Jean-Luc Trompette

Jean-Francois Lahitte

Influence of the Counterion Nature on the Stability Sequence of Hydrophobic Latex Particles

The aggregation kinetics of negatively charged polystyrene latex particles in the presence of monovalent electrolytes have been investigated. The inferred coagulation critical concentrations were compared to establish the stability sequence. With the same representative co-ions, this sequence is reversed when using kosmotrope sodium and chaotrope potassium cations. The results have been ascribed to a variable competition of the co-ions toward the hydrophobic surface depending on the lyotropic nature of the associated counterion. They provide new insights into the implication of ionic specificity in the stability behavior of aqueous dispersions of charged colloids.

INTRODUCTION

Although Hofmeister (or ion-specific) effects have been known for more than 120 years and were shown to play a significant role in the behavior of many colloidal domains where aqueous salt media are involved, the underlying reasons remain unclear. [START_REF] Bostrom | Specific Ion Effects: Why DLVO Theory Fails for Biology and Colloid Systems[END_REF][START_REF] Kunz | The Present State of Affairs with Hofmeister Effects[END_REF] What is remarkable is that they offer the opportunity to modulate the desired properties of a system by simply changing the electrolytes' nature of even the same valency, either the cation or/and the anion. [START_REF] Parsons | Why Direct or Reversed Hofmeister Series? Interplay of Hydration, Non-Electrostatic Potentials, and Ion Size[END_REF] At the fundamental level, ion-specific effects pose a challenge to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, [START_REF] Hiementz | Principles of Colloid and Surface Chemistry[END_REF] describing the qualitative behavior (stability/aggregation) of colloidal systems through knowledge of the total energy of interaction resulting from the assumed algebraic sum of the van der Waals attraction (U A ) and the electrostatic double-layer repulsion (U R ). Recognized limitations, notably at short separation distances and/or for electrolyte concentrations greater than 0.1 M, were justified since ions are considered as point charges in this theory. Moreover, solventsurface, surface-ion, and solvent-ion interactions are not taken into account. Additional non-DLVO contributions, such as the dispersion energy (U Disp ) and the hydration energy (U Hyd ), were thus proposed to interpret the experimental findings, although it is still open to debate. [START_REF] Salis | Models and Mechanisms of Hofmeister Effects in Electrolyte Solutions, and Colloid and Protein Systems Revisited[END_REF][START_REF] Bastos-Gonzaĺez | Ions at Interfaces: the Central Role of Hydration and Hydrophobicity[END_REF][START_REF] Santos | Ion Specificity and the Theory of Stability of Colloidal Suspensions[END_REF] Nevertheless, qualitative trends have been reported when studying the ability of ions to promote aggregation of aqueous dispersions of nanoparticles. [START_REF] Loṕez-Leoń | Hofmeister Effects in Colloidal Systems: Influence of the Surface Nature[END_REF][START_REF] Dishon | From Repulsion to Attraction and Back to Repulsion: The Effect of NaCl, KCl, and CsCl on the Force between Silica Surfaces in Aqueous Solution[END_REF][START_REF] Yang | Structures of Water Molecules at the Interfaces of Aqueous Salt Solutions and Silica: Cation Effects[END_REF][START_REF] Schwierz | Anionic and Cationic Hofmeister Effects on Hydrophobic and Hydrophilic Surfaces[END_REF][START_REF] Merk | In Situ Non-DLVO Stabilization of Surfactant-Free, Plasmonic Gold Nanoparticles: Effect of Hofmeister's Anions[END_REF] Various factors such as the hydrophobic/hydrophilic character of the particles, the sign of the particle surface charge, and the kosmotrope/chaotrope (lyotropic) nature of ions were shown to exert an influence. [START_REF] Oncsik | Specific Ion Effects on Particle Aggregation Induced by Monovalent Salts within the Hofmeister Series[END_REF][START_REF] Peula-García | Inversion of Hofmeister Series by Changing the Surface of Colloidal Particles from Hydrophobic to Hydrophilic[END_REF][START_REF] Schwierz | Reversed Anionic Hofmeister Series: The Interplay of Surface Charge and Surface Polarity[END_REF][START_REF] Cao | Aggregation of Colloidal Particles in the Presence of Hydrophobic Anions: Importance of Attractive Non-DLVO Forces[END_REF] Among the different investigated systems, the case of negatively charged hydrophobic particles is particularly representative, as for instance with polystyrene lattices. On the basis of aggregation kinetics experiments, the influence of anions, acting as co-ions, was notably evidenced. The coagulation critical concentration (ccc), representing a stability indicator (or gauge) of a colloidal system, was determined with sodium salts. [START_REF] Loṕez-Leoń | Hofmeister Effects in the Stability and Electrophoretic Mobility of Polystyrene Latex Particles[END_REF][START_REF] Loṕez-Leoń | Ion-Induced Reversibility in the Aggregation of Hydrophobic Colloids[END_REF][START_REF] Loṕez-Leoń | Ion-Specific Aggregation of Hydrophobic Particles[END_REF] The ranking with monovalent co-ions was as follows

≤ < < < < - - - - - - IO F Cl NO I SCN 3 3
When the ccc is higher, it reflects that higher amounts of electrolytes are required to definitively suppress any repulsive contribution. This stability sequence was shown to be inverted when the surface charge of the latex was positive (with hydrophobic particles) or when the surface turned hydrophilic (with negatively charged particles). [START_REF] Bastos-Gonzaĺez | Ions at Interfaces: the Central Role of Hydration and Hydrophobicity[END_REF] The above sequence follows what is defined as the indirect Hofmeister series for anions, where chloride Cl -is usually considered as a reference ion. On the right side of chloride, anions are structure-breaker or chaotrope ions where the immediately adjacent water molecules are weakly attracted to but are sufficiently enough to perturb their organization in the bulk. Anions on the left side of chloride are associated with structure-maker or kosmotrope ions, where the surrounding water molecules are strongly attracted to and are more closely organized among themselves than in pure water. Contrary to the kosmotropes, chaotrope ions have the propensity to easily remove their hydration shell. This kosmotrope or chaotrope nature of the ions can be conveniently distinguished according to the sign of the viscosity B coefficient appearing in the Jones-Dole relationship 20

η η = + × + × A c B c 1 w (1)
where η is the viscosity of aqueous salt solution at molar concentration c, and η w is the viscosity of pure water, which is valid usually up to 0.5 M. The coefficient A, which is positive for all salts, corresponds to an electrostatic term originating from the interaction between the ions. The value of B, which is either positive or negative, is a direct measure of the strength of ion-water interactions normalized to the strength of the water-water interactions. Positive B values are associated with kosmotrope ions, whereas negative B values correspond to chaotrope ions. The B values of ions are reported in Table 1.

To interpret the above trends, an accumulation/exclusion mechanism of ions near surfaces has been proposed. [START_REF] Bastos-Gonzaĺez | Ions at Interfaces: the Central Role of Hydration and Hydrophobicity[END_REF][START_REF] Loṕez-Leoń | Hofmeister Effects in Colloidal Systems: Influence of the Surface Nature[END_REF] It corresponds to a parallel approach of the previous concept "like adsorbs like". [START_REF] Beŕube | Adsorption at the Rutile-Solution Interface[END_REF][START_REF] Dumont | Influence of the Point of Zero Charge of Titanium Dioxide Hydrosols on the Ionic Adsorption Sequences[END_REF] It is related to the microscopic water structure around both the hydrated ions and particle surfaces. It predicts qualitative attraction/accumulation between ions and surfaces when they have similar water arrangements or repulsion/exclusion between them when they have dissimilar water arrangements.

Nevertheless, it has to be remarked that the previous trends with polymer lattices were inferred from the comparative behavior of monovalent salts where the counterion was restricted to kosmotrope sodium cations. The influence of ionic specificity on many colloidal systems has been often investigated by either changing the anion nature of a same cation (often sodium or potassium) or the cation nature of a same anion (very often chloride). [START_REF] Colic | Effect of the Size and Affinity for Water of Excluded Ions on Interparticle Forces[END_REF][START_REF] Franks | Ion-Specific Strength of Attractive Particle Networks[END_REF][START_REF] Livney | Interactions Between Inorganic Salts and Polyacrylamide in Aqueous Solutions and Gels[END_REF][START_REF] Finet | The Hofmeister Effect as Seen by SAXS in Protein Solutions[END_REF][START_REF] Zhang | Specific Ion Effects on the Water Solubility of Macromolecules: PNIPAM and the Hofmeister Series[END_REF][START_REF] Van Der Linden | Microscopic Origin of the Hofmeister Effect in Gelation Kinetics of Colloidal Silica[END_REF][START_REF] Higashitani | Ionic Specificity in Rapid Coagulation of Silica Nanoparticles[END_REF] However, the crossover study with co-ions and counterions of different lyotropic nature has been scarcely reported in the literature. [START_REF] Loṕez-Leoń | Hofmeister Effects in the Stability and Electrophoretic Mobility of Polystyrene Latex Particles[END_REF][START_REF] Simonsson | The Specific Co-Ion Effect on Gelling and Surface Charging of Silica Nanoparticles: Speculation or Reality? Colloids Surf[END_REF] Following the accumulation/exclusion mechanism and as polystyrene lattices are assumed to be model hydrophobic colloids, it may be interesting to examine whether the kosmotrope/chaotrope character of the counterion exerts a sensitive influence in the presence of representative co-ions with a different lyotropic nature.

