1.31 (m, 28H, CH 3 -(CH 2 ) 7 -CHg 2 -CH 2 -N + -), 0.91 (t, 6H, CH 3 -(CH 2 ) 9 -N + -).

2.

3. Synthetic procedures for tertiary 10-2-10 gemini surfactants 10-2-10 ter I. To

  bromodecane (8.29 mL, 39.95 mmol) was mixed with DMEDA (2 mL, 18.58 mmol) and DIPEA (6.95 mL, 39.95 mmol) in acetonitrile (30 mL). The reaction mixture was stirred at 40 °C for 2 days. The completion of the reaction was monitored with 1 H NMR (MeOD), the absence of peaks of DMEDA at 2.67 and 2.38 ppm indicated that the dialkylation was complete.After reaction the solvent was evaporated and extraction was used for separation of amine and Hünig's base. For the extraction water was used to extract the base, whereas amine compound remained in CHCl 3 . The organic phase containing 10-2-10 amine was collected and dried over MgSO 4 , filtered and evaporated to give the 10-2-10 amine compound.

	1 H NMR (300 MHz, D 2 O, 25˚C): δ = 3.65 (s, 4H, CH 3 -NH + -(CH 2 ) 2 -NH + -CH 3 ), 3.24 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -NH + -), 2.96 (s, δ = 2.82 (s, 4H, CH 3 -N-(CH 2 ) 2 -N-CH 3 ), 2.65 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N-), 2.46 (s, 6H, CH 3 -N-(CH 2 ) 2 -N-CH 3 ), 1.57 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -N-), 1.34-1.30 (m, 28H, CH 3 -(CH 2 ) 7 -CHg 2 -6H, CH 1 H NMR (300 MHz, MeOD, 25˚C): CH 2 -N-), 0.90 (t, 6H, CH 3 -(CH 2 )

a solution of DMEDA (2 mL, 18.58 mmol) in CH 3 CN (40mL), 1-iododecane (8.72 mL, 40.88 mmol) was added and the reaction mixture was strirred for 24 hours at 40 °C. After completion, the solvent was removed under vacuum and obtained compound was purified by crystallization from MeOH (10 mL). White crystals were collected and dried under vacuum. 3 -N-(CH 2 ) 2 -NH + -CH 3 ), 1.78 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -NH + -), 1.36-1.30 (m, 28H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -NH + -), 0.88 (t, 6H, CH 3 -(CH 2 ) 9 -NH + -). 10-2-10 amine. 1-9 -N-).

10-2-10 ter Br.

To an acetone solution (15 mL) of 1 eq. of 10-2-10 amine (2 g, 5.23 mmol) under stirring was added 2.2 eq. of HBr 48 wt% (1.35 mL, 24.86 mmol). A white precipitate was formed, collected and purified by crystallization from a hot mixture (60 °C) of MeOH/CH 3 CN 22 mL (1:10 ratio). The mixture was cooled down to room temperature and crystals were collected and dried under vacuum.

1 H

NMR (300 MHz, D 2 O, 25˚C):

  AgAc (1.72 g, 10.31 mmol) in 30 mL of MeOH. The reaction was stirred for 24 h at 40 °C. Then the suspension was filtered over celite to remove precipitate of AgBr and unreacted AgAc. Solvent was removed under vacuum. Black color of the compound indicates that additional purification of the compound was needed (filtration on the celite from acetone). After all the trace of Br was removed 10-2-10 ter Cl can be synthesized. -10 amine was solubilized in 5 mL of acetone and an excess of HCl 37 wt% (1.06 mL, 34.8 mmol) was added under the stirring. The obtained powder was purified by filtration on celite from methanol, crystallized in MeOH, crystals were collected and dried under vacuum.

