Niki Baccile
email: niki.baccile@upmc.fr

Thomas Fontecave

Cédric Boissière

Inge N A Van Bogaert

Hierarchical Porosity in Silica Thin Films by a One-Step Templating Strategy using a Stimuli-Responsive Bioderived Glycolipid Authors

Hierarchical porosity in thin films obtained with the Evaporation Induced Self-Assembly strategy is a major challenge that is generally accomplished by combining different templating compounds or strategies, each one typical for a given size range. For instance, for small-medium sized pores (below 20 nm) a library of surfactants exists, while colloids, polymers, as well as phase separation and water condensation strategies are used for large mesopores to macropores (up to several microns). Here, we report the pH-dependant templating effect using a class of entirely bioderived, functional, glycolipids obtained from the culture broth of the yeast Starmerella bombicola. We show that these compounds can be used to achieve a micro-to-macro hierarchical porosity in a one-step process by simply tuning the solution pH and without using any additional co-surfactant, co-solvent or pore-swelling agent. SAXS, TEM, SEM and ellipsoporosimetry data show the multi-scale porous texture of silica thin films. We attribute this behaviour to the unusual self-assembling properties of these compounds as a function of pH and which varies, at the same surfactant concentration, from dispersed micelles to micronic aggregates.

INTRODUCTION

The synthesis of porous silica-based materials obtained by a bottom-up, chemistrybased process (gels, powders, thin films, fibbers) represents an exciting challenge for many applications like catalysis, sensing and drug delivery, in which a large specific surface area is a crucial characteristic. [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] So far, the developed strategies introduce either monomodal micro (< 2 nm), meso (2-50 nm), macro (> 50 nm) or multiscaled porosity at once. Independently on the scale, one common goal has always been the development of an efficient strategy to achieve the three-dimensional, long-range organisation of the pores, even if rather than order, hierarchy in size and pore interconnection have recently been judged the most important parameters for the improvement of diffusion, percolation or adsorption phenomena within a material. [START_REF] Innocenzi | A Hierarchical Mesoporous Films: From Self-Assembly to Porosity with Different Length Scales[END_REF][START_REF] Meng | Triblock copolymer templated semi-crystalline mesoporous titania films containing emulsion-induced macropores[END_REF] In the field of multifunctional thin films, the hierarchical distribution of pores is very important, for instance, to develop highly performing sensors (both chemo-and bio-) and catalytic surfaces. More specifically, extensive work has been performed to achieve a multiscale porosity on thin films obtained by the sol-gel process and a recent review paper, to which we direct the reader for more details, makes a nice summary of such a field of research.

Leaving aside all physical, top-down methods that use high-energy beams or chemical etching, a short point on the various types of templating strategies from the smallest to the largest pore sizes is presented hereafter. Micropores (< 2nm) are obtained using small molecules, such as quaternary ammonium salts or ionic liquids. Mesoporosity (2-50 nm) is generally achieved using surfactants (cationic or non-ionic), where the pore size is generally related to the surfactant dimensions. Quaternary ammonium or sulfates-based surfactants provide small mesopores while block-copolymers, the size of which is tunable at will (though at high cost), is generally in charge for the larger mesopores up to about 20-30 nm.

Surfactants are also well-known to provide a long range mesoscopic order. The region between large mesopores and small macropores (30-100 nm) is probably the most challenging one. The easiest way to obtain such pore sizes is to use either organic or inorganic nanoparticles (colloidal silica, mesoporous or not, polystyrene and polymethylmethacrylate beads) and in some cases unshaped polymers like polyethylene or polypropylene glycole (PEG, PPG). Above 100 nm, other methods exist: large-scale phase separation of polymers, use of salts crystals, water droplets, decomposition gases and ice-templating. Each one of these methods is summarized in Table 1, where, for sake of clarity, we disregard the nature of the inorganic support, which is most often composed of SiO2 or TiO2. The combination of two or more approaches eventually produces very impressive hierarchical materials. Yang et al. 6 have for instance used block copolymers and latex spheres in combination with micromolding to obtain materials with long range porous order in the 10 nm to µm size range.

In the same line but using a different physico-chemical approach, Malfatti et al. [START_REF] Malfatti | One-Pot Route to Produce Hierarchically Porous Titania Thin Films by Controlled Self-Assembly, Swelling, and Phase Separation[END_REF] have combined the use of block copolymers with polymer-induced porosity enhancement by swelling and phase separation to make titania thin films with bimodal porosity between 10 and 150 nm, a range which is notoriously difficult to achieve with standard templating methods. A close look at Table 1 indicates that the medium and large ranges of porosities (6-20 nm) cannot be achieved with a surfactant templating strategy due to the obvious size-limiting factors, unless laborious and expensive organic synthesis provides well-designed block copolymers. For this reason, polymers, polymeric (or inorganic) colloids or those other strategies listed in Table 1 must be used. In addition, if one wishes to introduce ecoconception criteria (e.g., non-toxic materials, renewable resources, biodegradability) in the material synthesis process, as recently outlined in various works, [START_REF] Baccile | Introducing ecodesign in silica sol-gel materials[END_REF][START_REF] Gérardin | Ecodesign of ordered mesoporous silica materials[END_REF] most of these approaches should probably be revised and substituted by others.

In this work we would like to show the porogenic effect of a single biosurfactant at multiple scales. We have previously reported that acidic sophorolipids (SL), a family of entirely bio-derived glycolipids with low environmental impact, [START_REF] Rau | Sophorolipids: a source for novel compounds[END_REF][START_REF] Tulloch | Structure and reactions of lactonic and acidic sophorosides of 17-hydroxyoctadecanoic acid Canad[END_REF][START_REF] Asmer | Microbial production, structure elucidation and bioconversion of sophorose lipid[END_REF][START_REF] Zhou | Supramolecular assemblies of a naturally derived sophorolipid[END_REF][START_REF] Develter | Sophorolipids and Rhamnolipids[END_REF] can be used as structure-directing agent for silica thin films [START_REF] Baccile | Sophorolipids: a yeast-derived glycolipid as greener structure directing agents for self-assembled nanomaterials[END_REF] using the evaporation induced self-assembly (EISA) process. [START_REF] Brinker | Evaporation-Induced Self-Assembly: Nanostructures Made Easy[END_REF] In that work, we have shown the possibility to obtain small mesopores under acidic conditions. Here, we go much further as we show that by tuning the initial conditions (concentration, pH), it is possible to obtain an interconnected porous network between 1 nm and 1 µm. The most striking point is that the largest pores seem to be related to the peculiar assembling properties of SLs; in fact, we do not use any additive (block copolymers, polymers, colloids) nor any highly volatile solvent (THF, chloroform). Indeed, we have shown that SL display an unusual pH-dependent self-assembly behaviour and, in particular above pH= 10, most micelles are disrupted in favour of micronic aggregates, [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] which we believe to be the origin of the multi-scale porogenic effect. In summary, this work shows that by exploiting the self-assembly properties of a single, bioderived, glycolipid it is possible to achieve a hierarchical porosity within thin silica films.

