Bejoy Thomas

Florence Babonneau

Thibaud Coradin

Inge N A Van Bogaert

Niki Baccile

One-step introduction of broad-band mesoporosity in silica particles using a stimuli-responsive bio-derived glycolipid Authors

Keywords: Porous silica, Sophorolipids, Anionic Mesoporous Silica (AMS), Aminopropyltriethoxysilane (APTES), Biosurfactants, Glycolipids

Stimuli-responsive glycolipid biosurfactants belonging to the family of acidic sophorolipids (SL) have been used to introduce a broad range of pore size in the mesoscale regime (2 nm -30 nm) in silica particles using a one-pot co-assembly sol-gel route in water. The pore size distribution is tailored by the sole interaction between an amino-modified silane, aminopropyltriethoxy silane (APTES) and SL. No additional compounds (e.g., block copolymers, polymers, organic solvents, pore-swelling agents) have been used to promote the formation of mesopores larger than 2 nm. Increase of the basicity of the medium has a strong impact on the glycolipid assembly, which is known to form large supramolecular assemblies at high pH. Materials morphology and porosity have been characterized by high resolution TEM, SEM and nitrogen physisorption while the interaction between the glycolipid and silica is demonstrated by FT-IR and solid state NMR.

Introduction

Porosity in materials has been deeply explored because of the large number of applications that are concerned, such as catalysis, sensing, drug delivery. In the specific field of sol-gel chemistry, this task is nowadays mastered by the well-known bottom-up templating process that can provide, among others, gels, powders, thin films and fibrous shaped materials. [START_REF] Qian | Application of mesoporous silicon dioxide and silicate in oral amorphous drug delivery systems[END_REF][START_REF] Lee | Multifunctional Mesoporous Silica Nanocomposite Nanoparticles for Theranostic Applications[END_REF][START_REF] Slowing | Mesoporous silica nanoparticles: structural design and applications[END_REF] Independently of the scale of porosity (micro < 2 nm, meso between 2 and 50 nm and macro, above 50 nm), recent research works have been focused on developing methods to achieve a three-dimensional, long-range, organisation of the pores of different sizes. Hierarchy in size and pore interconnection have been judged to be equally, if not more, important in terms of final application because of the much easier control that one can establish on the diffusion properties inside the pores. [START_REF] Sel | Principles of Hierarchical Meso-and Macropore Architectures by Liquid Crystalline and Polymer Colloid Templating[END_REF][START_REF] Fujita | Preparation of hierarchical porous silica and its optical property Micro[END_REF][START_REF] Chiu | Friedel-Crafts alkylation properties of aluminosilica SBA-15 meso/macroporous monoliths and mesoporous powders[END_REF] Typically, organic templates are currently used to obtain micro-or meso-scale porosity such as in zeolites (use of quaternary ammonium salts) or in MCM-41 or SBA-15 materials (use of surfactant micelles). [START_REF] Beck | A new family of mesoporous molecular sieves prepared with liquid crystal templates[END_REF][START_REF] Huo | Surfactant Control of Phases in the Synthesis of Mesoporous Silica-Based Materials[END_REF] Ordered macroporosity is easily achieved using micrometer-sized colloidal packing. The main idea behind each templating process is that the pore size is strongly related to the size of the template, if one does not consider any additional effect driven by thermal treatment or use of organic swelling agents. 4,9,10, Other typical porogens can be: colloids, emulsions, gases, vesicles, salts, water, polymers and a wide combination of these to eventually achieve a hierarchical porous network. [START_REF] Innocenzi | Hierarchical Mesoporous Films: From Self-Assembly to Porosity with Different Length Scales[END_REF]12 Removal of the template takes place with soxhlet washing, calcination or other techniques based on the stimuli-driven dissociation of coacervates. [START_REF] Baccile | Ecodesign of Ordered Mesoporous Materials Obtained with Switchable Micellar Assemblies Angew[END_REF][START_REF] Che | A novel anionic surfactant templating route for synthesizing mesoporous silica with unique structure[END_REF] If sustainability is taken as an important criterion in the synthesis process, the number of works is drastically reduced. A recent review paper nicely outlines all possible sustainable ways to obtain mesoporous oxides, and use of bio-sourced porogenic agents was shown to be an important issue with a limited amount of work. [START_REF] Gérardin | Ecodesign of ordered mesoporous silica materials[END_REF] In this context, we have recently investigated the self-assembly and structuring properties of sophorolipids (SL), a family of entirely bio-derived glycolipids. [START_REF] Baccile | Sophorolipids: a yeast-derived glycolipid as greener structure directing agents for self-assembled nanomaterials[END_REF][START_REF] Baccile | pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants[END_REF][START_REF] Baccile | Surface charge of acidic sophorolipid micelles: effect of base and time Soft Matter[END_REF] These natural compounds are obtained in large amounts (several hundred gram per liter) by yeast culture in presence of several carbon sources, like glucose, fatty acids but even alkanes and waxes. 19 Due to their biosynthesis with reduced environmental impact, confirmed by a recent life cycle analysis study, [START_REF] Develter | Surfactants from Renewable Resources[END_REF] sophorolipids (SL) have attracted a fair attention of the home and skin-care industry 21 even if other applications are being explored, as antimicrobial [START_REF] Shah | Sophorolipids Having Enhanced Antibacterial Activity[END_REF] and anticancer [START_REF] Fu | Sophorolipids and their derivatives are lethal against human pancreatic cancer cells[END_REF] agents, surface stabilizer for nanoparticles. 24 More specifically, their selfassembly properties are being explored with particular interest 16,19d,25,26 and in the context of drug release, an ongoing work in our group puts in relationship the silica templating properties of self-assembled sophorolipids and their eventual controlled release in solution. 27 Acidic SL, which contain a free COOH group, are also of particular interest because of the pH-dependent self-assembly behaviour. 16,19d In particular, the size of the supramolecular aggregates strongly increases from acidic to basic pH. We have also used them as structuredirecting agents in the formation of silica thin films using the evaporation induced selfassembly (EISA) process. We could then show the formation of silica thin films exhibiting a wormlike organization of the pore but, more importantly, their size strongly increases with pH. [START_REF] Baccile | Sophorolipids: a yeast-derived glycolipid as greener structure directing agents for self-assembled nanomaterials[END_REF] In the present work, we discuss the templating and porogenic effect of SL in the synthesis of silica powders obtained by the precipitation technique in water. The process of self-assembly in solution is driven by the organic/inorganic interactions between the surfactant and the silicate species. This effect is largely observed in the synthesis of MCM and SBA-type materials, where direct (MCM-41), indirect (SBA-3) electrostatic or hydrogen bond (SBA-15) interactions drive the precipitation of mesostructured silica powders with long-range order. [START_REF] Baccile | Solid-state NMR characterization of the surfactant-silica interface in templated silicas : acidic versus basic conditions[END_REF] Sophorolipids in water practically behave as non-ionic compounds and they can be associated to the most common polyethylene oxide based surfactants, precursors of SBA-15 materials. However, when sophorolipids are used as structure directing agents under the typical conditions (acidic water solution) required to form these materials, a gel is systematically obtained. This is a strong hint that the interactions between sophorose, the main hydrophilic moiety of SL, and silica oligomers in solution are not strong enough to trigger silica condensation. Nevertheless, SL contain a COOH group, the role of which in their self-assembly has been described previously, [START_REF] Baccile | pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants[END_REF] making them effectively anionic amphiphiles. It is then possible to employ the structuring strategies commonly used for the synthesis of Anionic surfactant templated Mesoporous Silicas (AMSs), based on charge matching between anionic surfactants with an amino-modified silane, (e.g., APTES), [START_REF] Che | A novel anionic surfactant templating route for synthesizing mesoporous silica with unique structure[END_REF][START_REF] Han | Anionic surfactant templated mesoporous silicas (AMSs)[END_REF][START_REF] Atluri | Nonsurfactant Supramolecular Synthesis of Ordered Mesoporous Silica[END_REF] a co-reactant in the formation of a hybrid organic-inorganic silica scaffold.