In this study, the comparative stability behavior of negatively charged polystyrene lattices in the presence of monovalent electrolytes with potassium, ammonium, and sodium counterions has been investigated.

EXPERIMENTAL SECTION

2.1. Materials. Sodium thiocyanate (NaSCN), sodium chloride (NaCl), sodium iodate (NaIO 3 ), potassium thiocyanate (KSCN), potassium chloride (KCl), potassium iodate (KIO 3 ), ammonium thiocyanate (NH 4 SCN), ammonium chloride (NH 4 Cl), and lithium chloride (LiCl) were purchased from Sigma-Aldrich or Prolabo and were used as received. Deionized water (18.2 MΩ•cm) was used as a solvent to prepare the electrolyte solutions, whose pH values ranged between 6 and 7.

2.2. Synthesis of Polystyrene Latex. Stable polystyrene lattices were synthesized from surfactant-free emulsion polymerization according to classical methods reported in the literature. [START_REF] Kotera | Colloid Chemical Studies of Polystyrene latices Polymerized Without Any surface-active Agents[END_REF][START_REF] Goodwin | Studies on the Preparation and characterisation of Monodisperse Polystyrene laticee[END_REF] Nitrogen gas was bubbled during 10 min into a solution containing 0.0347 g of potassium persulfate (K 2 S 2 O 8 , 270 g•mol -1 ) dissolved in 100 mL of deionized water. The sample was placed into a double-jacked reactor thermostated at 70 °C through a regulator bath (Lauda Eco Silver), and the solution was moderately agitated (300 rpm) with a magnetic stirrer. The inhibitor present in liquid styrene was removed by filtration across a bed of activated alumina powder. Then, 1.5 mL of purified styrene (C 8 H 8 , 104.15 g• mol -1 , density: 0.906 g•mL -1 ) was added into the previous solution, and the reaction was stopped after 24 h. After synthesis, 30 mL of the obtained latex was poured in a dialysis bag (12-14 kDa, regenerated cellulose) that was immersed in 1.8 L of deionized water under gentle agitation. The conductivity of the contact water was checked over time, and deionized water was changed until the conductivity of contact water reached 1 μS•cm -1 .

The conductivity, pH value, and dry content of the latex sample passed, respectively, from 1.1 mS•cm -1 , pH 4, and 1.22% before dialysis to 59 μS•cm -1 , pH 5.2, and 1.07% after dialysis. The density of the dialyzed sample was 0.9981 g•mL -1 at 25 °C, corresponding to a particle volume fraction of 0.01 (density of polystyrene: 1.055 g•mL -1 ).

A Malvern NanoZS model was used for size and zeta potential measurements. Scanning electron microscopy (SEM) micrographs were obtained with a JEOL JSM 7100F model operating at 10 kV.

2.3. Kinetics Measurements. The dialyzed polystyrene latex was diluted with deionized water at a 1:8 ratio, and then 0.2 mL of the diluted sample was added into a quartz cell (1 cm in length) containing 2 mL of aqueous salt solution to obtain the desired electrolyte concentration (whole volume of 2.2 mL). In these conditions, the volume fraction of polystyrene particles corresponds to 114 × 10 -6 ; the pH value of the resulting dispersions remains the same as that of the prepared electrolyte solutions (i.e., between 6 and 7). As described in other studies, [START_REF] Loṕez-Leoń | Hofmeister Effects in Colloidal Systems: Influence of the Surface Nature[END_REF][START_REF] Peula-García | Inversion of Hofmeister Series by Changing the Surface of Colloidal Particles from Hydrophobic to Hydrophilic[END_REF] the aggregation kinetics of the latex particles were followed at 25 °C with a spectrophotometer (Shimadzu) by recording the variation of the absorbance A at 600 nm wavelength as a function of time t. In each experiment, the initial slope k during the first 40 s was determined. For a given electrolyte, the slope increases with concentration until a maximum value k f is reached, corresponding to the coagulation critical concentration (ccc). This indicates the onset of the fast or diffusion-limited aggregation regime (DLA), whereas for concentrations lower than the ccc, the slow or reaction-limited aggregation regime (RLA) is characterized by an increasing slope k s with salt concentration. The stability behavior of a colloidal system is classically assessed through the stability ratio W. It reflects the collision efficiency, that is, the ratio of the number of collisions to the number of efficient collisions, and it is conveniently obtained from the slope measurements through

= ≥ < = ( ) ( ) W k c k c (at ccc) (at ccc) A t A t fast slow d d f d d s (2)
The determination of initial slopes at increasing concentrations enables ccc detection. W was then calculated by dividing the highest slope (corresponding to the ccc) to the slope at each concentration, and it was further plotted in a log scale as a function of salt concentration. The concentration where W = 1 corresponds to the ccc (a horizontal line at W = 1 is drawn on the plots to locate easily its position). Below the ccc (i.e., RLA regime), there is a repulsive energy barrier so that W > 1: each collision between particles does not result in [START_REF] Marcus | Ion Properties[END_REF] and static polarizability (α 0 ) 2,5,8 of ions.

sticking. The kinetics measurements were repeated twice and were focused to accurately determine the ccc values (average values were taken).

RESULTS AND DISCUSSION

3.1. Latex Characterization. Within the synthesis conditions, the molar ratio of monomer styrene (0.1286 M) to initiator potassium persulfate (12.66 × 10 -4 M) is 102, ensuring the hydrophobic character of the latex particles. The stability of polystyrene particles originates from their electrostatic repulsion due to the negative charges carried on the hydrophobic surface. They result from the sulfate free radicals (SO 4 * -) generated consecutively to the thermal decomposition of persulfate anions. In the following step, the radical entity is transferred to the styrene monomer through chemical bonding from which the polymerization reaction then propagates. It has been reported in the literature that some sulfate groups may transform in carboxyl groups, owing to eventual differences in preparative, cleaning, or characterization procedures, although there is some conflicting evidence. [START_REF] Hearn | Polymer latices as Model Colloids[END_REF] Once dialysis was performed (see Section 2.

2), the zeta potential and z-average diameter of polystyrene particles of diluted sample in 10 -3 M NaCl were, respectively, -53 mV and 468 nm with a 0.08 polydispersity index (given from the cumulants method). As an indication, a SEM micrograph of the polystyrene latex is shown in Figure 1. The average diameter and polydispersity index (i.e., [standard deviation/ average diameter] 2 ) evaluated from the picture are, respectively, 451 nm and 0.02. The difference originates from the used procedure; analysis from light scattering technique is based on an intensity response, whereas analysis from SEM picture is based on a number response, giving higher emphasis on the smallest components.

Effect of the Lyotropic Nature of Counterions.

In the classical description of the interface between a negatively charged solid surface and an aqueous phase, the ionic atmosphere surrounding the charged surface is composed of cationic counterions that are favorably attracted, whereas the anionic co-ions are repelled. This distribution and random thermal motion of ions lead to the formation of an electric double layer: [START_REF] Hiementz | Principles of Colloid and Surface Chemistry[END_REF] an inner (Stern) layer rich in cations in the immediate vicinity of the negatively charged surface and an outer (diffuse) layer where some cations and a majority of anions are present, whose thickness, the Debye length κ -1 , represents the range of electrostatic repulsion. When enough salt is added, the extent of diffuse layers is sufficiently reduced so that the attractive van der Waals contribution becomes dominant. The aggregation kinetics are related to the total energy of interaction; the more attractive the total interaction, the faster the aggregation. When the ccc is reached, the aggregation regime is fully attractive (i.e., there is no more repulsive contribution).

The influence of the nature of the counterion on the colloidal stability of the synthesized latex was investigated by using chloride electrolytes with monovalent cations exhibiting a different kosmotrope/chaotrope character (see Table 1). The aggregation kinetics were carried out at different electrolyte concentrations to appreciate the slope variation until the ccc was reached, that is, the concentration where the slope remains the highest. As an example, the case with a LiCl electrolyte is shown in Figure 2.

As a characteristic parameter in aggregation kinetics studies, the half-time of aggregation t 1/2 may be evaluated. According to Smoluchowski's theory, [START_REF] Hiementz | Principles of Colloid and Surface Chemistry[END_REF] it is given by

= × t k N 1/2 1 r 0
, where N 0 is the initial number of particles per unit volume, and k r is the second-order rate constant for diffusion-controlled aggregation that corresponds to 5.48 × 10 -18 m 3 •s -1 in the dilute aqueous phase at 25 °C. As the particle volume fraction in aggregation kinetics measurement is 114 × 10 -6 and the average diameter of a spherical particle is taken as 468 nm, it gives N 0 = 2.1 × 10 15 particles/m 3 . The corresponding halftime of aggregation t 1/2 is about 80 s, accrediting that kinetics measurements are carried out in the early stages of aggregation during the first 40 s (see Section 2.3).