EXPERIMENTAL METHODS

Sample Preparation. The synthesis of acidic sophorolipids (Figure 1) was described in Ref. presented in the text correspond to the following amount of SL/TEOS molar ratio: S1≡ 0.06, S2= 0.08, S3= 0.10, S4= 0.12, S5= 0.14, S6= 0.16, S7≡ 0.18.

Figure 1. Molecular structure of acidic sophorolipids

The effect of pH was tested by adding micromolar amounts of a 1 M NaOH solution to the solution immediately before dip-coating, thus preventing its gelification. These specific samples are coded as such: NaOH_0, for which no base is added, NaOH_1 and NaOH_2, the NaOH/TEOS molar ratio composition of which is given in Table 2.

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. Energy Dispersive X-ray Spectroscopy was done using an Oxford X-Max (area: 20 mm 2 ) detector installed on the just described SEM instrument.

Grazing incidence Small angle X-ray scattering (GISAXS) 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 meters. The applied voltage and filament current were 40 kV and 50 mA respectively. Environmental Ellipsometric Porosimetry analyses were recorded with a M2000 Woolam ellipsometer. The sample was fixed in a controlled atmosphere cell in which the relative humidity was controlled by mass flow controllers. The analysis protocols and the setup are described in Reference 33. A simple Cauchy's equation was used for modeling the optical properties of the porous thin films. The probe beam incidence angle was fixed at 70°. Dynamic Light Scattering (DLS) was run on a Zetasizer Nano ZS Malvern Instrument using the Zetasizer software for data acquisition and size distribution by intensity.

RESULTS

Effect of Sophorolipid Concentration

The effect of increasing SL concentration (S1 to S7) on the mesophase formation with no base added (samples NaOH_0 in Figure 2, where the integrated SAXS patterns are shown in log(I)-log(q) scale. A broad correlation peak, indicative of a wormlike structure and the position of which is related to the intermicellar distance, d= 2π/q, is observed for S1. Its intensity decreases from S1 to S7 and its position varies as follows: dS1= 5.23 nm, dS2= 4.58 nm, dS3= 4.24 nm, dS4= 4.08 nm, dS5=

4.05 nm and dS6= 3.93 nm. The SL/TEOS= 0.12 (S4 sample) seems to be the upper molar ratio limit above which the quality of the mesostructuration decreases. If compared with literature results, this value rather constitutes the lower range for silica; if compared to the system templated by the cationic CTAB surfactant, mesostructuration is kept up to surfactant/TEOS ratios as high as 0.35, even if the 0.12-0.18 can probably be considered to be the most stable interval. [START_REF] Grosso | Fundamentals of Mesostructuring Through Evaporation-Induced Self-Assembly[END_REF] The difference can probably come from the difference in the occupied volume between the two compounds. In fact, sophorolipids contain a comparable C18 chain but a bulkier sophorose polar head group, thus making the expected volume fraction of incorporated surfactant lower. A similar case occurs for silica films templated by the larger Pluronic block-copolymers (typical molar mass are larger than 5000 Da), where the typical Pluronic/TEOS ratios are in the 0.001/0.030 range. [START_REF] Zhao | Continuous Mesoporous Silica Films with Highly Ordered Large Pore Structures[END_REF][START_REF] Grosso | Two-dimensional hexagonal mesoporous silica thin films prepared from block copolymers : detailed characterization and formation mechanism[END_REF] At SL/TEOS= 0.18 (S7 sample), the correlation peak is replaced by a pronounced signal in the low-q region, characterized by a -1 slope of the scattered intensity in the log(I)-log(q) spectrum (Figure S1).

This value is typical for a cylindrical shape and a qualitative fit of the spectrum reveals the presence of channels, the typical geometrical parameters of which are: radius= ~ 1 nm, length= ~ 7 nm, as directly extracted from the fit shown in Figure S1 Such data can indicate the presence of elongated, most likely interconnected, micropores. We will only consider in the following part of the work a selected set of data concerning samples S1, S5 and S7, which are representative of the pH-dependant templating effect reported in this manuscript. 2.

The typical 2D GISAXS patterns related to non-calcined S1, S5 and S7 are shown in Figure 3a-c. The continuous ring in (a) confirms that the scattering pattern is typical of a wormlike mesophase. For S5, the overall quality of the scattering pattern is definitely poorer, even if its morphology (the signal is concentrated on the right and left-hand side of the beam stop) seems to indicate a possible micellar alignment perpendicular to the film surface. Finally, for S7, the mesophase is most likely lost as well as any pore micellar morphology. The corresponding integrated patterns (in 2θ scale) are shown in Figure S2a. 2.

The Effect of Base Addition. The effect of pH on the mesostructure is shown on Figure 3d-i, while the corresponding integrated spectra are presented on Figure S2c-h. For all samples, the addition of small amounts of NaOH (NaOH_1 in Table 2) has a modest influence on the initial mesoscopic order (Figure 3d -Figure 3f), when compared to the corresponding, basefree, patterns (Figure 3a -Figure 3c). The same conclusion can be drawn by looking at the corresponding 1D integrated spectra on Figure S2c, 4e, 4g exception made for the S7 sample (red line, Figure S2g), for which low-q scattering is completely disrupted. This indicates that the small amount of base affects the constitution of the micro-scale channels. A six to ten fold (see Table 1 for molar ratios related NaOH_2) increase in the base has, on the contrary, an important effect on all samples. Interestingly, this effect has similar consequences on the GISAXS patterns (Figure 3g-i), which show the complete disruption of the micellar network in favor of a strong signal very close to the center of the beam. Even if the amount of base required to observe this phenomenon slightly differs from S1 with respect to both S5 and S7

(Table 1), the same effect is observed from a NaOH/SL> 0.5. This correlates well with our previous investigations concerning the effect of the ionization degree of the COOH group on the self-assembly of SL in water. [START_REF] Baccile | pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants[END_REF] In fact, at low ionization degrees (COO -/COOH), SL forms ellipsoidal micelles [START_REF] Baccile | Surface charge of acidic sophorolipid micelles: effect of base and time Soft Matter[END_REF] with typical size of few nanometers. Upon full ionization of the COOH group, the system also forms large micronic, ill-defined, assemblies, as also shown by DLS data in the discussion section of this paper. In this study, the theoretical ionization degree estimated from the NaOH/SL molar ratio given in Table 2 is below 0.3 for all NaOH_1 samples while that increases above 0.5 in the NaOH_2 samples. Even if the parallelism between the work in ref. 31 and this one is not possible due to at least two important factors (presence of ethanol, silica condensation is catalyzed by the presence of NaOH), the presence of larger amount of base clearly has a strong effect on the final structuration and pore size distribution, as preliminary shown here by GISAXS and confirmed later on by microscopy and porosimetry analysis.