We are able here to go one step further because the NH2/SL ratio also influences the nature of the pH-sensitive sophorolipids assemblies, [START_REF] Baccile | pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants[END_REF][START_REF] Baccile | Surface charge of acidic sophorolipid micelles: effect of base and time Soft Matter[END_REF] having a strong impact on the final mesoporous texture. The final pore size distribution varies between 3 and 30 nm, suggesting the formation of SL large aggregates. Interestingly, in this specific case, a biomodal pore size distribution can be achieved, with larger pores being accessible without removal of the template. Our results demonstrate that it is possible to use a single type of bioderived surfactant to obtain comparable porosity to what can be obtained by mixing two different surfactants, such as CTAB and a polyethylene oxide-based block copolymer, [START_REF] Sel | Principles of Hierarchical Meso-and Macropore Architectures by Liquid Crystalline and Polymer Colloid Templating[END_REF] thus showing that the sustainability principles in the synthesis of porous oxides [START_REF] Gérardin | Ecodesign of ordered mesoporous silica materials[END_REF] can be successfully put to practice.

Experimental

All products (excluding sophorolipids) used in this work were purchased from Aldrich. Raw sophorolipids were produced by Starmerella bombicola (ATCC 22214). They have been eventually hydrolyzed to obtain only acidic sophorolipids (SL), as described elsewhere. [START_REF] Baccile | Sophorolipids: a yeast-derived glycolipid as greener structure directing agents for self-assembled nanomaterials[END_REF][START_REF] Baccile | pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants[END_REF] Silica particles synthesis procedures have been adapted from ref. 30. Samples SL1 and SL2: a specific amount of SL (0.390 g for SL1 and 0.456 g for SL2) was dissolved in 20 g of water of MilliQ purity under stirring at room temperature overnight. The system is then brought to 60°C under stirring and under these conditions, 0.484 g of (3-aminopropyl)triethoxysilane (APTES) were introduced, immediately followed by 1.004 g of tetraethyl orthosilicate (TEOS); this was kept under static conditions for 12 h at 60°C. The batch was eventually treated under static hydrothermal conditions for 6h at 100°C. The powder was collected by filtration, washed with distilled water and air-dried. For sample SL3, a similar protocol was followed except that 1.351 g of SL were introduced in 45 g of water of MilliQ purity and a larger amount of TEOS (1.808 g) was used, the amount of APTES being constant. The SLfree sample was obtained using the same synthesis procedure as for SL1 and SL2, but no SL was added. Relative molar concentrations are summarized in Table 1.

A two-step calcination procedure was used for organics removal. The sample is initially kept at 300°C for 2h before the final treatment at 500°C for 5h under air. Heating rate is 2°C/min. Adsorption/desorption isotherms were recorded on a Belsorp-Max Instrument (BEL-Japan Inc.) using nitrogen as probe gas. t-plot analysis was performed on the basis of an empirical mathematical expression based on adsorption measurements obtained on model materials, as detailed in the supplementary information material in section S1.0 and in corresponding Figure S1 and Figure S2. T) spectrometer using a 3.2 mm rotor spinning at 22 kHz, where recycle delay is 3 s and tc= 0.5 ms, which was chosen to explore short-range carbon-proton proximities. Two-pulse phase-modulated (TPPM) proton decoupling was applied during acquisition while quadrature detection in t1-dimension was realized using the STATES method. Cross-polarization transfers have been optimized under adiabatic tangential ramps. [START_REF] Hediger | NMR cross polarization by adiabatic passage through the Hartmann-Hahn condition (APHH)[END_REF][START_REF] Hediger | Adiabatic passage Hartmann-Hahn cross polarization in NMR under magic angle sample spinning[END_REF]

Results and discussion

Sophorolipid-containing porous silica powders can be readily synthesized in the presence of APTES, the role of which is of being a promoter of the interaction between the amino group and the carboxylate in SL, thus driving the precipitation phenomenon. Described below is the specific analysis of these APTES-containing silica materials obtained at different NH2/SL ratio.

Electron microscopy studies

In Figure 1 and the material is mainly constituted by amorphous micrometer-scale blocks. Figure 1d shows a magnified image of sample SL2 obtained using a high resolution Field Emission Gun (FEG) SEM microscope. This image shows a developed roughness and a nanoscale porosity, which characterizes the entire outer surface of the particle. This result is confirmed by TEM imaging of SL1 and SL2 samples, shown in Figure 2 a-d as it shows the existence of an interconnected and apparently accessible porosity over the whole particle volume. A closer look at SL1 sample (Figure 2c) evidences the presence of 10-20 nm sized interconnected particles constituting the larger spheres. When SL is not added to the precursor solution, the obtained powdery material is constituted by sub-micrometer sized amorphous particulate that does not display any porosity. This is in agreement with previous observations of APTEScatalyzed silica particles 34 and indicates that the here-obtained morphologies cannot be attributed to APTES only. It is important to note that small angle X-ray diffraction (not shown here), typically employed to put in evidence a long-range mesoscopic order, did not provide any indication for any special organization of the porous network, as one would expect for typical surfactant-derived silica solids and as previously found for EISA-derived sophorolipid-based silica thin films. 16

Specific surface area and porosimetry analysis

Despite the lack of long-range organization of the pores, the most interesting feature to put in evidence is the apparent porosity of the materials in the absence of a calcination step.

N2 adsorption/desorption isotherms presented for the as-synthesized (Figure 3a) and calcined (Figure 3c) SL1, SL2, SL3 and SL-free samples supports this assumption in a direct way.