	δ = 3.65 (s, 4H, CH 3 -NH + -(CH 2 ) 2 -NH + -CH 3 ), 3.26 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -NH + -), 2.96 (s, 6H, CH 3 -N-(CH 2 ) 2 -NH + -CH 3 ), 1.74 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -NH + -), 1.36-1.30 (m, 28H, CH 3 -10-21 H NMR (300 MHz, D 2 O, 25˚C): δ = 3.63 (s, 4H, CH 3 -NH + -(CH 2 ) 2 -NH + -CH 3 ), 3.24 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -NH + -), 2.95 (s, 6H, CH 3 -N-(CH 2 ) 2 -NH + -CH 3 ), 1.76 (m, 4H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -NH + -), 1.36-1.30 (m, (CH 2 ) 7 -CH 2 -CH 2 -NH + -), 0.88 (t, 6H, CH 3 -(CH 2 ) 9 -NH Chapte II. Synthesis and characterization of cationic gemini surfactants 28H, CH 3 -(CH 2 ) 7 -CH 2 -CH 2 -NH + -), 0.88 (t, 6H, CH 3 -(CH 2 ) 9 -NH + -).
	78

+ -).

10-2-10 ter Cl. Purification: 1 eq. of 10-2-10 amine (1.9 g, 5.15 mmol) was mixed with 2 eq.

  Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level

	°=	( . + )	-	/	(III-18)
				87

Zana in his paper ref.

[103]

described ΔG°M for the different types of ionic surfactants. For gemini amphiphiles with two polar head groups (monovalent) linked to two tails and two counterions (monovalent), the free energy of micellization per mole of alkyl chain is:

Table III -1. CMC a , CMC b , α Z , α E and -ΔG°M for quaternary 10-2-10 gemini with inorganic counterions.

 III

	Counterion Temperature, °C CMC a , mM	CMC b , mM	α Z	α E	-ΔG°M, kJ•mol -1
	Br	30	6.5	6.4	0.21	0.15	15.69
	Cl	30	12.8	12.4	0.43	0.21	12.88
	F	30	34.9	31.7	0.68	0.29	9.28
	NO 3	30	6.4	6.3	0.24	0.15	15.69
	MeSO 3	30	15.3	15.2	0.44	0.23	12.04
	PH	30	26.4	26.0	0.49	0.25	10.18
	C2	30	23.3	22.7	0.52	0.26	10.53
	TFA	30	6.4	6.4	0.17

Table III -

 III 2.

Table III -2. The correlation coefficient R 2 of the dependence CMC on the different ion properties.

 III Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution level 92 counterion. However, although obtained CMC values also have a good correlation with the hydration number, n H , (Figure III-3 B), except that PH significantly go out from the trend, R 2 gives just 0.526.

	Ion property	-ΔG hyd	n H		ν S	α	pKa	N lyo
	R 2	0.876	0.526	0.204	0.261	0.555	0.883
	ΔG hyd -free energy of hydration (kJ·mol	-1 ), n H -hydration number, ν S -partial molar volume (cm	3 ·mol	-1 ),
	α -polarizablility (Å

3 ), N lyo -lyotropic number

Analyzing correlation coefficients, it seems that the free energy of hydration and lyotropic number (originally correlates well with Hofmeister series) are the two parameters which correlate better with the CMC and consequently the self-assembly of surfactant with corresponding

1.2.2. Quaternary 10-2-10 gemini with carboxylate counterions

The results obtained for 10-2-10 gemini with carboxylate counterions via conductivity measurements are presented in Table III-3: CMC, α Z , α E and -ΔG°M. Figure III-6 shows the CMC (A) and α (B) as a function of counterion chain length. We can see that the CMC and α decrease

Table III -3. CMC, α Z , α E and -ΔG°M for quaternary 10-2-10 gemini with carboxylate counterions.

 III

	Counterion Temperature, °C CMC, mM	α Z	α E	-ΔG°M, kJ•mol -1
	C1	30	21.7	0.49	0.23	10.88
	C2	30	22.7	0.52	0.26	10.25
	C3	30	20.0	0.38	0.22	11.30
	C4	30	14.3	0.33	0.21	12.09
	C5	30	9.4	0.28
	C6	30	5.7	0.22
	C8	30	1.0	0.10

Table III -4. CMC a and CMC b for tertiary and quaternary 10-2-10 gemini with Br -and Cl -as counterions.

 III

	Counterion	Temperature, °C	CMC a , mM	CMC b , mM
		tertiary ammonium 10-2-10 gemini (10-2-10 ter X)
	Br	30	0.54	0.40
	Cl	30	0.34	0.33
		quaternary ammonium 10-2-10 gemini (10-2-10 X)
	Br	30	6.5	6.4
	Cl	30	12.8	12.4

). After second dissociation reaction neutral amine is forming (determined by Ka 2 ).