To better understand the origin of the intense signal at low angle observed in Figure 3 g-i, we draw the attention on the corresponding 1D integrated patterns on the right-hand side of Figure S2 (b,d,f,h), where the only the 0° < 2θ < 2° region is highlighted and where the Yscale is fully shown. In (b), the direct x-ray beam is detected at 2 θ< 0.15° in the base-free reference systems. This is still the case for the SAXS experiments recorded under the NaOH_1 conditions (red pattern in d, f, h). For larger base amounts (blue patterns, NaOH_2), an additional intense peak is detected at 2θ> 0.15°, which corresponds exactly to the lowangle diffusion just described in the 2D patterns (Figure 3). Since the position of this peak is invariant for all samples, one can deduce that it is not directly related to a well-defined correlation distance but rather to a strong, ultra low-angle scattering effect detected close to the beam stop. In fact, the lowest q-value in the SAXS apparatus used here is about q= 0.013 Å -1 , which identifies a typical size of about 48 nm (d= 2 π/q). However, Figure S2 shows that important low-angle scattering already starts at typical sizes slightly above 15 nm. This shows that the to low-angle scattering contains the contribution of all pores larger than about 15 nm, as verified hereafter by TEM and SEM analysis. If SEM nicely shows the effect of base addition on the surface texture of the silica films, TEM can bring additional information on the eventual porosity inside the silica walls, as shown in Figure 5, where we only selected samples S1 and S5. In the base-free systems, (images a and d), a mesoporous structure is observed in the S1 sample, as already suggested by the GISAXS patterns discussed in Figure 3, while, as expected, the organization is more difficult to detect for S5. Upon addition of the base (NaOH_2), TEM shows two families of pore distribution for sample S1. In (b), one finds spherical pores in the 100 nm range, which correlate well with SEM data, but a lower-scale porosity in the 10-20 nm range is also detected underneath the silica surface, as pointed out in the higher magnification image in (c). No small organized mesopores, possibly related to the presence of micelles, are indeed found, which also confirms the GISAXS data for this sample. Similar results are obtained for the base-treated S5 sample (images e and f), where unorganized porosity in the 5-20 nm range is also observed.

Analysis of the

Additional TEM images for both S1 and S5 samples shown in Figure S4 confirm this description. More specific porosimetry experiments, performed by following the variation of the silica thin film refraction index (measured using ellipsometry) as a function of the relative humidity, RH%, support the data above and contribute to better describe the porous network of these materials (refer to Table 3 for numerical values). The adsorption-desorption isotherms recorded for samples S1, S5 and S7 under the NaOH_0, NaOH_1 and NaOH_2 conditions are shown on Figure 6 a-c, while the entire set of experiments is shown in Figure S5. When no base is added, isotherms are of type IVa, [START_REF] Gregg | Surface Area and Porosity[END_REF][START_REF] Rouquerol | Adsorption by Powders and Porous Solids: Principles, Methodology and Applications[END_REF] which is typical for interconnected pores heterogeneous in size and shape. Upon addition of small amounts of base (NaOH_1 in Figure S5), the nature of the isotherm does not change. On the contrary, in the NaOH_2 systems (Figure 6 a-c), isotherms change completely and reflect the complex nature of the porous network. In all cases, the H4 hysteresis loop seems the best suited to describe the desorption behaviour. The capillary condensation occurs in the 0.7 < P/P0 < 0.8 range, where nominal values increase consequently between S1 and S7. A second capillary condensation is also detectable above P/P0 = 0.9, which is due to the presence of macropores. This last sample displays a well characteristic desorption branch, most likely typical of an array of slit-shaped pores in the microporous range. The double desorption step below P/P0 = 0.2 may actually not be related to the simple pore emptying but to specific water adsorption phenomena which depend either on the surface chemistry or the roughness of the material. [START_REF] Gregg | Surface Area and Porosity[END_REF] The pore size distributions calculated from the adsorption branches are shown in Figure 6 d-f. The S1 sample shows a relatively narrow distribution with respect to S5 and S7, which reflects the better mesoscale order observed by SAXS experiments. A common feature for all samples is the increase in pore size upon addition of the base. For NaOH_1, pore diameter is smaller than about 4 nm. Additional increase in the base for S5 and S7 (not shown here) induce a further increase in diameter up to 6 nm. Then, in the NaOH_2 systems and for all samples, the pore size is much broader and it reaches the values of 8 nm for S7.

The thickness of the silica films can be calculated from the ellipsometry data. Without base addition, thickness goes from 127 nm for S1 to 187 nm for S7 (Table 3 and Figure S6). This trend can most likely be related to a supposed increase in the solution viscosity due to the larger glycolipid amount. Upon base addition, the thickness systematically increases for all samples, a fact that could reasonably be explained by the increased viscosity of the sol-gel solution due to quicker based-catalyzed silica condensation. The direct proportional relationship between viscosity and thickness is well described by the Landau-Levitch equation [START_REF] Landau | Dragging of a liquid by a moving plate Acta Physicochim[END_REF] for non-evaporating fluids. The case of evaporating fluids in a withdrawal regime above 1 mm . s -1 for sol-gel-based thin films, a situation which best describes our systems, has been discussed in ref. 39, where authors also comment the effect of surfactant concentration and silica condensation rate on thickness.

In this work, the thickest sample is about 200 nm (S5 and S7) while the thinnest is 137 nm (S1), values which are quite common for mesoporous silica films obtained from blockcopolymer (Pluronic F127) surfactants at pulling rates of about 1 mm . s -1 . [START_REF] Faustini | Preparation of Sol-Gel Films by Dip-Coating in Extreme Conditions[END_REF] Additional interesting aspects on the thin film stability can also be deduced from the ellipsoporosimetry data. The variation of the thickness with relative humidity, RH%, tells about the mechanical stability of the film; in fact, it has been shown that capillary condensation due to water adsorption in mesopores is responsible for strong local pressure with values of young modulus up to several GPa. [START_REF] Boissiere | Porosity and mechanical properties of mesoporous thin films assessed by environmental ellipsometric porosimetry[END_REF] If, at the end of each humidity cycle, the initial thickness is unvaried (or it has varied little), then the film has kept its structural integrity. This is the case for the sophorolipids' templated thin films shown in this study, as presented in the thickness vs.