As an indication, the variation of the absorbance during the first 40 s in the case of LiCl at each concentration is reported in Figure 3. The highest initial slope corresponding to the ccc value occurs at 0.35 M; the initial slopes allow the stability ratio W to be calculated at each concentration from eq 2 (see Section 2.3).

The plot of the stability ratio W as a function of electrolyte concentration is presented in Figure 4. The corresponding ccc values are, respectively, 0.35, 0.32, 0.27, and 0.25 M for Li + , Na + , K + , and NH 4 + cations. These weak but noticeable variations are in accordance with the accumulation/exclusion mechanism, which is based on hydration state compatibilities between ions and surfaces. It can be assumed that the interfacial hydrogen-bonding network of water molecules at the hydrophobic polystyrene surface is poorly developed so that the structure-breaker cations (that disrupt the water structure around them) are preferentially attracted in this favorable environment than the structuremaker cations, contributing to an enhanced screening of the electrostatic repulsions between particles. This is also consistent when the dispersion energy U Disp is considered. It is expected to be attractive when the refractive index n of the substrate is larger than that of water (n = 1.3325), which is the case for polystyrene particles since n = 1.5916, and its magnitude should increase as the static polarizability of ion in water, α 0 , is higher [START_REF] Bostrom | Specific Ion Effects: Why DLVO Theory Fails for Biology and Colloid Systems[END_REF][START_REF] Loṕez-Leoń | Hofmeister Effects in Colloidal Systems: Influence of the Surface Nature[END_REF] (see the reported values in Table 1). A better screening of the surface charge, and thus a promoted destabilizing effect, is therefore expected as the counterions are chaotropes and are more polarizable. The ccc value increases effectively according to the Hofmeister series of cations: NH 4 + < K + < Na + < Li + . This is in agreement with earlier studies on negatively charged hydrophobic latex surfaces. [START_REF] Oncsik | Specific Ion Effects on Particle Aggregation Induced by Monovalent Salts within the Hofmeister Series[END_REF] The same trends have been also reported in the presence of negatively charged silica nanoparticles at pH values much greater than the isoelectric point (IEP), that is, 2 (pH value for which the zeta potential is zero). Within these conditions, where fewer neutral SiOH silanol groups are present at the silica surface, the adjacent aqueous layers are not well structured so that an enhanced accumulation of alkali cations with increasing chaotrope character was observed. [START_REF] Van Der Linden | Microscopic Origin of the Hofmeister Effect in Gelation Kinetics of Colloidal Silica[END_REF] 3.3. Comparative Effect of Co-Ions with Counterions of Different Lyotropic Nature. The comparative colloidal stability of the polystyrene latex has been investigated through aggregation kinetics measurements with sodium, potassium, and ammonium salts of identical anions. The three chosen coions were the same as those in a previous study [START_REF] Trompette | Influence of Co-Ion Nature on the Gelation Kinetics of Colloidal Silica Suspensions[END_REF] since they correspond to representative anions covering a wide range of B values. Thiocyanate (SCN -) is a well-recognized chaotrope anion of high polarizability, chloride (Cl -) is a slightly chaotrope anion that is considered as a central ion in the Hofmeister series, and iodate (IO 3 -) is a quite kosmotrope anion, [START_REF] Baer | Is Iodate a Strongly Hydrated Cation?[END_REF] however, with a large polarizability (see Table 1).

It has to be mentioned that, owing to solubility considerations, the study of colloidal stability in aqueous electrolyte solutions at 25 °C is limited to concentrations lower than 0.478 and 0.428 M, respectively, for NaIO 3 and KIO 3 salts. That for ammonium iodate (NH 4 IO 3 ) is precluded since its solubility is too low (i.e., 0.155 M). Because of the experimental procedure described in Section 2.3, the tested concentrations could not exceed 0.42 and 0.38 M, respectively, for NaIO 3 and KIO 3 . As a consequence, the charge density of the synthesized particles should not be too high to allow the study in the authorized concentration range of these salts. This was achieved when disposing of stable polystyrene latex particles with a zeta potential in the range -50 to -60 mV.

The results obtained with the sodium salts are presented in Figure 5.

The corresponding ccc values are, respectively, 0.38, 0.32, and 0.3 M for SCN -, Cl -, and IO 3 -. This sequence is similar with that reported in previous studies using polystyrene particles, [START_REF] Peula-García | Inversion of Hofmeister Series by Changing the Surface of Colloidal Particles from Hydrophobic to Hydrophilic[END_REF] and the accumulation/exclusion mechanism allowed interpretation of this stability sequence. [START_REF] Bastos-Gonzaĺez | Ions at Interfaces: the Central Role of Hydration and Hydrophobicity[END_REF] When the surface is hydrophobic, interfacial water molecules experience a peculiar arrangement at its contact so that adjacent aqueous layers exhibit a lower density than that in the bulk. Therefore, chaotrope anions with larger polarizability are more favorably accumulated even on these negatively charged hydrophobic surfaces than the kosmotrope anions. As a result, the effective negative charge on the particles' surface is increased so that stability is enhanced and higher concentrations of salts are thus required to reach the ccc from IO 3 -to SCN -. Even though the polarizability of iodate anion (IO 3 -) is large, this co-ion should not accumulate to the hydrophobic particle surface, owing to its pronounced kosmotrope character [START_REF] Peula-García | Inversion of Hofmeister Series by Changing the Surface of Colloidal Particles from Hydrophobic to Hydrophilic[END_REF] (see Table 1). This indicated, in fact, that hydration state compatibilities between ions and surfaces prevailed over polarizability (dispersive energy) considerations. [START_REF] Loṕez-Leoń | Hofmeister Effects in Colloidal Systems: Influence of the Surface Nature[END_REF][START_REF] Peula-García | Inversion of Hofmeister Series by Changing the Surface of Colloidal Particles from Hydrophobic to Hydrophilic[END_REF] The results obtained with potassium and ammonium salts are presented, respectively, in Figures 6 and7.

With potassium, the corresponding ccc values are, respectively, 0.2, 0.27, and 0.32 M for SCN -, Cl -, and IO 3 -. This sequence is the opposite from that of sodium cations. The same ccc was obtained with NH 4 SCN and KSCN (i.e., 0.2 M), and that with NH 4 Cl was 0.25 M. The comparative ccc values for the studied salts are summarized in Table 2.

From the accumulation/exclusion mechanism, it is difficult to consider that the higher ccc value with iodate anion (IO 3 -) in the presence of potassium cation would result now from a better accumulation of this kosmotrope co-ion to the hydrophobic polystyrene surface (so as to increase significantly the negative surface charge). In the case with thiocyanate anion (SCN -), there is a striking difference in the ccc values: it corresponds to the highest with Na + (i.e. 0.38 M), whereas it corresponds to the lowest with K + or NH 4 + cations, that is, 0.2 M. As it was confirmed that accumulation of chaotrope counterions (potassium or ammonium) is enhanced when compared to the kosmotrope counterion (sodium), it suggests that chaotrope thiocyanate anions may experience a quite different accumulation toward the hydrophobic surface, depending on the lyotropic nature of the counterion. Assuming that accumulation of both chaotrope counterions and co-ions should be promoted near a hydrophobic surface, this leads to consider a possible competition between ions within the inner layer to interpret the obtained results.

It has to be emphasized at this level that the knowledge of the precise solution structure near charged interfaces is a difficult task. The concentration profiles of counterions and coions are clearly dependent on the surface chemistry (nature and extent of electrical charge and hydrophobic/hydrophilic character of the surface) and the bulk ionic strength. Moreover, the water molecules have a paramount role in establishing this solution structure, owing to their very large number and their ability to promote or inhibit hydrogen bonds around surfaces and ions depending on their nature. Nevertheless, qualitative trends may be assumed to interpret the results. It may be suggested that, when co-ions have a marked chaotrope character, together with an appreciable polarizability such as thiocyanates, they may significantly compete with the kosmotrope sodium counterions within the inner layer close to the hydrophobic surface. The kosmotrope cations may be seen as being permissive to such co-ions in this case. As a result, the screening of the negative surface charge should be effectively decreased, and higher amounts of sodium salts are required to reach the ccc. With chloride, which is slightly chaotrope, and with a lower polarizability, this competitive aspect should be less pronounced. As iodate anions (IO 3 -) are quite kosmotropes, they should reside preferably in the diffuse (outer) layer.