First of all, the isotherm related to the SL-free sample indicates the absence of pores larger than 2 nm before and after thermal treatment, which is consistent with the TEM image (Figure 2 e,f), and typical of a dense material. However, microporosity is detected after calcination (sharp nitrogen uptake at P/P0< 0.01). As for sample SL3, the isotherms recorded for the assynthesized and calcined samples reflect the typical behaviour of an organically-templated mesoporous network, where porosity is not accessible before template removal whereas a well-defined type-IV isotherm with a H2 hysteresis loop is observed upon calcination. This is characteristic for a complex network of interconnected pores of different shapes and sizes. [START_REF] Rouquerol | Adsorption by Powders & Porous Solids: Principles, Methodology and Applications[END_REF] On the contrary, SL1 and SL2 samples show a well-defined type IV isotherm with a H3 hysteresis loop before any template removal (Figure 3a). Upon calcination, small-scale porosity becomes accessible, as demonstrated by the increase in the adsorbed volume at low P/P0 < 0.1. In parallel, the shape of the isotherm at higher relative pressure (P/P0> 0.5) is almost not affected by the calcination step for both samples. A closer look at the N2 isotherms and corresponding BJH plots for SL1 and SL2 are provided in Figure S3, where the isotherm for the as-synthesized SL2 sample has been multiplied by a factor 2 for convenience. The change in the isotherm shape, and consequently of the adsorption behaviour, is mild at high relative pressure. For SL1 (Figure S3a), the shape of the hysteresis loop is very similar before and after calcination with two desorption steps at P/P0= 0.44 and 0.85. After calcination, desorption above P/P0> 0.7 is practically unchanged while the desorbed volume at P/P0= 0.44 has slightly increased. The corresponding BJH pore size distribution in Figure S3c shows that the overall shape of the distribution does not change before and after calcination (especially the band at dp= 20.7 nm) but it actually becomes more resolved (peak at dp= 12.7 nm) with overall larger dVp/dlogdp values. Since no variation in size is detected before and after calcination, these data could suggest that a fraction of the pores is filled with organic matter before calcination. For SL2, the nitrogen isotherms are slightly different before and after calcination (Figure S3b), but under both conditions one can still observe a clear hysteresis loop. As for SL1, the upper part (P/P0> 0.7) of the isotherm is less influenced by the calcination step, as seen in the BJH pore size distribution (Figure S3d), in which the position of peak at dp= 11.1 nm is not affected. Smaller pores are, however, emptied after calcination and the shape of the adsorption branch indicates a large dispersion in size, discussed in more detail later.

These data mainly indicate that calcination does not affect the size of pores larger than 8 nm but it only affects their overall amount and, mainly for sample SL3, their connectivity. Table 2 gathers the BET specific surface area, SBET, values for all samples, before and after calcination. As expected, the as-synthesized SL1 and SL2 samples have non-zero, though small, SBET values of 23 and 42 m 2 .g -1 respectively, that increase to 136 and 367 m 2 .g -1 after calcination, while SBET for calcined SL3 is close to 500 m 2 .g -1 . The SL-free sample also displays a fair surface area (SBET= 240 m 2 /g) mainly due to microporosity. The effect of calcination is well-known as it contributes to remove the organic matter and gives access to the porous network. If one considers that the size of a sophorolipid molecule is not more than 2.5 nm and the corresponding micellar diameter not more than 5 nm, [START_REF] Baccile | Surface charge of acidic sophorolipid micelles: effect of base and time Soft Matter[END_REF] one also expects to measure porous properties close to those found in standard-pore MCM-41 materials. In reality, SL1 can be rather compared to either block-copolymer templated silica SBA-15 [START_REF] Zhao | Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures[END_REF] or large-pore CTAB-derived MCM-41, obtained after 3 days of hydrothermal treatment. [START_REF] Sayari | Characterization of Large-Pore MCM-41 Molecular Sieves Obtained via Hydrothermal Restructuring[END_REF][START_REF] Kruk | Application of Large Pore MCM-41 Molecular Sieves To Improve Pore Size Analysis Using Nitrogen Adsorption Measurements[END_REF] On the contrary, the nitrogen adsorption features of SL2 and SL3 are closer to that of a variety of other materials obtained under different conditions: hydrothermally treated SBA-15 (pore size of 8.9 nm) [START_REF] Zhao | Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures[END_REF] , monoliths obtained by a mixture of large block copolymer F127 and furfuryl alcohol, [START_REF] Drisko | One-Pot Synthesis of Hierarchically Structured Ceramic Monoliths with Adjustable Porosity[END_REF] silica-based materials templated using a mixture of two surfactants, CTAB and block copolymer Pluronic F127. 4 t-plot, for which standards of similar surface chemistry to our samples have been used

(refer to discussion in Section S1.0), is employed here to determine the presence of microporosity. The analysis of the t-plot curve (section S1.0.3 in the Supplementary Information) for both as-synthesized and calcined samples (Figure S2a -Figure S2c) shows a systematic upward deviation of the t-curve in the mid-t region, except for the calcined SL1 sample. This behaviour is very specific of a monolayer-multilayer adsorption phenomenon within mesopores. [START_REF] Rouquerol | Adsorption by Powders & Porous Solids: Principles, Methodology and Applications[END_REF] It also indicates that microporosity, that would be indicated by a downward deviation and a consequent extrapolation of positive Vads value at t = 0, is most likely absent from all samples. The only exception is constituted by the SL1 sample, in which the t-plot is represented by a straight line with almost no deviation. This may suggest the presence of a complex micro/mesoporous network. For this reason, the strong increase in Vads observed at low P/P0 after calcination (Figure 3c) cannot be related to micropore-filling.

Specific surface area (Sm) can be directly estimated from the t-plot slopes in the low-pressure region and values are given in Table 2. Sm is in agreement with SBET, confirming that the tplot analysis, and in particular the choice of the model curve, is correct. Figure 3b and Figure 3d show the pore size distribution for as-synthesized and calcined samples, respectively. As expected from the isotherms presented above, assynthesized materials are characterized by a broad distribution of pores going from 7 to 30 nm. In particular, both SL1 and SL2 show two poorly-resolved, yet detectable, maxima at 14 and 21 nm for SL1 (for which a shoulder at 10 nm can also be observed) and at 9 and 11 nm for SL2. Upon calcination, very similar values are still recorded for the same samples, as shown in Table 2, but a smaller pore size of 3 nm can additionally be seen for SL2. The presence of such small mesopores for SL2 is coherent with the conclusion drawn from t-plot analysis, where the presence of small mesopores instead of micropores was proposed. Finally, the pore size distribution of calcined SL3 shows a peak centered at 4 nm with a very sharp distribution and a broad shoulder at about 7 nm.

We have also evaluated the fractal dimension of the porous structure, [START_REF] Rouquerol | Adsorption by Powders & Porous Solids: Principles, Methodology and Applications[END_REF] as shown in Figure S2b and Figure S2d for as-synthesized and calcined samples respectively. It is known that plotting the log(Vnorm), where Vnorm is the normalized specific adsorbed volume, versus log(log(P0/P)), where P0/P is the inverse partial pressure, one can fit the linear section of the curve in the mid P0/P region and correlate the slope to the fractal dimension (FD). FD values are comprised between 2 and 3, where 2 corresponds to a smooth surface and 3 indicates a non-regular surface, which characterizes well mid-range mesopores. For large mesopores, the explored fractal dimension belongs to the void volume that is accessible after multilayer adsorption of nitrogen. For instance FD for aerosol silica spheres has a value of 2 while FD for silica gels and aerogels is close to 3. FD for calcined MCM and SBA materials is reported to be between 2.5 and 2.7. [START_REF] Prouzet | Roughness of mesoporous silica surfaces deduced from adsorption measurements Microp[END_REF] Here, for as-synthesized SL1 and SL2, FD is about 2.4, while it increases to about 2.7 after calcination, indicating that internal surface roughness is already present before calcination and therefore confirming the high-resolution SEM and TEM images.