RH% curves shown in Figure S6 for selected samples: in all cases, there is no considerable variation in the initial thickness upon a cycle. In comparison, silica thin films templated by classical cationic (CTAB) or block-copolymer (F127) surfactants are mechanically less stable, because the original thickness cannot be fully recovered after a single humidity cycle, [START_REF] Boissiere | Porosity and mechanical properties of mesoporous thin films assessed by environmental ellipsometric porosimetry[END_REF] thus making these materials more resistant to local stress than samples with monomodal longrange ordered mesoporosity.

The homogeneity of the mesopores inside the films can also be commented. In fact, the model used for fitting the experimental data required only one layer for achieving a good simulation, whatever the value of the relative humidity. This proves that no significant variations of the optical properties exist as a function of the depths of films, the resolution of which is better than 10 nm. [START_REF] Boissiere | Porosity and mechanical properties of mesoporous thin films assessed by environmental ellipsometric porosimetry[END_REF] These arguments are also supported by the fact that the probed surface is 27 mm 2 (size of the light beam: 9 mm x 3 mm), a dimension which is more than satisfactorily larger than any local lack in homogeneity at the nano and micro scale.

DISCUSSION

The experiments presented above show that the basic medium contributes to change the local structure of the silica network promoting the formation of a distribution of micro-meso-and macropores between 1.5 nm and 1 µm, whereas the calculated size of the SL molecule is not larger than 2.5 nm. [START_REF] Baccile | pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants[END_REF] If we compare the results found here with the known porogenic mechanisms summarized in Table 1, one can realize that none of those fully explain our results. First of all, our macroscale porosity cannot be related to the presence of colloidal particles, as we neither introduce nor form any. Neither can the surface porosity be related to the presence of other organic templates, such as large block copolymers, like polybutadieneb-polyethyleneoxide, which is known to produce organized cavities in the meso-macro limit range perpendicularly oriented to the film surface. [START_REF] Faustini | Antireflective, Self-Cleaning, and Antifogging Sol-Gel Coatings: An Example of Multifunctional Nanostructured Materials for Photovoltaic Cells[END_REF] Another hypothesis could be the formation of "breath figures", driven by water condensation at the film surface. [START_REF] Maruyama | Mesoscopic patterns of molecular aggregates on solid substrates[END_REF][START_REF] Sakatani | Coupling Nanobuilding Block and Breath Figures Approaches for the Designed Construction of Hierarchically Templated Porous Materials and Membranes[END_REF] However, this phenomenon would be difficult to explain because it is known to occur when water is mixed with a highly volatile solvent, like chloroform or THF. Two factors make us exclude this hypothesis, at least the way it is actually explained in the literature: 1) we do not have a highly volatile solvent, but only ethanol; 2) The size of the surface pores found here increases with the amount of employed SL (Table 3), while in general that is relatively independent of the concentration in organic matter (polymer, surfactant). The only exception occurs for very low concentration, at which the amount of compound is not enough to stabilize the water droplets. [START_REF] Maruyama | Mesoscopic patterns of molecular aggregates on solid substrates[END_REF] Additionally, we do not observe micronic surface porosity at acidic pH values for the same SL concentration, which strengthens the hypothesis that they cannot be related to the ethanol/water mixture. If we exclude all other hypotheses which obviously do not apply to our system (templating by ice, gas and salt), there are two last possible explanations.

Many authors have reported the formation of micronic surface patterns by exploiting phase separation effects induced by polymers (PEG, PPG, PLA). [START_REF] Malfatti | One-Pot Route to Produce Hierarchically Porous Titania Thin Films by Controlled Self-Assembly, Swelling, and Phase Separation[END_REF][START_REF] Kajihara | Preparation of macroporous titania films by a sol-gel dip-coating method from the system containing poly(ethylene glycol)[END_REF][START_REF] Fuertes | Processing of Macroporous Titania Thin Films: From Multiscale Functional Porosity to Nanocrystalline Macroporous TiO2[END_REF][START_REF] Zhao | Water-assisted fabrication of honeycomb structure porous film from poly(L-lactide)[END_REF] This method is very effective to make crack-free nested meso-and macropores, with diameters controllable in the 10-200 nm range. [START_REF] Malfatti | One-Pot Route to Produce Hierarchically Porous Titania Thin Films by Controlled Self-Assembly, Swelling, and Phase Separation[END_REF][START_REF] Fuertes | Processing of Macroporous Titania Thin Films: From Multiscale Functional Porosity to Nanocrystalline Macroporous TiO2[END_REF] The proposed mechanism for PEG and PPG-templated TiO2 thin films involves the formation of a stable complex between the ethylene glycol units and the Ti-oxo clusters, where the macropores would then be directly linked to the phase separation of the solvent. [START_REF] Kajihara | Macroporous Morphology of the Titania Films Prepared by a Sol-Gel Dip-Coating Method from the System Containing Poly(Ethylene Glycol). I. Effect of Humidity[END_REF][START_REF] Kajihara | Macroporous Morphology of the Titania Films Prepared by a Sol-Gel Dip-Coating Method from the System Containing Poly(Ethylene Glycol). II. Effect of Solution Composition[END_REF] Similar effects were obtained for the synthesis of bulk porous monolith in SiO2/PEG [START_REF] Nakanishi | Phase Separation in Silica Sol-Gel System Containing Poly(ethylene oxide). I. Phase Relation and Gel Morphology Bull[END_REF][START_REF] Nakanishi | Phase Separation in Silica Sol-Gel System Containing Poly(ethylene oxide) II. Effects of Molecular Weight and Temperature Bull[END_REF] systems and have been explained via a so-called "chemical cooling", as opposed to physical cooling; in the latter, macroscopic phase separation (the two-phase region) is achieved by cooling down the system temperature while in the former molecular movements are restrained by increasing the number of chemical bonds. Chemical cooling has been largely discussed in sol-gel systems. [START_REF] Nakanishi | Pore Structure Control of Silica Gels Based on Phase Separation[END_REF] If some key elements found in chemical cooling are lacking from this study, such as polymers and/or additional solvent (besides water and ethanol), this mechanism was reported for other systems composed of SiO2/CTAB. [START_REF] Nakanishi | Supramolecular Templating of Mesopores in Phase-Separating Silica Sol-Gels Incorporated with Cationic Surfactant[END_REF] Whether this mechanism applies or not to the system studied here, it cannot be excluded, even if it is still uncertain for the following reasons: 1/ Chemical cooling has been largely described for the formation of monoliths, where evaporation of the solvent is slow, which is not the case for fast-evaporating systems composed of silica thin films; 2/ a large literature exists on the synthesis of porous silica thin-films (some examples given in respectively corresponding to the size of CTAB micelles and phase separation effects while here we experience the formation of pores larger than 10 nm; 4/ in chemical cooling, it exists an inverse proportionality between the amount of non-volatile organic compound (e.g., PEO, CTAB) and macropore size [START_REF] Nakanishi | Supramolecular Templating of Mesopores in Phase-Separating Silica Sol-Gels Incorporated with Cationic Surfactant[END_REF][START_REF] Nakanishi | Hierarchically Porous Materials Optimized for High-Performance Liquid Chromatography Separations Acc[END_REF] and a direct one between the amount of solvent and macropore size. [START_REF] Nakanishi | Pore Structure Control of Silica Gels Based on Phase Separation[END_REF] This is not the case for our system, where the macropore size increases from sample S1 to S7, that is, with the amount of sophorolipid in solution (Table 3).