When the counterions have a chaotrope nature, the inner layer near the hydrophobic surface is expected to be more densely constituted by such cations such as potassium or ammonium. They may be seen as being much less permissive to the co-ions, inhibiting their accumulation close to the hydrophobic surface. As a result, the screening of the negative surface charge is enhanced by these counterions, and a large majority of co-ions should remain in the diffuse layer where they experience however a distinct affinity with the solvent water molecules. The observed differences in ccc values may originate in this situation from variable repulsive hydration effects (U Hyd ) depending on the lyotropic character of the coions. When the ionic concentration is not negligible, the presence of strongly hydrated co-ions such as the iodates should exert a more enhanced water ordering and cohesiveness within the contracted diffuse layer when compared to the case with the chaotrope chloride and thiocyanate co-ions. As a result, more energy should be supplied upon closer approach (aggregation) of the nanoparticles to disrupt this outer interfacial water structure stabilized by the confined kosmotrope iodate anions, aiming at minimizing the extent of broken hydrogen bonds. When the contracted diffuse layer is populated by chaotrope co-ions, the magnitude of this repulsive dehydration contribution is expected to be significantly reduced as aggregation proceeds so that the electrolyte concentration required to reach the ccc should be lowered, especially as the co-ion has a marked chaotrope character such as thiocyanate. These trends seem to coincide with the reported influence of the same co-ions of potassium electrolytes on the gelation kinetics of silica nanoparticles at pH much higher than the IEP. [START_REF] Trompette | Influence of Co-Ion Nature on the Gelation Kinetics of Colloidal Silica Suspensions[END_REF]

CONCLUSIONS

In this study, the influence of the lyotropic nature of monovalent counterions on the inversion of the stability sequence of negatively charged hydrophobic latex particles has been evidenced. For the same representative co-ions, the ordering of the ccc values has been found to be reversed when using comparatively the kosmotrope sodium and the chaotrope potassium or ammonium counterions. This result advocates in favor of a competitive behavior of the implied co-ions and counterions within the inner layer at the hydrophobic particle-aqueous solution interface.

■ AUTHOR INFORMATION

Corresponding Author *E-mail: jeanluc.trompette@ensiacet.fr. Phone: +33 05 34 32 36 34.

Figure 1 .

 1 Figure 1. SEM micrograph of a dried sample of the synthesized polystyrene latex.

Figure 2 .

 2 Figure 2. Variation of the absorbance at 600 nm as a function of time during aggregation kinetics at 25 °C of polystyrene particles in the presence of LiCl electrolyte at 0.1 M (open box), 0.15 M (open diamond), 0.2 M (solid box), 0.25 M (open triangle), 0.3 M (times sign), 0.32 M (solid diamond), 0.35 M (open circle), and 0.375 M (minus sign).

Figure 3 .

 3 Figure 3. Variation of the absorbance at 600 nm during the first 40 s at 25 °C in the presence of LiCl electrolyte at 0.1 M (open box), 0.15 M (open diamond), 0.2 M (solid box), 0.25 M (times sign), 0.3 M (open triangle), 0.32 M (solid diamond), 0.35 M (open circle), and 0.375 M (minus sign).

Figure 4 .

 4 Figure 4. Plot of the stability ratio (log scale) as a function of concentration with LiCl (open circle), NaCl (open square), KCl (open triangle), and NH 4 Cl (solid diamond) electrolytes.

Figure 5 .

 5 Figure 5. Plot of the stability ratio (log scale) as a function of concentration with NaSCN (solid diamond), NaCl (open square), and NaIO 3 (open triangle) electrolytes.

Figure 6 .

 6 Figure 6. Plot of the stability ratio (log scale) as a function of concentration with KSCN (solid diamond), KCl (open square), and KIO 3 (solid triangle) electrolytes.

Figure 7 .

 7 Figure 7. Plot of the stability ratio (log scale) as a function of concentration with NH 4 SCN (solid diamond) and NH 4 Cl (open square) electrolytes.

Table 1 .

 1 Representative Parameters of the Studied Ions a

	ions	SCN -	Cl -	IO 3 -	Li +	Na +	K +	NH 4 +
	B (M)	-0.103	-0.005	0.138	0.146	0.085	-0.009	-0.008
	α 0 (Å 3 )	6.61	3.59	8.00	0.03	0.13	0.79	1.86

a Viscosity coefficient value (B)

Table 2 .

 2 Coagulation Critical Concentration (in M) of the Corresponding Electrolytes

	ions	SCN -	Cl -	IO 3 -
	Na +	0.38	0.32	0.30
	K +	0.20	0.27	0.32
	NH 4 +	0.20	0.25


Yves Queneau
email: yves.queneau@insa-lyon.fr

Stéphane Chambert

Rouba Cheaib

Arkadiusz Listkowski

Alain Doutheau

Stéphane Trombotto

Carboxymethyl Tri-O-acetyl-α-D-glucopyranoside 2-O-Lactone -A Synthon for the Preparation of Neoglucoconjugates

published or not. The documents may come

Introduction

Being involved in the use of carbohydrates as organic raw materials, 1 we have studied the reactivity of isomaltulose (6-α-D-glucopyranosyl-D-fructofuranose, 1) and found that its oxidation conveniently provided carboxymethyl tri-O-acetyl-α-D-glucopyranoside (CMG, 2). 2 Also, its 2-O-lactone (CMGL, 3) proved to easily open by reaction with nucleophilic species leading to neoglucoconjugates as depicted in Figure 1. Such molecular constructs involving a carbohydrate moiety are interesting chemical systems, most often because of their biological significance. Indeed, various types of glycoconjugates appear to be essential species in many biological phenomena and have attracted considerable attention in terms of drug-discovery. [START_REF]Carbohydrate-based Drug Discovery[END_REF] Other applications lie in the high polarity of a glucidic moiety which brings improved water solubility to the whole molecule with eventual self association properties. Among synthons designed for glycoconjugate synthesis, carboxymethyl glycosides have been used

efficiently either as carbohydrate provider in the preparation of glycoconjugates analogues or as carbohydrates scaffolds able to serve as a polyfunctional and structurally constrained backbone. In the same idea, we have demonstrated that carboxymethyl glycoside lactones could be used as an activated analogues of carboxymethyl glycoside able to deliver a carbohydrate moiety under extremely mild conditions. In this first section, we will detail the synthesis and the uses of carboxymethyl glycoside lactones from isomaltulose and review some other studies of the literature in which carboxymethyl glycosides have been used as synthons as well as the main methods for their preparation.

Synthesis of carboxymethyl glucoside and its lactone from isomaltulose

Isomaltulose (1) and trehalulose (5), 6-α-D-glucopyranosyl-D-fructofuranose and 1-α-Dglucopyranosyl-D-fructopyranose respectively, are obtained in one step from sucrose (4) by bioconversion (Figure 2). [START_REF] Weidenhagen | [END_REF] The chemistry of these available carbohydrates has been recently reviewed. Upon oxidation by air under basic conditions, isomaltulose is known to provide glucosyl-α-Darabinonates or more carboxylated derivatives under platinum-catalysed oxidation conditions. [START_REF] Kunz | Carbohydrates as Organic Raw Materials[END_REF] Such compounds might be interesting as potential cation sequestering agents. [START_REF] Besemer | Carbohydrates as Organic Raw Materials III[END_REF] We focused our efforts on the use of aqueous hydrogen peroxide, because it is easy to handle, easily available and generates no by-products. The efficiency and the outcome of the oxidation of carbohydrates by hydrogen peroxide depends on the occurrence of hemiacetalic centres, the basic or acidic conditions and the presence of additives such as metal salts. [START_REF] Arts | For a review on hydrogen peroxide oxidation of carbohydrates[END_REF] Some of these reactions are degradative oxidation-decarboxylation sequences since the first oxidation products are themselves oxidisable, and so on, leading to short acids as final products. What we found is that under acidic conditions, the hydrogen peroxide oxidation of isomaltulose led to α-CMG (2). The reaction was also studied in the presence of sodium tungstate, known to promote the oxidative cleavage of glycols via peroxotungstate species when used in combination with hydrogen peroxide. 9 Applied to starch and maltodextrins, erythronic acid-terminated oligoglucosides were obtained. 10 α-CMG can also be obtained from isomalt, the hydrogenation product of isomaltulose, and in this case, the presence of tungstate salts proved to be indispensable. 11 A typical procedure is to treat isomaltulose with excess hydrogen peroxide in acidic conditions (pH 2) at 90 °C. The reaction can also be performed under basic conditions, but the formation of α-CMG is much slower. The possible routes towards CMG from these disaccharidic substrates are depicted in Figure 3. It is related to the work by Isbel and Frush who studied the base catalysed oxidation of other disaccharides with hydrogen peroxide leading to mixtures of oxidised products, among which carboxymethyl β-D-glucopyranoside was identified when cellobiose was used as starting material. 12 The product 2, which can be obtained at the 5-10 g scale in ca. 35% yield, was identified and characterised as its methyl ester 6 and the corresponding tetraacetyl derivative 7.

Figure 3

We also found that the direct treatment of α-CMG under acetylation conditions led to a new product which was identified as the triacetyl lactone 3 (α-CMGL, Figure 4). 1 H and 13 C one-and two-dimensional NMR spectroscopic analyses were consistent with the presence of the α-carboxymethyl linkage, and HMBC C-H correlations were observed between H-7ab and C-1 and between H-7ab and C=O. The most likely hypothesis for the formation of this lactone is an intermediate mixed anhydride formed first, followed by the cyclisation by reaction with OH-2. The formation of such a glucoside lactone had never been described, although it was suggested as an intermediate in the case of a β-lactoside. 13 A comparable structure has been described among intermediates towards the synthesis of a lipid-A analog in a recent patent. 14 Likewise, a phostone was observed during the acetylation step of a phosphono-C-glycoside. 15 The diverse side products observed after acetylation are also a way to have a clearer view of the outcome of the oxidation step.