		Br	Br					Br
	H H 3 C	N	N	H CH 3	+	H 2 O	H 3 C H	N	N	CH 3	+ H 3 O + Br -
		C 10 H 21	C 10 H 21				C 10 H 21	C 10 H 21
		N	N				H 3 C	N	N	CH 3
		C 10 H 21	C 10 H 21				C 10 H 21	C 10 H 21

Table III -5. The binding energies ΔG B between positively charged cationic 1-2-1/1-2-1 ter and counterion. Compound Binding energy ΔG B , kcal/mol

 III

		Cl	Br	I
	1-2-1	-3.17	0.006	0.83
	1-2-1 ter	-16.0	-11.6	-8.21

Table III -6. CMC and N for 2xCMC and 3xCMC for the 10-2-10 X and 10-2-10 ter X surfactants obtained from TRFQ using 1-methylpyrene as a probe and CPC as a quencher.

 III

	Chapter III. Contribution of ion specific effects to the physical properties of micellar aggregates at the bulk solution
					level
	and the standard deviation in micelle aggregation numbers (from Equation (III-31)) are given in
	Table III-6.
	Counterion	CMC, mM	N av	N W 2xCMC		N av	3xCMC	N W
				quaternary ammonium 10-2-10 gemini (10-2-10 X)
	F	31.7	27	28	8.3	37		57	59
	PH	26	25	28	13.6	38		75	78
	C2	22.7	23	24	7	25		35	49
	MeSO 3	15.2	28	30	18.4	31		37	33
	Cl a	12.4	34	35	10	40		40	17
	Br b	6.4	35	35	9	35		36	13
	TFA	6.4	33	33	5.4	39		38	9.5
	1-Methylpyrene NO 3 6.3	33	34 tertiary ammonium 10-2-10 gemini (10-2-10 ter X) 4.5 35 36 Cetylpyridinium chloride	6.3
	Br	0.4	16	23
					109

N av -averaged aggregation number, N W -weight averaged aggregation number, a -average of two independent measurements, b -average of three independent measurements

Table III -7. CMC and N for 2xCMC and 3xCMC for the 10-2-10 X surfactants obtained from TRFQ using 1-methylpyrene as a probe and CPC as a quencher.

 III -7.

	Counterion	CMC, mM	N av	N W 2xCMC		N av	N W 3xCMC
	C1	21.7	28	31	19	37	47	42
	C2	22.7	23	24	7	25	35	49
	C3	20.0	24	25	6	26	31	25
	C4	14.3	23	27	21	27	37	37
	C5	9.4	25	26	7	30	34	21
	C6	5.7	26	27	10	28	32	21

N av -averaged aggregation number, N W -weight averaged aggregation number

Table III -8. Summary table for the results obtained with conductivity measurements.

 III

	Gemini	T	CMC a	S 1	a	S 2	a	CMC b
		(C°)	mM					mM

Table IV -1. HPLC average peak areas, observed and normalized yields for reaction of 16-ArN 2 + in 10-2-10 Br micelles from 10 mM to 250 mM at 25 C. [HBr] = 1 mM d [10-2-10 Br] mM

 IV

		Average Peak Areas (10 6 vs) a		Observed Yields (%)		Normalized Yields (%) c
		16-ArOH 16-ArH	16-ArBr	16-ArOH	16-ArH	16-ArBr	Total b	16-ArOH N 16-ArBr N
	10	7.832	0.1482	4.269	69.1	1.2	26.8	98.3	72.4	27.6
	20	8.143	0.1083	4.553	71.8	0.8	28.6	102	71.7	28.3
	30	7.807	0.1035	4.549	68.9	0.8	28.6	99.0	70.9	29.1
	50	8.484	0.1132	5.230	74.8	0.9	32.9	109	69.7	30.3
	100	6.936	0.4618	4.674	56.7	3.7	27.2	91.3	68.9	31.1
	150	3.224	0.3003	2.228	53.0	4.7	25.9	88.3	69.0	31.0
	250	3.137	0.4578	2.348	51.6	7.3	27.3	93.5	68.3	31.7

a. 100 μL sample injections. Peak areas are average of triplicate injections. Eluting solvents: 65%MeOH/35%i-PrOH; Flow rate: 0.4 ml/min; Detector wavelength: 220 nm. b. % Total = %16-ArOH + %16-ArBr + 2 %16-ArH c. % 16-ArBr N = 100 (%16-ArBr)/(%16-ArOH + %16-ArH + %16-ArBr); % 16-ArOH N = 100 (%16-ArOH + %16-ArH )/(%16-ArOH + %16-ArH + %16-ArBr). d. Reaction time ca. 48 h to ensure the complete dediazoniation reaction. The concentrations of 16-ArN 2 BF 4 were around 10 -4 M but vary in each experiments.