Without excluding completely the chemical cooling hypothesis, we propose another mechanism. Sophorolipids in water under acidic pH conditions form micelles, but upon pH increase, in particular at low concentrations, micronic assemblies can also be formed, [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] as shown by complementary DLS experiments shown in Figure S7.. Even if some caution must be employed when comparing the behavior of sophorolipids in pure water and in the presence of silica, ethanol or during the EISA process, one can still argue that these aggregates could be responsible for the pore size above 5 nm observed on samples S1, S5 and S7 studied here.

For the largest, micron-sized, porosity, other wettability effects must probably be considered.

To go further and test the wettability effect of a SL in water, we prepared a solution containing SL at a NaOH/SL= 1; this was used to coat a silicon substrate and the corresponding SEM images are shown in Figure 7c,d. Interestingly, large micronic aggregates, which correspond to the bright texture in (c) (for sake of clarity, colors have been artificially inversed with respect to the original image), are observed and actually look closely to typical dewetting patterns found in other surfactant-based systems. [START_REF] Bernardes | Morphology and self-arraying of SDS and DTAB dried on mica surface[END_REF] Coating of a silicon substrate at NaOH/SL= 0 results in a homogeneous thin film with no dewetting phenomenon.

The EDAX analysis of the surface patterns is given in (d): the full analytical spectrum shows the presence of the following elements: silicon (from the substrate only), carbon and oxygen (from SL but also from impurities) and sodium (from the NaOH base solution). Even if EDAX should be used carefully when analyzing carbon and oxygen, the Kα edge of carbon (in red) describes very well the texture (bright signal) observed in its corresponding SEM image. For completeness, we also provide the corresponding Kα edge of oxygen (in green), which, on the contrary, results to be less probing than the carbon edge. Considering these data, we can formulate several hypotheses in terms of the mechanism of formation of the multiscale porosity for the SL-templated SiO2 films under basic conditions.

In the first one, these assemblies constitute the direct template for silica at all scales. For pore size below 50 nm, it is possible that SL aggregates observed in DLS constitute the template while for pore size above 500 nm, the peculiar SL/surface interaction giving rise to the patterns in Figure 7c, 8d could be the porogenic driving force. In the second hypothesis, a possible phase separation mechanism as described above for the TiO2/PEG materials may occur where silica oligomers strongly interact with the micronic sophorolipid aggregates, thus repelling the solvent (water/ethanol) outside. Finally, a third, cooperative, effect could take place, where the solvent evaporation rate and the relative humidity in the dip-coating chamber could have important effect on water condensation effects at the film surface, thus making an atypical "breath-figure"-like mechanism.

The poor knowledge about the origin and stability in water [START_REF] Baccile | Surface charge of acidic sophorolipid micelles: effect of base and time Soft Matter[END_REF] (and even less in a water/ethanol mixture) of the supramolecular SL assemblies makes any possible clear-cut explanation hardly feasible. However, we believe that there seems to be a strong correlation between them and the multiscale, hierarchical, porosity and for this reason we make the first hypothesis highly possible. In this context, the second hypothesis, that is the formation of strong silica/SL interactions, could justify the micronic-scale surface pattering. For the third one, we do not believe that "breath figures" do actually form in the way they have been explained so far in the literature (no highly volatile solvent is used in this work) but relative humidity, not studied here, in the dip-coating chamber could probably plays an important role on water evaporation rates and its influence on the micronic-scale porosity.

CONCLUSIONS

In this work we show how a biobased functional glycolipid surfactant can be efficiently used to introduce a micro-to-macro scale porosity in a single step in EISA-processed silica thin films, without using co-solvents and co-surfactants. When prepared in solution under acidic conditions, sophorolipids (SL) form micelles that are able to form a mesoscale wormlike porosity, the typical size of which (about 2 nm) is compatible with the typical size of the sophorolipid chain length. Upon addition of a strong base to the initial solution and keeping the same SL concentration, one can increase the pore size up to 4 nm but, beyond an upper threshold which depends on the initial SL concentration, large mesopores (8-30 nm) and macropores (0.15-2 μm, according to the initial SL amount) are immediately formed. The former are interconnected within silica while the latter are oriented perpendicularly to the support. Their size and interconnectivity is highly dependent from the initial SL concentration. SAXS data recorded at different SL and NaOH amounts and combined with elliproporosimetry, SEM and TEM experiments clearly show the textural properties and porous network of the sophorolipids's templated silica thin films. The fact that we neither use any co-surfactants (e.g., block copolymers) nor polymers (e.g., PEG, PPG) and (volatile) cosolvents (e.g., THF, chloroform), makes this process very atypical if compared to the existing methods to make thin films with hierarchical porosity and which generally combined more than one compound and/or porogenic strategy. At the same time, the explanation is not straightforward. It is known that, at low SL concentrations in bulk water, micelles, generally formed under acidic conditions, are mostly disrupted at high pH. This phenomenon puts in evidence, at the same time, the formation of large, micronic, supramolecular assemblies, as reported elsewhere but presented again in this work using DLS experiments. We also show how the large assemblies formed at unitary NaOH/SL ratios can be deposited on a silicon surface in the absence of both ethanol and silica precursors and that they form interesting surface patterns. In terms of the mechanism, we believe that pores below 50 nm are formed by direct SL templating, as also found for silica particles. For the micronic pores we exclude the existence of "breath figures" (no volatile solvent is used here) but we make the hypothesis that surface patterns actually form due to strong SL/silicates interactions, probably enhancing a dewetting effect rather than a chemical cooling process, even though this is not excluded.