Notably, another lactone (8) arising from incomplete oxidation was formed when the reaction is not carefully maintained at 90 °C and glucose pentaacetate ( 9) can be present when the pH is not well controlled. Also, acetylated carboxymethyl glucosides arising from the opening of the lactone were observed, either with four acetyl groups (10) formed during the acetylation step or with OH-2 still unprotected (11), thus formed during the purification procedure.

Figure 4

The ability of lactone 3 to open up in the presence of a nucleophilic species was first observed when traces of ethyl ester 31 (vide infra, Figure 11) were identified during a recrystallization in ethanol. This led us to investigate in a more general way whether lactone 3 could be used as a synthon for connecting a carbohydrate moiety to other systems, as detailed in the following sections. Indeed, opening of carbohydrate lactones is a popular way for such connective strategies, which can be considered as an alternative to glycosylation. It has been used for the synthesis of a variety of conjugates such as amphiphilic derivatives, 16 carbohydrate terminated dendrimers 17 and hybrid polymers. 18 Only uronic acid lactones or more complex bicyclic structures based on carbohydrates have been used in similar strategies, with the purpose of preparing either surfactants or glycopeptide analogs. 19,20

Alternative accesses to carboxymethyl glucosides

Carboxymethyl glycosides can be prepared by different routes. The most simple one is the direct Fischer glycosylation of glycolic acid by glucose in the presence of hydrochloric acid, described by Petersson et al., leading to an 70:30 α/β mixture of the glycosides in a 6 % yield (Figure 5, path a). 21 The method described in section 1.1., which leads exclusively to the α anomer, constitutes path b. 2 A major alternative involves intermediate allyl glycosides, obtained either via Fischer or Koenigs-Knorr type glycosylations, which can be oxidized either by the RuCl 3 -NaIO 4 method, [22][23][24][25][26] developed by Sharpless and co-workers, 27 by bishydroxylation followed by glycolic cleavage, [28][29][30] or by ozonolysis (path c). 31,32 Following Fischer and Helferich early report who described ethoxycarbonylmethyl β-Dglucopyranoside and its subsequent saponification to the carboxylic function in 1911, 33 glycolic acid esters were directly used (path d) in glycosylations involving activated glycosyl donor instead of allyl alcohol (glycosyl bromide, [34][35][36][37] fluoride, 38 trichloroacetimidate 13,31,39 ). The anomeric configuration of the carboxymethyl glycoside relies therefore on the classical parameters which control the selectivity of the glycosylation step. The more stable tetrabenzylated carboxymethyl glycoside can be obtained by acid catalysed reaction with glycolic acid ethyl ester. 29 An example using the silver (I) salt of glycolic acid has been described. 40 Also, the bisdimethylacetal of glycolylaldehyde has been used, with a subsequent oxidation step by NaClO 2 . 41 Finally, let us mention that the carboxymethylation of alcohols or amines using αhalogenoacetic acid derivatives is a very common reaction, widely used in polysaccharide chemistry for example. Typical conditions are sodium chloroacetate in basic solution, or bromoacetonitrile in acetonitrile in the presence of NaH, followed by transformation of the nitrile group to a carboxylic acid. 42 Some esters (tbutyl, benzyl) of bromoacetic acid are also often used, but there has been only rare mention of their use for anomeric alkylation (path a).

Uses of carboxymethyl glycosides

Carboxymethyl glycosides have been used in different syntheses of peptidomimetics constructed on carbohydrate scaffolds (Figure 6). For example, Nicolaou and co-workers designed non-peptide mimetics 12 of cRGDFV pentapeptide, a potent angiogenesis inhibitor. 38 In the field of cell adhesion inhibition, related with inflammatory processes, Kessler and co-workers have prepared a series of peptidomimetic integrin antagonists constructed on a carboxymethyl mannoside backbone 13. 41 A multifunctionalised scaffold (14) was prepared by Ghosh and co-workers for the construction of broad screening libraries, based on a N-acetyl glucosamine carboxymethyl glycoside, also bearing an azido residue at C-6. 22

O MeO

BnO

I O O H N NH 2 NH CO 2 H O O O OBn O O CO 2 H OMe OMe 12 13
O BnO HO AcHN N 3 O CO 2 H 14

Figure 6

Glycodendrimers have stimulated considerable interest in the recent years. 44 Glycoconjugate libraries (including compound 15) were prepared by Lockhoff with carboxymethyl glycosides serving as the carboxylic acid partner in four components Ugi reactions (Figure 7). 31 This reaction was also used by Ziegler and co-workers for the preparation of neoglycoproteins ( 16), 34 and by Westermann and Dörner, and Li and co-workers for the synthesis of glycoclusters 17 and 18, designed for RNA and lectin binding properties. 23,24 Virta and co-workers have connected carboxymethyl glycosides to cyclopeptidic scaffolds, leading thus to diverse di and trivalent glycoclusters (including compound 19) using a solid support methodology. 25 Other types of amphiphilic glycopeptidic clusters (20) were prepared by Grandjean and co-workers. 45 A tetrameric glucose based cluster (21) was prepared by Binder and Schmid on pentaerythritol as central polyol (Figure 8). 28 Using a carboxymethyl lactoside, Toyokuni et al. synthesized multivalent lactosyl clusters (22) designed as potential tumour metastasis inhibitors. 13 Here again, carboxymethyl glycosides are useful synthons for the connection of the external carbohydrate function to the internal multivalent core. For example, Kitaoka and co-workers prepared PAMAM dendrimers decorated with cellobiose (23), designed for studying the accessibility of cyclodextrin phosphorylase. 35 Multivalent systems such as 24, involving specific disaccharides were prepared by Khan, Pieters and co-workers for the study of their properties as E-Coli antiadhesion agents. 39 The carboxymethyl residue at the anomeric position has been also used in other types of conjugates. In a solid-phase approach to galactose based trisaccharide epitopes involved in hyperacute rejection in xenotransplantation, Elofsson, Kihlberg and co-workers used a carboxymethyl linkage for the connection of the first carbohydrate with the resin (25) (Figure 9). Basu and Santacroce prepared disaccharidic fatty amides (including compound 26) linked at the anomeric position via amide formation from the corresponding carboxymethyl glycosides. The carbohydratecarbohydrate interactions of these synthetic glycolipids with natural gangliosides and sphingolipids were investigated in the context of a study of the activity of cell surface glycolipids as mediators in cell adhesion. 32 A carboxymethyl glycoside moiety was used by Mandai and co-workers as a water-solubility promoter for anticancer taxoids, i.e. CMG-docetaxel (27). 29 This approach was also shown to provide prolonged duration of action of some peptide drugs. 46 Let us mention two last examples of the use of carboxymethyl residues. The strategy was mentioned for the preparation of complex phosphine oxides (28) designed for metal coordination properties as well as water solubility 47 and carboxymethyl glycosides were used by Stoodley and co-workers as starting material towards highly oxygenated dienes (29) used in asymmetric hetero-Diels-Alder reactions (Figure 10). 48 H

Figure 10

Finally, the properties of carboxymethyl glycoside itself were investigated among other glycosides, in the context of cation complexation studies by Van Bekkum and co-workers, 36 and for the rate of the glycosidic bond hydrolysis by Timell. 37

Reactions of carboxymethyl glucoside lactone with alcohols

Alcohols react with α-CMGL (3) to provide the corresponding glycosyloxyacetylated compounds which have unsubstituted OH-2 49 (Figure 11, Table 1). The reaction proceeds with either base, acid or lanthanide salts catalysis, using either alcohol as solvent (or mixed with CH 2 Cl 2 ) or with stoichiometric amount in CH 2 Cl 2 . A series of alcohols were used, including simple aliphatic alcohols, glycol, cholesterol, and a protected carbohydrate having only one OH available, 1,2,3,4-di-O-isopropylidene-α-D-galactopyranose. Methanol led to the fully deacetylated product.

When the chloroacteyl analog 40 of the lactone is used, concomitant ester deprotection is observed. Therefore, the chloroacetyl lactone is a more convenient starting material for providing the deprotected conjugates (Figure 12, Table 2).

Reactions of carboxymethyl glucoside lactone with amines

The main applications of CMGL as a carbohydrate connecting synthon were obtained when amines were used as the nucleophilic species for opening the lactone. Enhanced nucleophilicity of amines compared to alcohols makes the reaction general and easy to perform, even under very mild conditions. The high stability of the amide function which is thus created permits easy further elaboration of the adducts (functionalisation, deprotection). The reaction was applied to the synthesis of various types of conjugates such as glycoaminoacid hybrids, 49 pseudo disaccharides, 50 glycosylated porphyrins 51 and glycosteroids. 52

Glycoaminoacid hybrids

Glycopeptides are an important class of biomolecules, involved in many physiological and pathological processes. Therefore various strategies have been established for the synthesis of their analogs and mimetics. 22,38,41,53,[START_REF] Gruner | For recent reviews on sugar-aminoacid conjugates[END_REF] We thus investigated the potential of CMGL in similar strategies and examined its opening by variously protected aminoacids (Figure 13). 49 Glycine methyl ester was first used as a simple model to give the amide 44.