Sophorolipid/silica interactions

Considering that SL1 and SL2 show significant porosity before calcination, one can wonder about the presence of sophorolipids in the materials. Figure 4 gathers a series of complementary analytical tools used to identify and quantify sophorolipids for each sample.

TGA (Figure 4a) indicates that the total organic content is no more than 10 wt% for SL1, and increases up to 17 wt% and 24 wt% for SL2 and SL3, respectively. From elemental analysis (N, C) the NH2/SL molar ratio in the final material is calculated, increasing from ca. 2.5 to a maximum of ca. 9.5 from SL3 to SL1. Quantitative [START_REF] Han | Anionic surfactant templated mesoporous silicas (AMSs)[END_REF] Si single pulse solid state NMR spectra shown in Figure S4 allows the calculation of the T/Q ratio, where T species include all RSiOx(OH)3-x units (R= aminopropyl) and Q corresponds to SiOx(OH)4-x species. If all initially added aminopropyl groups were present in the final material, T/Q value should be 0.45. In the absence of sophorolipids (SL-free sample), T/Q = 0.34 while T/Q = 0.30 for both SL1 and SL2. This shows that the incorporation of APTES precursors in the silica network is not complete, whether SL are present or not, in agreement with previous reports, where similar T/Q values are found. [START_REF] Ottenbrite | Self-Catalyzed Synthesis of Organo-Silica Nanoparticles[END_REF] Finally, TGA, elemental analysis and [START_REF] Han | Anionic surfactant templated mesoporous silicas (AMSs)[END_REF] Si solid state NMR data indicate that the highest amount of SL is found in SL3, the NH2/SL ratio being the lowest among the three samples and close to 2.

It is now possible to attempt to correlate silica morphology and porous structure with the NH2/SL ratio. We recently have deeply studied the effect of basic pH on the self-assembly of sophorolipids and, in particular, specifically observed the effect of both strong (NaOH, pKb= 0.2) and weak (NH3, pKb= 4.75) bases in solution. [START_REF] Baccile | pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants[END_REF][START_REF] Baccile | Surface charge of acidic sophorolipid micelles: effect of base and time Soft Matter[END_REF] In the presence of a large excess of NaOH with respect to sophorolipids (ionization degree of 1 of the COOH group in the SL), large aggregates are observed in solution with respect to simple micelles, that are more stable at acidic pH. [START_REF] Baccile | pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants[END_REF] When aqueous ammonia is used, larger amounts of the base with respect to sophorolipids are necessary to obtain similar results. In our previous work, [START_REF] Baccile | Surface charge of acidic sophorolipid micelles: effect of base and time Soft Matter[END_REF] the highest investigated NH3/SL ratio was 1.5 and, in that case, micelle formation was still favored over micron-scale aggregation. At higher ratios, micronic structures are generally formed instead of micelles. In this work, we use APTES, the pKb of which can reasonably be compared with the one of propylamine, pKb= 3.33, making APTES a base slightly stronger than ammonia (and of course weaker than NaOH). For these reasons, the effect of APTES on SL can reasonably be compared and discussed with respect to the NaOH and NH3 effects.

Accordingly, SL3 with the lowest NH2/SL ratio close to 2, behaves like a typical micelletemplated mesostructured material where pores are only accessible after surfactant removal with a narrow pore size distribution (after removal) centered at 4 nm is in agreement with the size of a sophorolipid micelle (about 4.5 nm). [START_REF] Baccile | Sophorolipids: a yeast-derived glycolipid as greener structure directing agents for self-assembled nanomaterials[END_REF][START_REF] Baccile | pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants[END_REF] In contrast, in SL2 and SL1, the NH2 content is much larger with respect to SL (molar ratios: NH2/SL ≈ 5.5 and 9.0, respectively) (Figure 4b). For this reason, a more complex interaction between the aminopropyl group and sophorolipids can be expected. The 13 C NMR spectrum of the reference APTES-SiO2, sophorolipid-free, powder is shown in Figure 4d, displaying the typical fingerprint of the organosilane propyl carbons: δ = 10.7 ppm, 23.4 ppm and 43.7 ppm for the α, β and γ carbons, respectively (refer to Figure 4 for attribution). [START_REF] Albert | Characterization of chemically modified glass surfaces by 13C and 29Si CP/MAS NMR spectroscopy[END_REF] The spectra of SL2 and SL1 are also shown in Figure 4d, where the corresponding peak attribution for SL2 with respect to the SL molecule is presented on top of the spectrum, Hence it appears that the templating process does not affect the SL molecular structure. Indeed, the [START_REF] Baccile | Ecodesign of Ordered Mesoporous Materials Obtained with Switchable Micellar Assemblies Angew[END_REF] C NMR spectra of SL2 and SL1 also contain the signals of the propyl group, that is easily identified by comparison with the reference sample. Noticeably, these signals are predominant in the SL1 spectra, in agreement with the elemental analysis. To demonstrate the proximity between SL and the silica network, and in particular between SL and the aminopropyl group, we ran a 2D 1 H-13 C HETeronuclear CORrelation (HETCOR) experiment, shown in Figure 5. This experiment is based on the dipolar coupling between 1 H and [START_REF] Baccile | Ecodesign of Ordered Mesoporous Materials Obtained with Switchable Micellar Assemblies Angew[END_REF] C, which is dependant on 1/r [START_REF] Slowing | Mesoporous silica nanoparticles: structural design and applications[END_REF] , where r is the internuclear distance. One can thus probe inter-nuclear proximities. The 2D map in Figure 5 is dominated by cross-peaks that are directly related to the CHx chemical bond for all resonances, something expected for the selected short contact time (tc= 0.5 ms). Nevertheless, the additional cross-peaks, numbered 1 through 5, deserve a specific attention. Cross-peak No.1 is due to the C-H bonds present in the sophorose head and in particular those related to carbons C'2-5 and C''2-5. Cross-peaks No.5, 4

and 2 indicate a specific through-space interaction between the Cα, Cβ and Cγ of the aminopropyl group with the H-groups of the sophorose head. Cross-peak No.3 states a through-space interaction between the sophorolipid aliphatic chain (here identified as Cx and corresponding to the signals of C4-C7 and C12-C16 carbons) and the sophorose head. This experiment proves that SL and the aminopropyl group are very close in space and, in particular, that the C2-C5 carbons of the sophorose head are the more exposed to the propyl moiety. Meanwhile, parts of the aliphatic chain of SL interact with sophorose as well, thus suggesting its outward, silica-directed, bending. The outward bending of the end-chain moiety of sophorolipids was also hypothesized for sophorolipid micellar solutions alone based on neutron scattering arguments. [START_REF] Baccile | Surface charge of acidic sophorolipid micelles: effect of base and time Soft Matter[END_REF][START_REF] Penfold | Solution Self-Assembly of the Sophorolipid Biosurfactant and Its Mixture with Anionic Surfactant Sodium Dodecyl Benzene Sulfonate[END_REF] It would have been very interesting to get information on the exact localization of the carbonyl group of sophorolipids but unfortunately its resonance cannot be detected in the 2D HETCOR map, neither at short (0.5 ms, Figure 5) nor at long (10 ms, not shown here) contact time values.