ACKNOWLEDGMENT

The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No.

Biosurfing/289219. We warmly thank Mohamed Selmane and Isabelle Genois (Laboratoire de TOC

Figure 2 -

 2 Figure 2 -Log-log plot of the SAXS spectra recorded for non-calcined samples S1 to S7. The

Figure 3 -

 3 Figure 3 -Selected 2D SAXS patterns corresponding to non-calcined samples S1, S5 and

  Pore Size at Large NaOH/SL> 0.5 Figure4shows the SEM images of samples S1, S5 and S7 in the NaOH_2 regime (NaOH/SL> 0.5), which is the one presenting the largest influence of base onto porosity. For all samples, the surface of the silica films shows an open porous texture, the pore size and interconnectivity of which strongly varies with the initial amount in SL. For S1, pores are estimated to be 220 ± 70 nm. For S5, we measure larger pores with a broader distribution (500 ± 200 nm) while for S7, the pore size ranges in the 1.5 ± 1.0 μm. In this last sample, the interconnection between pores seems extremely large if compared to S1 and S5. Additional SEM images at different scales for the same samples are provided in FigureS3(a,b: S1; b,c: S5; d,e: S7), thus showing the extension and full coverage of the whole porous silica network over the silicon substrate. In the case when no base is added (images not shown), SEM is useless to characterize the silica thin film because this is constituted by a uniform flat surface. EDAX analysis for sample S7 combined with filtering for the Kα edge of oxygen confirms that the interconnected network visible on the SEM images is actually composed of SiO2 (purple box in Figure4d).

Figure 4 -

 4 Figure 4 -SEM images recorded for samples (a) S1, (b) S5 and (c) S7 at large amounts

Figure 5 -

 5 Figure 5 -TEM images recorded for samples (a-c) S1 and (d-f) S5 without base added

Figure 6 -

 6 Figure 6 -(a-c) Water adsorption-desorption isotherms measured using ellipsoporosimetry

Figure 7 .

 7 Figure 7. (a) SEM image of the 5 mg/mL SL in water solution at NaOH/SL= 1 (no SiO2)

Table 1 . Most Common Bottom-up Porogenic Strategies Employed for Sol-Gel Derived Thin Films (mostly, but not limited to, SiO2 and TiO2) The

 1

	Size (nm)	Strategy	Example reference
	< 2	Small molecules (e.g., ionic liquids)	8
	2-6	Surfactants: ionic (e.g., CTAB, SDS), small block copolymers (e.g., Brij)	9, 10
	6-20	Surfactants: large block copolymers (e.g., Pluronics, KLE, PB-PEO, PS-PEO)	10, 11
	20-100	Nanoparticles and small colloids (inverse opal strategy) Polymers (e.g., PEG, PPG)	12, 13 14
		Salt crystals	15
		Polymer phase separation (e.g., PEG, PLA)	16, 17
	> 100	Water (ice) Water/volatile solvents ("breath figures")	18 19, 20
		Large colloids (inverse opal strategy)	21
		Gas (nitrocellulose decomposition)	22

size domains are indicative.

Additional information can be found in ref.

4.

  31. Mesoporous silica thin films are prepared by dip-coating via the EISA technique.

	Tetraethyl orthosilicate (TEOS), ethanol (EtOH), concentrated HCl 37% and NaOH are
	purchased from Aldrich and used without further purification. An initial pre-hydrolyzed
	TEOS solution is prepared as follows: 0.53 mL of TEOS, 0.37 mL EtOH and 40 μL of a 0.77
	M HCl solution. This solution is stirred for 45 min and then 1.86 mL of EtOH, 0.19 mL of
	water, 50 μL of 1 M HCl are added. The amount of acidic sophorolipids (SL) ranges from
	0.088 g to 0.266 g, which corresponds to a SL/TEOS ratio ranging from 0.06 to 0.18. This is
	stirred for 15 min before dip-coating. Silicon substrates (thickness 400 μm) are dip-coated at
	constant relative humidity (≈ 19-22%), the pulling rate is set at a variable speed ranges from
	to 1.9 mm.s -1 to 1.4 mm.s -1 . Finally, after deposition, films are treated at 130 °C for one night

to consolidate the silica scaffold and then calcined at 350 °C under air for 1 h. Samples

Table 2 -List of Samples with Values for Each Component Given in Molar Ratios NaOH_0

 2 , NaOH_1 and NaOH_2 correspond to the sample codes used in this study and the values given indicate the molar amount of NaOH with respect to TEOS.

Table 1 )

 1 is depicted in

	Sample	SL	TEOS	NaOH_0 NaOH_1 NaOH_2 NaOH/SL
	S1	0.06	1	0	1.69 . 10 -2 6.78 . 10 -2	1.1
	S5	0.14	1	0	1.69 . 10 -2 10.2 . 10 -2	0.7
	S7	0.18	1	0	1.69 . 10 -2 10.2 . 10 -2	0.6

Table 3 . Structural Features (average pore size, type of mesoptaining

 3

	(NaOH_1,

Table 1 )

 1 but much fewer (e.g., ref.7 could be considered as one but the largest pores are much smaller than 1 µm) examples are reported on the use of chemical cooling on surfactant-based systems, for which the formation of breath figures is a most common mechanism; 3/ in the SiO2/CTAB porous monolith discussed in ref.47, only two ranges of pores have been reported, 2 nm and > 1 µm,

Chimie de la Matière Condense de Paris, Collège de France/UPMC/CNRS, Paris, France) for their kind help with some of the experiments.

SUPPORTING INFORMATION DESCRIPTION.


Niki Baccile

David Grosso

Clément Sanchez

Aerosol generated mesoporous silica particles

Organized mesoporous silica sub-micro spheres have been synthesized by spray drying solutions containing hydrated Si(OEt)4 as silica precursor and surfactants as texturing agent. The resulting powder exhibits controlled organized 2-D hexagonal honey-comb like porosity of 2,2 nm mean pore diameter, homogeneous size dispersion (the particle average diameter is around 0,3 µm and standard deviation σ = 1,89), and a high specific surface area (1000 m 2 /g). The long-range order porosity was achieved after controlling different parameters: surfactant type and surfactant/TEOS ratios, dilution, pH, temperature and relative humidity under laminar flux motion conditions of aerosol generation. Powders were characterized using X-ray diffraction, TEM, N2 isotherm adsorption/desorption cycles and FTIR.