The case of aspartic acid dimethyl ester permitted to verify that no loss of configurational integrity at the amino acid chiral center occurred during the reaction (45). Indeed, from either L-or the D-amino acids, two diasteroisomers are formed and NMR spectroscopy clearly shows that both compounds are different and pure. Lysine methyl ester, which possesses an amino group on the side chain, gave the N,N' -bis(glycosyl) diacetamide 50, with small amounts of the monoamide 49. This latter compound has still the side chain amino group available for incorporation into a peptide sequence. The alternative lysine monoamide could be prepared from benzyloxycarbonyl N-protected lysine methyl ester.

Figure 13

Removal of acetyl groups proceeded without loss of chirality at the α-amino acid centres using hydrazine in either methanol or methanol-dichloromethane, known to respect amino acid chirality [START_REF] Aoi | [END_REF] or acetyl chloride in methanol. This method proved to be more general because methyl ester protecting groups are not compatible with the hydrazine/methanol method. Again in this case, NMR spectroscopy clearly indicated that the chirality at the AA-center was not affected (Figure 14), unlike in the case of the NaOMe-MeOH reaction which led to 1:1 mixtures of epimers, although this latter method was reported to allow O-acyl group deprotection of a disaccharidic hexapeptide.

Pseudodisaccharides

Oligosacharides play key roles in many biological processes. Among analogues, amide-linked saccharidic structures have attracted some interest. Sugar amino acids (SAAs) chemistry has provided compounds such as antiviral activity against HIV and inhibitors of sialyl Lewis x-dependant cell adhesion and other peptidomimetics which are interesting because of restricted conformational behavior. [START_REF] Suhara | [END_REF] New amide-linked pseudodisaccharides (53, 56, 59) have been synthesised by reaction of aminodeoxy sugars with α-CMGL (Figure 15). 50 Taking into account the length (four-atom) of the inter-glycosidic linkage, these compounds are equivalent to trisaccharide mimetics. Similar examples have been described in the literature, such as linear or cyclic oligosaccharides with a four-carbon rigid connection, and a competitive inhibitor of the hydrolysis of p-nitrophenyl α-maltotriose by porcine alpha-amylase having a six-atom acyclic spacer between two glucose residues. 59 Unlike the case of alcohols, the reaction with aminodeoxy sugars does not require the presence of base. This has also the advantage of limiting the formation of the undesired N-acetylation of the starting aminosugar arising from competitive intermolecular O-to-N acetyl exchange. Best conditions proved to be THF with a slight excess of CMGL or DMF for some aminosugars with unprotected OH groups.

Deprotection of the obtained amide-linked pseudodisaccharide 58 was performed using Zemplén conditions or, in the case of compounds 52 and 55 having isopropylidene and acetyl groups, using a 0.5 M HCl solution at 50 °C (proving the satisfactory stability of the α-carboxymethyl linkage under acidic conditions).

The amide-linked sugar/nucleoside adduct 61 which could mimic the glucosyltransferases substrate (UDP-Glc) was obtained by the same strategy (Figure 16) from 5'-deoxy-5'-azidouridine (60). 60

NH

Glycosylated porphyrins

Porphyrins bearing glycosylated groups have potential interest as photosensitizers for cancer photochemotherapy. 61 The sugar moieties have been shown to modulate the amphiphilicity of the photosensitizers and specific membrane interactions, 62 and in some cases to increase their plasmatic life time. 63 The presence of carbohydrates on porphyrins is also known to allow cancer cell surface targeting throught specific binding to membrane receptors. 64 Use of synthetic carbohydrates as carriers in directed drug delivery could thus be an interesting approach in cancer cell targeting.

New glucosylated porphyrins 62 and 63, having an α-D linkage, have thus been prepared by reaction of CMGL (Figure 17) with aminopropylated monohydroxyphenyltritolylporphyrins. 51,[START_REF] Little | Starting porphyrins were prepared using the procedure reported by[END_REF] The decay of fluorescence observed for these porphyrins in H 2 O/THF (8/2) compared to THF suggests the formation of aggregates. 66 The in vitro photocytotoxicity of the new porphyrins was evaluated using a K562 chronic leukaemia cell line and compared to that of Photofrin ® . The ortho porphyrin 62 is much more active than para porphyrin 63, although less active than Photofrin ® at the same ponderal concentration. It induces probably early necrotic death more than secondary necrosis that could be attributed to the induction of apoptosis. However, the photoactivity of the glycosylated porphyrin was clearly improved compared to the non glycosylated one.

Neoglycolipids

Glycolipids have important properties in terms of biological or physicochemical viewpoints, these two aspects being sometimes correlated. For example, glycolipids are involved in complex mechanisms that are thought to involve membrane sub-domains, containing liquid-ordered phases, termed lipid rafts. 67 Being interested in the synthesis and study of the surface activity and thermotropic liquid-crystalline behaviour of a variety of synthetic glycolipids, notably arising from simple and available sugars, 1a,68 we also developed the CMGL strategy towards such materials. 49,52 A series of aliphatic amines (C 6 , C 8 , C 10 , C 12 , C 14 , C 16 ) were condensed with CMGL in THF. The resulting amides 64 were deacetylated in methanol using catalytic amounts of sodium methanolate in good yields (Figure 18). Some symmetrical diamines were also prepared and new bolaform carbohydrate based amides 65 were thus obtained. In terms of thermotropic properties, it was shown that the minimum chain length is of 10 carbon atoms, the C 10 , C 12 , C 14 , and C 16 exhibiting all lamellar phases. The stability of the amide linkage, as well as that of the α-glucosidic bond, allowed full cycles of heating up and down for the DSC analysis experiments, unlike other glycolipids such as sucrose esters or hydroxyalkylethers we had previously studied. 5, 7, 9, 11, 13, 15 Figure 18 Four different amino steroids were also coupled onto CMGL. The 3-aminocholesterols (α and β) were obtained by reduction of 3-azido-5-cholestene prepared itself by reaction of HN 3 with cholesterol or epicholesterol under Mitsunobu conditions. The saturated equivalent systems were prepared from cholestanol and involving the same Mitsunobu reaction conditions to establish the stereochemistry at C-3 of the steroid followed by reduction of the azido group (Figure 19). 69 Reaction with CMGL was performed in anhydrous THF leading to the steroidal amides in very good yields. The typical proton NMR patterns for the CMG conjugates were observed, notably H-2, H-3 and H-4 at 3.82, 5.28 and 5.04 ppm respectively. Final amphiphilic glycosteroids 66-69 were obtained by deprotection of acetyl groups performed under Zemplén conditions.

Only the saturated steroid amides 66 and 67 exhibited some liquid crystalline behaviour, in a very limited temperature range (181-230 dec and 202-230dec). This is often the case for steroidal glycolipids having direct connection or short spacer (1-4 atoms) between the carbohydrate moiety and the steroid backbone. 70 Furthermore, the relatively high melting points of these compounds increase their tendency to decompose at elevated temperatures through caramelization. The 3-β-cholestane amide 67 exhibits a wider temperature range of the liquid crystal phase with a lower melting point compared to the α one. The unsaturated analogues 68 and 69, less flexible, did not exhibit any liquid crystalline phase as they decompose at lower temperatures.

Miscellaneous

Other various other amines were used in the CMGL opening reaction. Notably, some amines with polymerisable residues provide amides such as the methacrylate 70 or allyl and propargyl amides 71 and 72 (Figure 20). 49 Also, some multivalent species were obtained such as the triamide 73 71 and an octa amide on a PAMAM backbone (74) was identified by mass spectrometry. Likewise, a resin bearing free amino functions was shown to react with CMGL and could be used for example to eliminate excess CMGL from the reaction mixture providing 75 which could be easily removed by filtration. Finally the reaction with monoamino-βcyclodextrin led to the corresponding amide 76. 72

Reactions of carboxymethyl glucoside lactone with vinyl magnesium bromide

A short exploration of the reaction with vinyl magnesium bromide proved that CMGL can also be used for carbon-carbon bond connections with the carbohydrate moiety. However, yields remained rather low because of the lack of stability of the ester groups, even when the more stable tripivaloylated lactone was used. Actually, the best was to directly remove all esters in order to simplify the mixture of products, allowing the identification of the bis-vinyl alcohol 77 (Figure 21). 73

O PivO

PivO HO OPiv O HO 77 Figure 21

Structural variations on CMG-adducts

A major interest of the CMGL approach is to provide an isolated OH group at position 2 after the opening step (Figure 22). The selective further transformation of this function can therefore provide 1,2disubstituted compounds in a very straightforward manner. An example is the reaction of amide 78 with dihydropyran under acidic catalysis which yield the acetal 79, from which the acetyl groups could be exchanged to benzyl ethers to yield 80. 74 Another example is the oxidation of the ester 32 under Swern conditions which give the non-isolable corresponding 2-keto derivative, which undergo immediate elimination of one acetic acid molecule leading to the enone 81. 75 Further studies taking advantage of this selective functionalisation are currently ongoing in our laboratory. the very available isomaltulose. The opening of CMGL by nucleophilic species, which occurs under very mild conditions, provides glucosyloxyacetylated compounds which can be seen as analogs of glucoconjugates. In particular, a wide variety of amides have been obtained with examples among pseudo disaccharides, gluco-aminoacids and neoglucolipids. More work is currently in progress in our laboratory to extend the scope of this strategy toward conjugates constructed on other carbohydrates, mono-or oligosaccharides. With this aim, new ways to prepare similar lactones are studied and the results will be reported soon.