Table V -1. Experimental and calculated interfacial concentrations (in M) for ions and water. The calculated ion interfacial concentrations refer to the peaks of their distributions, whereas the calculated water concentrations are those resulting from the projection of the location of these peaks onto the respective water concentration gradients (see graph in Figure V-10 for clarity). The experimental values are those corresponding at the salt stoichiometric concentrations of 2xCMC.

 V

		acetate	chloride	bromide
	X m (exp.)	1.2 (1.3)	1.5	2.2
	X m (calc.)	1.5	1.8	2.1
	H 2 O m (exp.)	50	48	43
	H 2 O m (calc.)	31	34	29

Table V -2. Comparison of calculated and experimental (α E ) ionization degrees. The three sets of calculated values are obtained from the first, second and third (when applicable) peaks of the RDF of the counterion with respect to nitrogen atoms of the micelle polar heads. The values between brackets refer to the points just beyond the "last" peak. For clarity see Figure V-11 and their insets.

 V

	Counterions g(r N-X ) 1 st min. g(r N-X ) 2 nd min. g(r N-X ) 3 rd min.	α E
	C2	0.51	(0.39)	-	0.29
	Cl -	0.41	0.33	(0.26)	0.21
	Br	-	0.37	0.31	0.23	0.16

Table VI -1. Values of the CMC and α E obtained by conductivity measurements at 30 °C, and Ф* obtained by rheological measurements at 20 °C for the 10-2-10 X gemini, where X = Br -, Cl -, NO 3 -, PH, C2, TFA) Counterion CMC, mM α E Ф*, mM

 VI

		Temperature 30°C		Temperature 20°C
	NO 3	6.3	0.15	71
	TFA	6.4		51
	Br	6.4	0.15	78
	Cl	12.4	0.21	377
	C2	22.7	0.26	425
	PH	26.0	0.25	465

Table VII -1. Crystallographic data for cationic gemini surfactants n-2-n with I -, Br -and Cl - as a counterion obtained from single-crystal mesurements.

 VII

		10-2-10 I (CH3CN)	10-2-10 I (H2O)	10-2-10 I (H2O) polymorph.	16-2-16 I (MeOH/ Acetone)	10-2-10 Br (CH3CN)	14-2-14 Br (MeOH/ Acetone)	14-2-14 Cl (ACN)	10-2-10 ter Br (H2O)
	Formula	C13H29IN	C13H29IN	C39H87I3N3	C38H82I2N2	C13H29BrN	C34H74Br2N2	C34H76Cl2N2O	C12H26.50Br2N2
	FW (g•mol-1)	326.27	326.27	978.82	820.86	279.28	670.77	599.87	358.67
	Temperature (K)	293	110	293	293	113	293	113	293
	Crystal system	triclinic	triclinic	triclinic	triclinic	triclinic	triclinic	orthorhombic	triclinic
	Space group	P-1	P-1	P-1	P-1	P-1	P1	Pbca	P-1
	Z	2	2	2	2	2	1	8	2
	Z'	0.5	0.5	1.5	1	0.5	1	1	0.5
	a (Å)	7.2858(4)	7.2048(6)	7.945(5)	8.891(2)	6.9176(5)	6.946(1)	9.8560(6)	5.6594(3)
	b (Å)	8.8012(5)	8.6132(7)	13.279(9)	13.564(4)	8.5365(6)	8.518(1)	14.4116(10)	6.20190(10)
	c (Å)	14.4660(10)	13.9385(13)	25.002(15)	19.125(6)	14.5033(10)	17.117(2)	52.110(4)	22.5865(11)
	α (degree)	73.270(5)	75.61(2)	93.02(4)	102.94(2)	75.50(1)	92.589(5)	90.00	94.036(9)
	β (degree)	79.475(6)	80.82(2)	94.94(3)	91.77(2)	81.43(1)	94.541(4)	90.00	92.741(12)
	γ (degree)	66.679(5)	67.76(2)	99.70(3)	102.51(2)	67.31(1)	113.753(4)	90.00	103.760(14)
	Volume (Å 3 )	813.28	773.44	2584	2187.0	763.65	920.7	7401.7	766.40
	R1 [I>=2σ (I)]	0.0633	0.0592	0.1435	0.0877	0.0830	0.0341	0.0736	0.0525
	wR2 (all data)	0.1648	0.1164	0.3432	0.3247	0.1969	0.0980	0.1860	0.1242
	density	1.332	1.401	1.258	1.246	1.215	1.210	1.077	1.554
	Number refl [I>=2σ (I)]	2158	1983	1169	2062	2144	3126	5786	2271
	Number parameters	140	136	409	380	139	344	360	129
	diffractometer	R-axis	FRX	FRX	R-axis	FRX	Proteum	FRX	FRX