Introduction

During the last ten years that followed the discovery of meso-organised templated materials by Mobil & Co. 1 , many groups developed novel nano-textured inorganic and organic/inorganic hybrid materials by combining the sol-gel chemistry and the structuring effect of self assembling liquidcrystal surfactants. Most of those works, to which a number of reviews has been dedicated 2,3,5 , focused on the precipitation in solution of silica based oxide networks around a great variety of charged surfactants 1,9 such as Cethyltriethylammonium Bromide (CTAB) or amphiphile block co-polymers such as PEO-PPO-PEO Pluronics 6, 7,8 . Once the template is removed from the framework through thermal treatment or solvent extraction, the porosity stands at the former location of the nano-segregated micelles. This technique allows a perfect control of the pore size (typically between 2,1 and 10 nm in diameter depending on the surfactant molecular weight) and a controlled pore morphology and self organization within the material (spheres, cylinders or Fig. 01 The process of spray dried mesoporous silica particles. The initial droplet contains Cetyltrimetilammonium bromide (CTAB) as structuring agent, Tetraethoxysilane (TEOS) as silica precursor. The inorganic condensation is achieved via the sol-gel process, whose fundamental steps are showed in the low part of the picture. The role of solvent evaporation in the mesostructuration is put in evidence.

Hydrolysis (RO)3-Si-OR + H2O

(RO)3-Si-OH + ROH.

Condensation (RO)3-Si-OH + HO-Si(RO)3 -Si-O-Si-+ H2O

Inorganic condensation via Sol-gel chemistry:

lamellae organized into mono-, bi-, or tri-dimensional arrays). In other words, surfactant micelles play the role of template of the porous network whose geometry is directly related to the surfactant molecule morphology 10 and concentration in the medium 11 .

Figure 01 illustrates the various steps involved in such a synthesis route. More recently, this technique was adapted to thin films, monoliths, and macro-spheres preparation through a controlled evaporation of initial isotropic mixtures of surfactant molecules and inorganic precursors in an appropriated highly volatile solvent. This new approach, addressed as the Evaporation Induced Self Assembly technique 12 (EISA), has great advantages over the precipitation one. Indeed, firstly the surfactant/silica stoichiometry is retained along evaporation; secondly the shaping into spheres takes place with a control on the diameter (aerosol); thin layers have a controlled thickness (liquid deposition) and finally large objects with controlled dimensions (slow evaporation) are possible. While powder precipitation has been well developed, the EISA method was firstly directed to the preparation of variously mesostructured and porous silica thin films 12 , which are interesting materials for applications in catalysis 13,14,15 , separation 16 , optoelectronic 17 and chemical sensing 18 .

The mechanisms involved during self-assembly were studied in details on thin films systems and are now well understood 4,19,20,21,22 . Critical parameters that govern the mesoporosity are the surfactant volume fraction, the inorganic extent of condensation when processing, the evaporation conditions such as humidity and the post synthesis thermal treatment aiming at eliminating the surfqctqnt and stiffening the inorganic network.

Powders have indeed important applications in catalysis and separation domains but control on the primary particle dimension remains a challenge in precipitation synthesis. An alternative path stands in the simple, industrially already well developed and cheap aerosol generation of particles in which the EISA follows the similar evaporation and self-assembly laws than those for thin films.

Few works have been already dedicated to mesoporous spheres prepared by spray-drying. The first one was provided by the Sandia National Laboratory 23 and concerns silica particles while the second was concerned the mesoporous crystalline anatase TiO2 homologue by Grosso et al. in 2003 24 . A third one has been published in 2003 by Bore and Coll. 29 . In this one, some more detailed informations about the influence of chemical parameters on the mesostructure were provided. Even if these works stretched out the enormous potential of combining spray drying to the template approach, none of them however gives detailed informations conceming the synthesis, the particle size dispersion and expecially the processing conditions and their influence on the mesostructure quality of the final rnaterial.

The present article is then dedicated to this topic and many future ones will undoubtedly follow. Typical silica mesoporous particles were prepared by hydrolysis and condensation of silicon alcoxide precursors [START_REF] Brinker | Sol-gel science -The physics and chemistry of Sol-Gel Processing[END_REF] in presence of CTAB, as depicted in Figure 01, through controlled evaporation of spray generated droplets from an initial sol. After surfactant elimination by calcination, the material exhibited an excellent degree of meso-ordering, a high specific surface area of more than 1000 m 2 .g -1 , a narrow pore size distribution centered at 2.3 nm and an excellent thermal stability. The process was then optimized by studying the influence of the chemical parameters (CTAB/Si molar ratio, pH, solvent quantity) and of the processing parameters (viscosity related to dilution, carrying gas composition, humidity percentage, pathway length). The optimal process was then extended to other silica particles templated with larger PEO-PPO-PEO block copolymers or CnH2n+1PEO amphiphiles to enable larger organized pores. It is worthed to remark that the EISA process is easily extensible to spray coating.

Experimental -preparation of particles

The particles were synthesized by spraying solutions containing Tetraethylorthosilicate (TEOS), surfactants, water (pH 1,2), ethanol of molar ratios 1:x:40:30 (where x is equal to surfactant moles) after vigorous stirring for 1 h. Four surfactants were used: C16H33N(CH3)3Br -CTAB -(0,35 > x > 0,08); C16H33(EO)20H -Brij 58 -(x = 0,06); (EO)20(PO)70(EO)20 -Pluronic P123 -(x = 0,0078), and (EO)106(PO)70(EO)106 -Pluronic F127 -(x = 0,0035). The spray drying apparatus was composed of a constant out-put atomizer (TSI model 3076), nominally providing droplet particles of 2000 nm in mean diameters (typical dry particle mean diameters vary from 20 to 300 nm depending on the concentration), at a fixed gas pressure of 2.5 . 10 5 Pa and a flux of 3 lt . min -1 . Particles were then carried by the gas flux to different elements where they stayed for various lengths of time that were controlled by the tubing lengths and diameter. These elements are the following:

(1) A nitrogen cold trap aiming at eliminating the alcohol vapor from the system.

(2) A drying chamber aiming at giving time to the particles to equilibrate with their environment and eventually to selforganize.

(3) A cylindrical furnace allowing the densification of particle outer shell (crust layer) preventing any future coalescence. (4) A 450 nm porous filter on which particles are collected.

(5) And finally a vacuum pump necessary to keep the flux constant in a laminar regime.