Finally, we checked by infrared spectroscopy (Figure 4c) that the interaction between SL and the amino group is of electrostatic nature. In fact, the stretching vibration for the COOH group in pure SL at 1708 cm -1 is shifted to1542 cm -1 , typical for the COO -form in SL1. Meanwhile, a signal at 1633 cm -1 identifies the NH3 + antisymmetric bending. This establishes a charge matching between SL and APTES in the synthesis conditions. No evidence for amide I and amide II bands, that would indicate covalent interactions between the amino group and SL, could be observed. The scheme given in Figure 6 visualizes the local interactions between sophorolipids and the aminopropyl-modified silica according to the results obtained in the FT-IR and 2D 1 H- [START_REF] Baccile | Ecodesign of Ordered Mesoporous Materials Obtained with Switchable Micellar Assemblies Angew[END_REF] C HETCOR NMR experiment and matching the results found for AMS's. 29 The possible effects of NH2 are multifold, both on silica and on sophorolipids selfassembly. The first one takes into account the well-known ability of APTES to take part to the formation of spherical silica particles. 34b, [START_REF] Ottenbrite | Self-Catalyzed Synthesis of Organo-Silica Nanoparticles[END_REF][START_REF] Rahman | Synthesis of organo-functionalized nanosilica via a cocondensation modification using γ-aminopropyltriethoxysilane[END_REF][START_REF] Chen | Sol-Gel Synthesis and Microstructure Analysis of Amino-Modified Hybrid Silica Nanoparticles from Aminopropyltriethoxysilane and Tetraethoxysilane[END_REF] This process was suggested to occur via an aggregative pathway where primary particles fuse together to form larger objects. [START_REF] Chen | Sol-Gel Synthesis and Microstructure Analysis of Amino-Modified Hybrid Silica Nanoparticles from Aminopropyltriethoxysilane and Tetraethoxysilane[END_REF][START_REF] Bogush | Uniform silica particle precipitation: An aggregative growth model[END_REF] By this mechanism, the final particles generally show a certain surface roughness, [START_REF] Chen | Sol-Gel Synthesis and Microstructure Analysis of Amino-Modified Hybrid Silica Nanoparticles from Aminopropyltriethoxysilane and Tetraethoxysilane[END_REF] but no specific porosity, 34b a fact that is actually confirmed by our own experiments on the SL-free materials.

Therefore, the presence of sophorolipids should interfere with the fusion process.

At equimolar NH2/SL ratios, we have already discussed the fact that sophorolipids form micelles that template silica. At higher ratios (SL1 and SL2 samples), SEM and TEM images in Figure 1 and Figure 2 show that silica spherical micrometer sized particles display an important roughness both at their outer surface (e.g., Figure 1d) and in their core (Figure 2a), a fact that nicely correlates with roughness estimation from nitrogen adsorption isotherms data. This result can then be related to the well-known aggregation model where particles below about 10 nm aggregate together to form a larger, condensed, silica spheres. [START_REF] Bogush | Uniform silica particle precipitation: An aggregative growth model[END_REF] From FTIR and solid state NMR we know that SL interacts with NH2 groups. Since it is known that APTES is highly concentrated at the particle surface, 34b,44 one can reasonably make the hypothesis of the formation of SL-stabilized silica nanoparticles, the coalescence of which is partially hindered by the presence of sophorolipids, forming a porous network. A possible depletion mechanism could in fact induce the attraction between sophorolipids molecules, thus inducing colloidal attraction between silica nanoparticles, as described before for the SDS/Ludox system. [START_REF] Kline | Interactions in binary mixtures: Partial structure factors in mixtures of sodium dodecyl sulfate micelles and colloidal silica[END_REF] The interparticle attraction for silica in the presence of a surfactant has also been described by Sharma et al. [START_REF] Sharma | Adsorption of Nonionic Surfactant on Silica Nanoparticles: Structure and Resultant Interparticle Interactions[END_REF] This picture eventually leads to the idea of a homogeneously SL-coated continuous silica network composed of fused primary particles.

Our second hypothesis consists of analyzing the effect of the excess of NH2 on the nature of the SL assemblies. In fact, we know from previous works [START_REF] Baccile | pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants[END_REF][START_REF] Baccile | Surface charge of acidic sophorolipid micelles: effect of base and time Soft Matter[END_REF][START_REF] Penfold | Solution Self-Assembly of the Sophorolipid Biosurfactant and Its Mixture with Anionic Surfactant Sodium Dodecyl Benzene Sulfonate[END_REF] that SL can form micrometersized networks under basic pH conditions so it becomes possible that silica is directly stabilized by such structures, which constitute the actual template. A similar hypothesis was emitted in the formation of large mesopores in sophorolipid-templated silica thin films under basic conditions. [START_REF] Baccile | Hierarchical Porosity in Silica Thin Films by a One-Step Templating Strategy Using a Stimuli-Responsive Bioderived Glycolipid[END_REF] The last scenario consists into taking into account a possible phase separation mechanism driven by the strong interactions between sophorolipids and silica oligomers in analogy to what has been observed in many systems described by the group of Nakanishi. [START_REF] Nakanishi | Hierarchically Porous Materials Optimized for High-Performance Liquid Chromatography Separations Acc[END_REF] In these works, meso-macroporous silica gels are commonly obtained upon addition of organic additives (generally polymeric compounds and, in few cases, surfactants) to induce phase separation. For silica-based systems, mesoscale porosity is systematically obtained by removing the organic additives (surfactants and polymers) by calcination while macroporosity is generally obtained by the so-called "chemical cooling" effect, where the two-phase domain is influenced by both silica condensation and specific inorganic (silica) -organic (polymers, surfactants, co-solvents) composition of matter. The solvent excess is eventually removed by evaporation thus leaving open macroscale porosity. The mechanism, texture and final size of the pores in the final material largely vary according to the composition of the gel. For silica, pores reach the approximate dimensions of few microns, even if these may depend on the relative silica/additive composition. Small mesopores can also be obtained if a surfactant is employed. This is obviously not the case in this work, as we do not observe large macropores and mesopores have a broad size distribution up to several nm.

The mechanism is slightly different for some specific metal oxides (metal center, M= Ti, Al), the inorganic polymerization reactions of which are much faster than in silica-based systems. [START_REF] Hasegawa | Facile Preparation of Hierarchically Porous TiO2 Monoliths[END_REF][START_REF] Tokudome | Structural characterization of hierarchically porous alumina aerogel and xerogel monoliths[END_REF] In this case, the gel is formed by the association of nanoscale primary particles, where braod-band mesoporosity is automatically generated without removing the organic additive. Nevertheless, the nature of the metal oxide/additive interactions and the additive/solvent role are far from being clear. According to the numerous data collected by Nakanishi et al. on silica, one should not expect porosity driven from the aggregation of primary particles, as seen for alumina aerogels and xerogels, [START_REF] Tokudome | Structural characterization of hierarchically porous alumina aerogel and xerogel monoliths[END_REF] but this is exactly what we do observe on SL1 and SL2 materials in this study. To account for this, we then must imagine such a strong interaction between SL and aminopropyl modified silica nanoparticles that eventually coalesce together and phase separate from the water solvent. Considering that both silica oligomers and sophorolipids are hydrophilic, the former can interact with condensing silicates both via the COO -and sophorose groups, thus leaving the aliphatic backbone oriented towards the water solution. In this case, silicate oligomers actually become slightly hydrophobic and can eventually undergo phase separation. Whether the interaction involves single sophorolipid molecules (as in the first hypothesis) or its micronic assemblies (as in the second hypothesis), which may be considered as polymer-like objects, is difficult to ascertain at this time. In addition, the fact that experiments are run at T= 60°C, followed by a treatment at T= 100°C, should not be neglected and a time-resolved study, from the moment of mixing the precursors until the final material, could help to better understand the origin of the porogenic effect.