Table VII -

 VII 2.

Table VII -2. Crystallographic data for cationic gemini surfactants n-2-n with I -, Br -and Cl - as a counterion obtained from single-crystal mesurements.

 VII

	Counterion	Object1	Object 2	10-2-10	12-2-12	14-2-14	16-2-16	10-2-10 ter
		1	N	4.14	4.14	4.11
		1	N*	4.33	4.34	4.20		3.19 HB
	Br	1 2	N# N'	4.40 4.14	4.36 4.14	4.36 4.16
		2	N#	4.33	4.34	4.32
		2	N*	4.40	4.36	4.35
		1	N	4.36			4.39
		1	N*	4.53			4.51
		1	N#	4.60			4.58
	I	2	N'	4.36			4.39
		2	N#	4.53			4.51
		2	N*	4.60			4.58

"penalty scores" give the information about the validity of the parameters. Penalties between 10 and 50 indicate that some basic validation is recommended; penalties higher than 50 usually are usually associated with parameters or charges that need additional optimization, see ref.168. Vanommeslaeghe, K., E.P. Raman, and A.D. MacKerell, Automation of the CHARMM General Force Field (CGenFF) II: Assignment of Bonded Parameters and Partial Atomic Charges. Journal of Chemical Information and Modeling, 2012. 52(12): p.

3155-3168.

Work described in this chapter was performed by our collaborators from Rutgers University, New Jersey, USA. The experiments were carried out by PhD students Changyao Liu and Xiang Gao under supervision of Prof. Larry Romsted.

Some of the crystal structures of the gemini surfactants discussed in this chapter were previously obtained by Dr. Sabine Manet in our group.

The computational work described in this chapter was performed by our collaborators from the Institut Européen de Chimie et Biologie, Pessac, France. Specifically, the molecular dynamics simulations were carried out by Dr. Massimiliano Porrini, formerly postdoc researcher in the team of Dr. Michel Laguerre.

Experimental section

Material and methods

The 10-2-10 X with different counterions were synthesized as was described in Chapter II.

The samples were prepared by weighing surfactants and adding the corresponding volume of water directly into sample vials. To get a homogeneous solution, heating at 60 °C for sufficient time and simultaneous stirring was necessary.

Measurements were performed for the following systems:

(1) 10-2-10 Br solutions in water with surfactant concentration from 6.4 mM to 256 mM Manager Ver. 4.41.0000 software was used for the data collection. For each measurement shear rate was adjusted according to the concentration. "Range calculator" from the software was used to estimate approximately the shear rate that can be applied for the certain system. The maximum waiting time was chosen as 500 s. Usually, the system reached equilibrium much faster than the set time. Before data collection was started, solution was homogenized for 300 s in order to reach the equilibrium at 20 °C.

Data analysis

The obtained plots, viscosity ( ) vs shear rate ( ) (log/log scale), were analyzed using HAAKE Rheo win Data Manager Ver. 4.41.0000. The data were extrapolated in order to obtain the zero shear rate viscosity ( 0) using the same software (Figure VI-5). There is always some deviation at the low shear rate due to the limitation of the sensitivity of the apparatus. However, it disappears with increasing the . While estimating 0 , we do not take into account the deviation at the low .