  Figure 1

Figure 2

 2 Figure 2

43

 43

  Figure 5

  Figure 7

26

 26

  Figure 8

  Figure 12

Figure 14 .

 14 Figure 14. NMR evidence for the configurational integrity at the AA centre as observed at the CH 2 -AB system of CMG-(L or D)-ASP: (left) deprotection with AcCl-MeOH of 45 and of its (D)-ASP-analog (right); mixture obtained by deprotection of 45with MeONa-MeOH.

  Figure15

  Figure16

  Figure 17

  n = 7 : 74 iso n = 9 : 81 smectic A* 94 iso n = 11 : 89 smectic A* 122 iso n = 13 : 92 smectic A* 178 iso n = 15 : 100 smectic A* 192 iso n

  Figure 19

  Figure 20

Figure 22 6.

 22 Figure 22

Table 1 .

 1 Reaction of CMGL 3 with alcohols.

		OAc								OAc					OAc
	AcO	O							AcO	O					AcO	O
	AcO 30: R=CH 3 HO O 31: R=C 2 H 5	O	O	R			AcO	HO 33 O	O	O			AcO	HO O	O 34	O	O	O
	AcO AcO 32: R=C 3 H 7 O AcO OAc	O HO OAc O	O 35	O	C 10 H 21 OH	AcO AcO	OAc O HO O 36	O	O	AcO AcO	OAc O HO O	O 37	O
	AcO	HO	O	O O	O							OAc
							O			AcO		O
		38		O			O	O			AcO		HO	O	O	O	39
											Figure 11

Table 2 .

 2 Reaction of chloroacetyl lactone 40 with alcohols.

	Alcohol	Catalyst	Equvalents Reaction	Temperature Obtained	Yields
				time (h)		adducts	(%)
	EtOH	DMAP	2	5	RT	41	60
	Propan-1-ol	DMAP	5	5	RT	42	40
	Allyl alcohol	DMAP	5	3	RT	43	57

Acknowledgments References

Acknowledgments

Financial support from CNRS, MENSER and TEREOS is gratefully acknowledged. We thank Cargill SA for a gift of isomaltulose. AL and RC thank CNRS and Région Rhône-Alpes for grants.


R Thamankar

T L Yap

K E J Goh

C Troadec

C Joachim

Low Temperature Nanoscale Electronic Transport on the MoS 2 surface

Two-probe electronic transport measurements on a Molybdenum Disulphide (MoS 2 ) surface were performed at low temperature (30K) under ultra-high vacuum conditions. Two scanning tunneling microscope tips were precisely positioned in tunneling contact to measure the surface current-voltage characteristics. The separation between the tips is controllably varied and measured using a high resolution scanning electron microscope. The MoS 2 surface shows a surface electronic gap (E S ) of 1.4eV measured at a probe separation of 50nm. Furthermore, the two-probe resistance measured outside the electronic gap shows 2D-like behavior with the twoprobe separation.

Two dimensional (2D) atomic crystals have shown promising electronic properties which can be used for future mesoscopic electronic devices 1,2,3,4,5 . Monolayers of transition metal dichalcogenides (MX 2 where M = Metal, X = S, Se and Te) are currently in the spotlight because they show interesting 2D electronic properties and atomically clean surfaces can be obtained by the mechanical exfoliation technique. A model system in this dichalcogenide family is molybdenum disulphide (MoS 2 ). It has an indirect bulk electronic band gap E B ~ 1.3eV and an indirect -direct band gap transformation occurs when MoS 2 is thinned down to its monolayer limit. In this case, calculations suggest that E B increases from its bulk value of 1.3eV to 1.9eV for a single MoS 2 layer 6,7,8,[START_REF] Splendiani | Nanoletters[END_REF]10 . Various studies have been reported using few monolayers of MoS 2 as active material in devices, such as field effect transistors 11,12,13 sensors 14 , phototransistors 15 and integrated circuits based on bilayer MoS 2 transistors 6 .

The MoS 2 surface holds promise for the construction of surface atomic scale circuits. By means of a scanning tunneling microscope (STM), sulphur atoms can be extracted one by one from the MoS 2 surface 16 . The missing surface atom then creates a localized electronic state in the surface electronic gap 17,18 . By systematically removing the sulphur atoms along the surface, it was calculated that long atomic wires can be formed by sulphur atom vacancies and calculations show that this will introduce a surface conduction band located in the MoS 2 surface electronic gap 19,20 . This approach has the advantage of working with a stable atomic wire at room temperature as compared to the recently reported dangling bond atomic wires constructed on passivated semiconductor surfaces 21 . In the prospect of using MoS 2 surface atomic wires to construct atomic scale surface boolean logic circuits 22 or to connect a single molecule in a planar configuration 23 , we demonstrate the measurement of transport properties on MoS 2 surface at a nanoscale regime comparable to the surface dimension of those atomic scale electronic circuits under ultrahigh vacuum (UHV) conditions.

In this letter, electronic properties of the MoS 2 surface measured using STM two-probe current-voltage (I-V) spectroscopy is demonstrated. The measurements were done on a UHV-Nanoprobe (Omicron Nanotechnology) which consists of four STM tips and a high resolution scanning electron microscope (SEM) 24 . Each STM tip can perform constant current imaging and I-V spectroscopy independently. The in-situ high resolution SEM (resolution~3.4nm) is operated at a large working distance (13mm to 15mm) to allow free and independent navigation of all the STM tips on the sample surface. We use chemically etched tungsten tips with an average apex diameter ranging between 30nm to 50nm as measured by SEM. All the measurements presented below were performed in UHV and at 30K which is the lowest temperature achievable on this instrument. Ex-situ room temperature Hall effect measurements show that our MoS 2 sample is natively p-doped (10 16 atoms/cm 3 ). No further heat treatment or cleaning procedure is applied to the surface before making the measurements. In conventional transport measurements involving devices using micro (or nano) lithography techniques to fabricate the surface contact pads, contamination induced by the resists or surface modifications induced by e-beam lithography cannot be neglected 25 . In order to avoid all these issues, we measure the transport characteristics in UHV by using individual STM tips as electrodes in top contact configuration. Furthermore, by using the high resolution SEM attached to the chamber (operated at low current), we can continuously vary the tip-to-tip distance down to a few tens of nanometers (50nm in the present case). The UHV cleanliness highlighted here is a pre-requisite for successful atom-by-atom extraction to construct the atomic wires and this is not typically catered for in standard nanolithography techniques 25 .

In order to measure the electronic energy gap of the bulk MoS 2 , experiments were done with a single tip in tunneling contact mode using an individual feedback loop. This is an essential step to identify possible bulk leakage current under the top MoS 2 layer. For this measurement, we intentionally maintained a tunneling barrier between the tip and the MoS 2 surface to form a nanoscale controllable metal-insulator-semiconductor (MIS) junction at each tip location. This tunneling barrier includes the tip-to-MoS 2 vacuum tunneling barrier and the remaining oxide (if any) on the tip apex. The length (L) of this tunnel barrier can be controlled by moving the tip away or closer to the surface with a precision better than 0.1nm at 30K. Notice that our objective is not to evaluate the maximum current flowing on the MoS 2 surface between the two tips, but to determine the bulk energy gap (E B ) and the surface electronic gap (E S ) in the nanoscale regime.

To tune a given tip to MoS 2 surface tunnel contact, its I-V characteristics were recorded at various tip-sample distances (L) using a single tip in the normal STM mode of operation. As presented in Fig. 1a, bulk MoS 2 presents an electronic energy gap (E B ) around 1eV for tunneling current set point between 10pA and 200pA. By increasing the tunneling set point current, the energy gap decreases from E B, 10pA = 1.2±0.1 eV to about E B, 200pA = 0.9±0.1 eV. This decrease is attributed to the change in the tip sample separation by more than 0.1nm. An interesting feature of the single tip I-V curves is the appearance of a shoulder in the range of 1.3eV to 1.8eV on the positive side of each I-V curves (see Fig. 1a). We attribute this to the Molybdenum (Mo) 4d and Sulphur(S) 3p states of the MoS 2 layer. Inverse photoemission spectroscopy on the MoS 2 surface showed similar features which are coming from the antibonding Mo 4d and S 3p states 26 .

Theoretical studies have also indicated that Mo d-S p hybridization dominates in the electronic properties of MoS 2 near the Fermi level 19,27 .