Conclusion

Bioderived glycolipids have been used here as supramolecular templates for the synthesis of silica particles with broadband mesoporosity. These compounds, belonging to the family of acidic sophorolipids, have an accessible COOH group making them pH-responding. In this work, we have exploited such a feature to promote the interaction with NH2-modified silica using a sol-gel reaction in water. Addition of the amino group via APTES is of paramount importance to promote the precipitation of a solid in solution, whereas a gel would be formed otherwise. By playing with the NH2/SL ratio it is possible to produce silica particles with different features. At low ratios, materials have no particular morphology and one need calcination to obtain a narrow pore size distribution centered at about 3 nm, a value that is coherent with the molecular size of the initial sophorolipid.

At higher ratio, on the contrary, porous micrometer-sized spherical particles are formed, where the porosity is actually accessible without the classical calcination step and its size distribution ranges from 5 nm to 30 nm. It is important to note that no additional compound (e.g., block copolymer, polymer, organic solvent, pore-swelling agent) has been added to the system to reach the pore size larger than 20 nm and as it is typically done in the literature on the synthesis of materials with large pores. If calcination is performed, only small pores (dp> 3 nm) are formed and the t-plot analysis realized on nitrogen physisorption data shows that no microporosity is actually formed. These data are confirmed by high-resolution TEM and SEM-FEG images, that show the presence of both surface roughness (also confirmed by an advanced analysis of the physisorption data) and particle core porosity, which seem to derive from the aggregation of primary nanoparticles. The strong interaction between acidic sophorolipids and amino-modified silica is probed using FT-IR, which show the presence of COO -and amino groups but no amide bonds, thus suggesting the electrostatic nature of the interaction itself, and solid state NMR spectroscopy. The last technique is quite powerful as it demonstrates, via a 2D CP-MAS 1 H- [START_REF] Baccile | Ecodesign of Ordered Mesoporous Materials Obtained with Switchable Micellar Assemblies Angew[END_REF] C HETCOR experiment at low contact times (tc= 0.5 ms), the local proximity between the aminopropyl group and the disaccharide (sophorose), which constitute the hydrophilic head of sophorolipids. In terms of the type of templating mechanism that could take place, we propose several hypotheses, which span from a direct template by the supramolecular assemblies that SL form under basic conditions to a possible spinoidal decomposition or a combination of both.

13 C{ 1

 131 H} Cross-Polarization (CP) Magic Angle Spinning (MAS) solid-state Nuclear Magnetic Resonance (NMR) experiments were performed on a Bruker AVANCE III 300 (7.05 T) spectrometer using a wide-bore superconducting magnet and a 4 mm MAS probe at a spinning frequency of 14 kHz. A cross-polarization contact time, tc, of 3.0 ms, a relaxation delay of 3 s and 8192 transients were applied for all the experiments. One pulse[START_REF] Han | Anionic surfactant templated mesoporous silicas (AMSs)[END_REF] Si MAS NMR studies were run on Bruker AVANCE III 500 (11.75 T) spectrometer employing a 7 mm zirconia rotors spinning at 5 kHz. A total of 240 transients were collected using a π/4 pulse and a relaxation delay of 60 s. The chemical shift values were calibrated with reference to tetramethylsilane (TMS) at 0 ppm. The 13 C{ 1 H} CPMAS two-dimensional HETero CORrelation (HETCOR) experiments were performed on a Bruker AVANCE III 700(16.45

Figure 1 .

 1 Figure 1. SEM pictures of samples a) SL3, b) SL1and c,d) SL2. Image (d) is obtained using a Field Emission Gun (FEG) SEM microscope.

  SEM images of the SL3 (a), SL1 (b) and SL2 (c,d) powder samples are shown. SL1 and SL2 samples are mostly composed of spherical micrometer-sized particles, the size distributions of which, obtained from direct counting on the SEM figures, are shown in the insets. SL1 shows particles having a log-normal distribution with a modal diameter centered at about 1.5 µm while SL2 particles show rather a Gaussian distribution having an average diameter of 2.5 µm. As for the SL3 sample, very few spherical particles are observed

Figure 2 -

 2 Figure 2 -TEM images a-c) SL1, d) SL2 and e-f) sophorolipid-free (SL-free) silica samples

Figure 3 -

 3 Figure 3 -N2 adsorption-desorption isotherms and derivate data analysis of (a-b) as synthesized and (c-d) calcined samples: (a,c) N2 isotherms; (b,d) BJH pore size distribution.

Figure 4 -

 4 Figure 4 -(a) TGA curves; (b) NH2/SL molar ratio obtained from (N, C) elemental analysis (the contribution of the propyl groups has been subtracted from that of sophorolipids; (c) typical FT-IR spectra for SL1 and pure, silica-free, sophorolipids (SL) powder; (d) 13 C{ 1 H} CP-MAS solid state NMR spectra for samples SL1, SL2 and sophorolipids-free APTES-SiO2. The Cx signal identifies the entire aliphatic chain (C4-C7 and C12-C16)

Figure 5 -

 5 Figure 5 -2D 13 C-1 H CP-MAS HETCOR experiment recorded on SL2 sample using a contact time, tc= 0.5 ms.

Figure 6 -

 6 Figure 6 -Scheme of the local interaction between sophorolipids and aminopropyl-modified silica. FT-IR results are based on results shown in Figure 4c while solid state NMR is based on the results obtained in the HETCOR experiment described in Figure 5

Table 1 -Molar ratio of samples SL1, SL2 and SL3. The [Si] column refers to the contribution of both TEOS and APTES. The [NH2] column refers to the contribution of APTES alone Sample SL [Si] [NH2]

 1

	SL1	1	11.2	3.5
	SL2	1	9.6	2.3
	SL3	1	5.0	1.0
	Experimental techniques.
	Transmission Electron Microscopy (TEM) was run on a FEI Tecnai 120 Twin
	microscope operating at 120 kV and equipped with a high resolution Gatan Orius CCD 4k x
	4k numeric camera while Scanning Electron Microscopy (SEM) was done on a Hitachi
	S3400N instrument while SEM Field Emission Gun (SEM-FEG) was done on Hitachi SU-70
	instrument at the Institut des Matériaux de Paris Centre (IMPC) facilities. Particle size
	distribution was calculated from SEM images using ImageJ 31 software. Elemental analysis (C,
	N) was performed at the Microanalysis Center of the Université Pierre et Marie Curie
	(France). Small angle X-ray scattering (SAXS) measurements on powder samples were
	performed on a pinhole type S-MAX 3000 RIGAKU Nanoviewer instrument using a
	monochromatic Cu-Kα radiation and equipped with a CCD detector. The sample-to-detector
	distance was 0.059 m. The applied voltage and filament current were 40 kV and 50 mA
	respectively. ThermoGravimetric Analysis (TGA) was run on a Netzsch STA 409 PC Luxx
	instrument using a 5°C/min heating rate. Fourier-Transform Infra-Red (FT-IR) spectroscopy
	has been operated on a Perkin Elmer 400 spectrometer using the universal ATR sampling
	holder.