The I-V characteristics in Fig. 1a can be interpreted using a metal-insulatorsemiconductor (MIS) junction model that takes into account the thermionic emission effect, the tunneling contact and the tip induced band bending. We consider all these effects via an effective Schottky barrier height (ı ı ) of this MIS contact [START_REF] Sze | Physics of Semiconductor devices[END_REF] . For a bias voltage V between the tip and the MoS 2 sample, the current through this MIS junction is given by:

ı ı ı , ∅ ı ı = ı ı * ı ı expı -ı ı ı ı ı ı ı - ∅ ı ı ı ı ı ı ı ı ı ı ı ı ı ı × ı 1 -ı ı ı ı ı ı ı ı ı ı (1)
where

ı * = ı ı ı * ı ı ı ı ı
is the effective Richardson constant. Due to the chosen tunnel contact mode, an exp(-ı ı ) term was introduced in (1) where β is the tunneling inverse decay length and L the effective length of the tip-MoS 2 surface tunnel barrier. ı is the Boltzmann constant, ı is the temperature and ı the ideality factor. In an MIS junction, the ideality factor indicates the deviation from ideal Schottky barrier (i. e. ı = 1). The effective mass of the electron ı * = 0.48ı ı of the bulk MoS 2 is considered here 27 and ı is the effective tunnel contact area of the tip -MoS 2 surface junction.

It is well documented that band bending induced by a metallic tip arises in the case of semiconductors surfaces 30 . In the present case of well controlled MIS junction, we compare the dI/dV curves for two set points: 10pA and 200pA. The dI/dV curves obtained from the numerical differentiation of the I-V characteristics are presented in Fig. 1b. Here the shoulders appearing in the I-V curves due to Mo 4d -S 3p have not been taken into account since we focus only on the onset of the bands. At a setpoint of 10pA, a linear onset of both the valence (VB) and conduction bands (CB) can be observed. Figure 1b shows that when the STM tip is brought closer to the MoS 2 surface by increasing this set point current to 200pA, the top of the MoS 2 VB edge is affected more than the CB edge by the metal tip approach. Following the analysis of Feenstra et.al [START_REF] Feenstra | [END_REF]30 , for a tip radius of 30nm, with a contact potential of -1.0eV and a tip-sample separation L = 1nm, the valence band bending in MoS 2 is rather large and can extend up to the tip Fermi level. Furthermore, using (1) and according to Fig. 1a, we estimate the value of the barrier height to be ı ı = 0.51eV which is comparable to the extracted values for various metals in contact with MoS 2 in transistor configurations 31 .

Two-tip surface I-V measurements were done by electrically isolating the back of the MoS 2 sample from the ground instead using the virtual ground of the two STM I-V convertors.

After every two-probe I-V measurements, the sample ground was automatically reconnected before the feedback loop of each STM tip was reactivated so as to control each tip surface distance. The lateral tip-to-tip separations were determined using the in-situ UHV-SEM of the Nanoprobe as presented in Fig. 2. Here, the SEM image shows two tips with an apex diameter of ~ 50nm (see Fig. 2c). For all the two-probe I-V measurements, Tip-2 was kept at a fixed position on the surface of the MoS 2 . Tip-1 was controllably moved keeping its feedback loop active and monitoring its position using the UHV-SEM. In this configuration, current flows from Tip-1 to Tip-2 in a floating ground mode of operation giving rise to I-V characteristics of the MoS 2 surface. Figure 2a shows a series of such I-V measurements using two tips separated by a distance of ~ 400nm. For this large separation, while keeping Tip-1 at 10pA feedback loop setpoint current, the surface electronic gap E S is 1.3eV as confirmed by its dI/dV curve presented in Fig. 2b. As the Tip-1 set point current was increased from 10pA to 250pA, Tip-1 approaches to the MoS 2 surface (inset Fig. 2a.) and this is reflected by the I-V curve changing drastically to shows a smaller surface electronic gap. The dI/dV curve at 250pA setpoint current show that the surface electronic gap is E S = 0.5eV. This reduction of the surface electronic gap is attributed to the band bending resulting from the approach of the metallic Tip-1 to the surface. As in the case of single tip measurements (Fig. 1), the surface valence band edge is affected more than the conduction band edge.

As already mentioned, each tip-MoS 2 junction can be modeled by a MIS junction. In the case of a tungsten tip-MoS 2 junction and for a doping level of 10 16 atoms/cm 3 , the surface depletion regions start to overlap for tip-to-tip separation around d ~ 150nm [START_REF] Sze | Physics of Semiconductor devices[END_REF] . For larger distance d, as presented in Fig. 2c and2d, the lateral tip induced surface band bending regions do not overlap. For tip separations smaller than the depletion width, an almost flat band situation occurs at both the CB and VB as schematically depicted in Fig. 2f. This leads to an almost symmetric two tip I-V curves. This effect was measured with a tip-to-tip separation of 50nm as presented in Fig. 2e where an abrupt increase of the tunneling current intensity is observed in both forward and reverse bias voltages. In this case, the measured MoS 2 surface electronic gap is now controlled by the overlap of the two depletion regions. This leads to an apparent electronic gap E S = 1.4eV which is larger than the bulk energy gap E B .

The measured two-tip I-V curves can be modeled by a linear combination of two back to back MIS contacts (Fig. 2d). Following (1), the resulting tunneling current intensity can now be written as:

ı ı ı , ∅ ı ı , ∅ ı ı ı = ı ı ı ı expı -ı ı ı exp ı - ∅ ı ı ı ı ı exp ı - ı ı ı ı ı ı ı ı 1 -exp ı ı ı ı ı ı ı + ı ı ı ı exp(-ı ı ) exp(- ∅ ı ı ı ı ) exp( ı ı ı ı ı ı ) {1 -exp( ı ı ı ı ) } (2)
Here we consider ∅ ı ı ~ (∅ ı +Δ1) and ∅ ı ı ~ (∅ ı +Δ2) as the effective Schottky barriers at the Tip1-MoS 2 and Tip2-MoS 2 junctions respectively where Δ1 and Δ2 are the local change in the barrier height at each junction due to the different contacts at the two tips on the surface. We consider ı ı and ı ı as the equivalent ideality factors, which contains the contribution from the non-ideal Schottky contact. This can also account for the small correction Δ1 and Δ2 for the smaller tip separation (~50 nm), the measured surface electronic gap E S =1.4eV. At large tip separations, the MoS 2 surface gap is impacted more by the tip-induced band bending, whereas at a small tip separation of 50nm, the tip induced band bending is minimized by the significant overlap of the surface depletion regions under the two tips. For bias voltages outside the surface electronic gap, the variation of surface resistance as a function of the tip-to-tip separation is in agreement with a 2D surface conduction mechanism. Our measurements clearly demonstrate that existence of a large surface electronic energy gap on MoS 2 even if nanoscale surface metallic contacts were present. This opens the way to characterize the electronic properties of surface atomic scale wires and circuits constructed on the MoS 2 surface.

Figure ( 1 )

 1 Figure (1): (a) Single tip I-V curves of p-MoS 2 at various tip-sample distances (corresponding to

Figure ( 2 )

 2 Figure (2): Surface electronic transport measurements on MoS 2 surface at 30K. (a) Two probe I-

Figure ( 3 )

 3 Figure (3): Dependence of the series resistance (R 2P ) with probe separation measured using two

Acknowledgements: Authors acknowledge financial support under the A*STAR's VIP Atom Technology project 1021100972 and EU ATMOL project contract number 270028 financial support. T. L. Yap thanks National University of Singapore, GLOBAL FOUNDRIES Singapore for the financial support during Ph.D. programme. We thank M. Bossman, for critical proofreading.

individual effective Schottky barrier relative to the already measured ∅ ı . As shown in Fig. 2e, the I-V characteristics can be very well fitted using (2). By fitting the ln(I) vs V curve, we obtain large values (~ 2.2) for the equivalent ideality factors (ı ı and ı ı ). This large value clearly shows that our tunneling contacts are far from being ideal Schottky barriers. Ideality factors greater than ı = 1.5 at room temperature and increasing at low temperatures have been measured for MIS junctions using molecular insulators 32,33 . In this case, the departure from ı = 1 is attributed to recombination effects at the interface 34,35 . Detailed understanding of non-ideality requires further investigation of such nanoscale MIS junctions which is beyond the scope of this Letter.

For bias voltages outside the electronic gap, the variation of the two-tip junction resistance (R 2P ) is plotted in Fig. 3 as a function of the tip separation (d) up to a very large distance d=2μm. As presented in Fig. 3, R 2P can be well fitted using a 2D-like surface resistance variation 36 :

Here R C is the effective contact resistance which includes the two MIS junctions and the surface resistance of the MoS 2 between the two tips. ı is the tip apex radius of curvature and 'd' is the separation between the tips on the surface. Assuming that the Tip-1 and Tip-2 apex radii are the same in our experiments i.e. ı = 30nm, one finds R C = 1.45GW for the best fit using (3). At small d, the 2D character of the large voltage surface resistance demonstrates how the leakage current is located at the MoS 2 top surface limiting the bulk contribution due to MoS 2 lamellar structure in the transverse direction.

In conclusion, we have determined the surface electronic gap of MoS 2 surface at the nanoscale down to a separation of 50nm between the tips. At large tip separations, the MoS 2 surface presents a surface electronic gap E S =1.3eV very close to the bulk MoS 2 band gap. At a