Table 2 -Values of the specific surface areas (S), pore size (dp), specific volume (V) and fractal dimensions (FD) for samples SL1, SL2 and SL3. Subscripts refer to the method employed to obtain the value. ° obtained from BET analysis; * obtained from t-plot analysis; § obtained from the adsorption branch of the isotherm; # Single-point value at P/P0= 0.95; ^ from BJH analysis;

 2

		Sample SBET° [m 2 /g] Sm* [m 2 /g]	dp § [nm]	Vads # [cm 3 /g]	Vads^ [cm 3 /g]	Vm* [cm 3 /g]	FD
		SL1	23	27	10/14/21	0.13	0.11	-	2.4
	As-synthesized	SL2 SL3 & SL-free	42 -	51 -	9/11 -	0.10 -	0.11 -	--	2.4 -
		SL1	136	132	(8)/12/21	0.21	0.17	-	2.7
	Calcined	SL2 SL3	367 599 / 478	338 455	3/7/11 4/(7)	0.37 0.51	0.36 0.41	0.33 0.40	2.6 2.4
		SL-free	240	-	-	-	0.05	-	-

Acknowledgement

The research leading to these results has received funding from the European Community's Si MAS NMR spectra. This information is available free of charge via the Internet at http://pubs.acs.org/.


Niki Baccile

Nadine Nassif

Luca Malfatti

Inge N A Van Bogaert

Wim Soetaert

Gerard Pehau- Arnaudet

Florence Babonneau

Sophorolipids: a yeast-derived glycolipid as greener structure directing agents for self-assembled nanomaterials

Sophorolipids, fully natural glycolipids, can form in water nm-size micelles of various geometries depending on their concentration as shown by Small Angle Neutron Scattering experiments. This property allows using them, for the first time, as structure directing agent in the synthesis of nanostructured silica thin film via the Evaporation Induced Self-Assembly (EISA) process.

Introduction

Alkylglycosides are a class of natural non-toxic surfactants whose properties have been largely studied and their commercial development is well-established [1]. Nevertheless, these industrially-produced compounds are generally synthesized by a multitude of chemical steps which limit the ideal "full green" approach suitable in the perspective of material synthesis via sol-gel process [2]. For this reason, we turned our interest towards sophorolipids (SL), a microbe-synthesized alkylglycoside (scheme 1), as structure-directing agent for mesoporous silica obtained via the sol-gel process using the well-established EISA [START_REF]SANS experiments performed on SL/D2O solutions (c= 2 and 5 w%)[END_REF] technique for thin film synthesis. Mesoporous materials aroused a growing interest because of their tunable and narrow pore size distribution and extremely high specific surface area [4]. These peculiarities make them ideal functionalizable materials for many applications such as sensing [5], photovoltaic electrodes [6], filtration [7] and catalysis [8]. So far, these materials are almost exclusively made with synthetic surfactants employed as structure directing agents and which arise important questions about sustainability and toxicity.

50

If compared to many other surfactants, SL are entirely obtained from renewable agroresources (rapeseed oil, oleic acid, carbohydrates) through a robust microbial synthesis with abundant production rates (up to 300-400 g . L -1 ); for this 55 reason, they have mainly attracted the attention of the cleansing industry (they can be found in Ecover© biobased products) even if recent studies showed the existance of interesting supramolecular assemblies [9c]. Their synthesis conditions, sources and applications have been recently 60 reviewed by Van Bogaert et al. [10]. Despite the potential interest for these compounds in dermatology [START_REF] Maingault | Use of sophorolipids and cosmetic and dermatological compositions[END_REF] and health care [START_REF] Fu | [END_REF] (more than 40 patents have been deposited on their synthesis and applications [13]), their self-assemblying properties in solution have only been partially explored [9c] 65 and their behaviour at the mesoscale level is still largely unknown. So far, they have never been used as structure directing agents. In this work, the acidic form of sophorolipid molecule (simply referred to as: SL) as shown in scheme 1 is synthesized from 70 glucose and oleic acid substrates using the yeast Candida bombicola (see experimental part in ESI). We show here that, in water, such molecules can form micellar aggregates of various shapes depending on their concentration and thus can be successfully exploited as structure-directing agents (SDA) Considered that very little is known about the self-assembling behavior of SL in water, we decided to investigate this point by mean of Small Angle Neutron Scattering (SANS) experiments performed on the SL in D2O. The knowledge of the optimal self-assembling conditions of SL are of paramount importance for their best employment in the synthesis of mesostructured silica thin films, as shown later.

SANS is an extremely powerful technique which is generally used to study the shapes, size, volumes and specific surface areas of colloids in general and surfactant-derived micelles in particular. According to the SANS theory (see ESI), the main contributions to the intensity I(q) come from the volume of the objects, their volumetric fraction, the contrast with the solvent, the form factor (related to the geometry of one scatterer) and their structure factor (related to their organization in space). In general, at low concentration values (indipendent objets with no mutual interactions), one can assume that the structure factor is equal to 1 and the form factor alone (well-known for simple objects like spheres, cylinders, etc) can be fitted to retrieve information on the shape of the micelle and its dimensions. In alternative, one can look more closely at the different parts of the SANS data set: 1) the low-q part of the curve gives some insights on the overall size (Guinier regime) and the geometrical features of the micellar object by looking at the slope (0, -1, -2 respectively for spheres, cylinders, disks); 2) the high-q region (Porod regime) is, on the contrary, more sensitive to the type of solvent/colloid interface (smooth, as in spheres, or fractal, as in polymer brushes) and to its dimension (crosssection).

Fig. 1 shows the variation of the absolute scattered intensity normalized by the concentration, I(q)/c, at different SL weight percentage concentrations prepared at their equilibrium pH (~ 4.5). As a general comment, we can say that the typical log(I) vs. log(q) plot represantation of our SANS data indicates the 40 existence of micellar objects of different geometries, as represented on the Fig. 1 itself. Low-q: the slope variation shows an evolution in the shape of the micelles as a function of concentration, as schematically represented in the inset of Fig. 1. Interestingly, no specific correlation peak is observed, 45 as one would expect at high concentration values; we will come back on this point later on in the discussion. High-q: superposition of the spectra in the region shows that the crosssectional dimension of the micellar objects does not consistently vary with concentration, as previously observed 50 in similar glycolipid systems [15]. A more detailed analysis follows.

SL concentration < 1 w%. In this regime, micelles can be considered as non-interacting monodispersed objects, whose form factor can be fitted with a spherical particle model (see 55 Fig. S1), which results in objects with a radius, R= 2.8 ± 0.2 nm, as further confirmed by Cryo-TEM experiments (Fig. S2 A). The micellar radius can also be estimated from the radius of gyration, RG, obtained from the Guinier plot, ln(I(q)) vs. q 2 (see eq.S1) at low-q. This approach is based on the following 60 two assumptions: the inter-micellar interaction is minimal, and the contribution of SANS intensity from large aggregates in the q region under interest is negligible (qR< 1). These hypothesis are verified here for q < 0.04 Å -1 . At c= 0.5 w%, R= 3.1 ± 0.3 nm which is consistent with the value obtained 65 above. Very interestinly, these values are also fully consistent with the R= 2.8 nm, which was obtained by Gross et al. [9c] after Dynamic Light Scattering (DLS) measurements, which, on the contrary, do not provide the exact micellar geometry.

From the analysis of the I(q) . q 4 values at high-q (porod 70 regime, eq. S2), some complementary information can be obtained about the total surface area per unit volume, SV, exhibited by the micelles. A simple calculation gives an estimated SV= 600 ± 50 m 2 /cm 3 and an aggregation number of about 50 SL molecules per micelle, if one estimates the 75 volume per molecule to about 950 Å 3 (see notes). Despite the lack of information about the exact configuration of the SL molecule in solution and, consequently, its real volume, these values appear to be self-consistent and coherent with an homogenous distribution of about 5.10 18 SL molecules . cm -3 80 (at c= 0.0048 g . cm -3 ) into spherical micelles having an average radius of 3.0 nm.

SL concentration  1 w%. In this regime, the slope at low-q (Fig. 1) increases and micelles are no-longer spherical. At c= 5 w%, a cylindrical model can be used to fit the form factor (see 85 fig. S1). Typical values for the cross-sectional radius are 1.6 ± 0.3 nm. At c  10 w%, the slope at low-q increases up to - 1.6 (inset in Fig. 1). These data show that the attribution of a defined shape is here an harsh task and a more appropriate model accounting for the whole q-range should probably be 90 employed, as reported in typical SANS studies for similar systems [15]. Nevertheless, even if the aggregation behaviour of SL has never been studied before, many studies exist for similar alkyl-glucosidic systems. For instance, Zhang et al. ). I(q), given in absolute scale (cm -1 ), is scaled by the w% concentration of the SL in solution. Inset: schematic evolution of slopes in (a) for 0.015 < q < 0.04.

and low-q regions. In general, alkylglucoside surfactants have the tendency to form both spherical and cylindrical objects according to the specific head-to-tail geometry, the concentration, c, in solution and the presence of a co-solvent.

In our case, micelles are spherical at low c while they start to 5 elongate from c> 0.5 w%. Isolated rigid rods would provide a -1 slope, while slopes between -1 and -2, as it is the case here, have been reported for long flexible cylindrical micelles [15,16]. Such a detailed analysis is extremely important for the better understanding of the self-assemblying properties of 10 SL in solution as it completes the previous study from Gross et al. [9c]; in fact, authors reported on the formation of both micellar and micrometer-sized supra-molecular assemblies as a function of pH and concentration. Despite the Small Angle X-ray Scattering (SAXS) and DLS analysis, no clear-cut answer could be provided on the shape of the micelles. Further proof of the cylindrical shape of the micelles is provided here by cryo-TEM experiments shown in Fig. S2 B,C.

Mesoporous thin film synthesis from SL.

20

The information obtained from the SANS study can be exploited in the synthesis of mesoporous thin films via the EISA process [START_REF]SANS experiments performed on SL/D2O solutions (c= 2 and 5 w%)[END_REF], where SL are used as a template. These experiments have been performed at c= 10 w%, which combines a good wettability of the film and which should provide flexible cylindrical micellar aggregates. 2d. This analysis reveals the presence of an anisotropic signal which is due to a coherent aligment of the pores. The strong influence of pH on the self-assembling behavior of SL in water is also confirmed by SANS experiments, as shown in Fig. 3. At pH= 6.5, where COO -species should be predominant, and for concentration values c  2 w%, an interaction peak centered at about q= 0.06 Å -1 (about 10 nm) dominates the SANS signal. This shows that micellar objects are now regularly spaced and their interaction is strong.

Further experiments are on-going to clarify the influence of pH on the self-assembly properties and on the morphology of the porous structure.

Conclusion 35

In conclusion, we show here a direct application of sophorolipids, a microbially produced, bio-engeneerable, glycolipid, in the synthesis of mesostructured silica thin films. Sophorolipids are able to form pH-responsive nanometersized micelles with spherical and worm-like shapes in water 40 as a function of their concentration and they reveal to be suitable alternatives to common petrochemical surfactants in the structure-directing agent process of inorganic materials.

Aknowledgements

We kindly aknowledge Prof. Walter Richtering (RWTH

  Scheme.1 Sophorolipids are commonly obtained from the culture broth of the yeast Candida bombicola. The acidic form of SL used in this work is obtained after processing the sophorose mixture (see experimental part).

Figure 1

 1 Figure1 -a) SANS curves of SL in D2O at variable concentrations and at SL equilibrium pH (~4.5). I(q), given in absolute scale (cm -1 ), is scaled by the w% concentration of the SL in solution. Inset: schematic evolution of slopes in (a) for 0.015 < q < 0.04.

Fig. 2 -Figure 2 -

 22 Fig.2-a shows the typical SAXS pattern of a SL-templated film. The presence of a broad peak at q= 0.13 Å -1 (d= 4.83 nm) indicates the existence of a porous network typical for a wormlike structure, as confirmed by Transmission Electron Microscopy (TEM) in figure2b. Interestingly, such an 65

1988, 65, 1460-1466 http://rsbweb.nih.gov/ij/developer/index.html O. Glatter, O. Kratky, Small Angle X-ray Scattering, 1982, Academic Press, London S. Förster, C. Burger, Macromolecules, 1998, 31, 879

Results and discussion

Self-assembly of SL Aachen University, Germany), Jose Teixeira (LLB, CEA/CNRS, France), Marianne Impéror-Clerc (LPS, Université d'Orsay, France) and François Ribot (LCMCP, Université Pierre et Marie Curie, Paris, France) for helpful discussions on SANS studies. We are also thankful to the 50 Industrial Yeast Collection, DBVPG (University of Perugia, Italy) for providing us the yeast Candida bombicola and their technical support. Gervaise Mosser (LCMCP) is kindly aknowledged for help in cryo-TEM experiments. Calculation of SV and aggregation number. SV is readily calculated from the I(q)q 4 plot from eq. S2, while the aggregation number can be 75 estimated from SV, the micellar radius obtained from the Guinier analysis, the density of the SL (= 1.092 g cm -3 ), the atomic weight of the acidic SL (622 g mol -1 ) and the irradiated volume (0.98 cm 3 , considering a 0.2 cm quartz cell, where the beamsize is 0.7 x 0.7 cm). The volume of one SL molecule (950 Å 3 ) is estimated from its density and molecular weight.

Notes and references