A global scheme is shown in Figure 02. Temperatures could be varied in the drying chamber and the oven and the relative humidity (RH) can be adjusted between 0 and 90 % in the drying chamber. As-prepared powders were then thermally treated at 400 °C to eliminate the organic templating phase. FTIR spectroscopy investigation insured the total thermal degradation of the template (Figure 03).

The particles meso-texturation was deduced from Xray diffraction (XRD) in a θ-2θ geometry (Cu kα, λ = 0,154 nm) (Philips PW 1820) investigations and confirmed on the whole sample by Transmission Electron Microscopy (TEM) (Jéol 100 JX/STEM). The elementary particle size dispersion was estimated from Scanning Electronic Microscopy (SEM) by manual counting. The Porosity was fully characterized by N2adsorption-desorption BET investigations.

Conditions of particle generation

Particles were generated under a homogeneous and constant regime. The Reynold's number (Ret) defines the regime of the particle transport within the system. Ret was calculated from U (average gas velocity which is 2 -3 lt.min -1 ), the tube minimal diameter (dt = 2 cm), the nitrogen gas density (ρ) and The particle generation follows a continuous regime since the Knudsen number has been evaluated to be (0.001 < Kn = 2λ/dp < 0,06; from Ref. 26, p.23), where dp is the experimental particle diameter which decreases from 2 to 0.3 µm by evaporation (in Figure 04 are reported the dried particle size distribution and a typical SEM picture of the particle is additionally provided) and λ is the nitrogen mean free path (λ = (kBT)/(2 1/2 σ 2 π p) ≈ 0,0097 µm, p being the gas pressure inside the pipe of (105 Pa) and σ the nitrogen molecular cross section at room temperature). In the continuous regime, particles are large enough to fully follow the carrying gas motion. Despite these ideal conditions of particle generation, we assume that ~ 40 % of the particles production rate (nominally given at 10 7 particles cm -3 ) are lost within the system as a result of the gravitational force, the hydrophilic nature of the glass pipes, the thermophoresis phenomenon at the oven exit enhanced by the complexity of the piping pathway geometry (abrupt flow direction changes).

Particle size distribution

The initial droplet size distribution and mean average size are strictly bound to the solution viscosity and the atomizer characteristics (pressure, dimension of the membrane hole). Taking into account that the optimal initial solution contains weight ratios of 9 % TEOS, 3 % CTAB, 58 % Ethanol, and 30 % hydrochloric aqueous solution at pH 1,2 (TEOS/CTAB/Water/Ethanol = 1/0,18/40/30 molar ratios), and that the final particles are composed of SiO2 microporous framework (ρSiO2 ≈ 1.8 g.cm -3 ) and CTAB micelles (ρCTAB ≈ 1 g.cm -3 ), the volume fraction of non volatile, condensed matter present in the solution is 4 %. Since Vpart/ Vdrop = 0.04 = dp 3 /dd 3 , then the dp/dd = 0.34 suggests that any droplet is expected to reduce its size of a factor of 0.34 after drying, when keeping its integrity.

Figure 04 exhibits the SEM picture of a typical mesoporous silica powder prepared from a solution with the typical composition given above. The corresponding particle size distribution is also given. The lognormal mean particle diameter was evaluated at dp = 0.29 µm with a standard deviation σg = 1.89 µm [Ref. 26, p.29]. These values suggest that the initial mean particle diameter is 0.29/0.34 = 0.85 µm. The nominal initial droplet diameter is given to be around 2 µm for water solutions. Here, the lower theorical value of 0.85 µm is likely related to the presence of 58 % ethanol which reduces the solution viscosity. However, the value of dp ~ 0.29 µm remains in respect with the nominal characteristics of the our atomizer. 2 show that the highest organization is reached in the central region of the graph, when the intensity is maximized and the FWHM is minimized.

Mesostructure formation (influence of chemical parameters)

The starting solution conditions have been chosen by taking into account the behaviour of TEOS silica precursor in ethanol/water mixtures well known in sol-gel chemistry. From previous studies on films it was demonstrated that optimal conditions of hydrolysis condensation exist to promote well organisation during EISA 21,[START_REF] Innocenzi | [END_REF] . Following these, the initial solution pH was fixed at 1,2 in order to favour high hydrolysis and low condensation rates, which enable highly hydrophilic but small silica entities within the solution during the pulverisation period. The optimal solvent composition and dilution were fixed at TEOS/Water/Ethanol = 1/40/30 (molar ratios) as to minimize the loss of matter into the system and the pulverisation time, and to keep a good compromise between the extent of organisation, solution stability and concentration (based on a trial and error method). These optimal conditions are kept unchanged in the following study.

As already reported for such systems 20,23 , the most important parameter that governs the formed mesophase is the CTAB/SiO2 volume ratio, since a high concentration in surfactant tends to form low curved interfaces while low surfactant concentrations lead to highly curved micelles that promotes extended interface of interaction between both phases 10,11 .

TABLE 1 gives a list of the samples synthesised at different surfactant ratios together with their corresponding structural characterisation deduced from XRD and N2-BET investigations. Figure 05 displays the X-ray diffractions of (a) a typical worm-like structure, and (b) a typical 2D-Hexagonal structure (space group p6m) composed of cylindrical micelles which in the second case are organised into a compact hexagonal array. Peaks have been attributed to the (10), (11) and (20) Miller indices of the hexagonal structures. The compact hexagonal structure is confirmed on the whole particle by TEM as shown by the [001] and [110] view planes of Figures 06 (b,c andd) respectively, where one observes that the elongated micelles tend to align with the spherical particle surface in the hexagonal organisation, while they are randomly oriented in the worm-like structure (Figure 06 (a)). This was already observed in thin films where cylindrical micelles tends to align with both surface and substrate interfaces during evaporation 20 .

A typical CTAB templated N2-BET adsorptiondesorption isotherm is shown in Figure 07 (a). It reveals that the material is highly mesoporous with narrow pore size distribution centred at 2.2 nm in agreement with TEM observations. The Relative pressure (P/P 0 ) silica wall between two pores was estimated to be close to 2 nm (calculated by subtraction of the pore diameter to the inter-planar distance, d10). The porosity was estimated to be above 0.5 cm 3 .g - 1 , and the specific surface area was calculated to be above 1000 m 2 .g -1 . These characteristics are similar to homologue materials prepared by precipitation 1,28 . The increase of the CTAB/TEOS ratio causes a shorter inter-micellar distance associated to a higher porosity and higher specific surface area, because of a larger surfactant volume fraction. The organisation into the p6m structure is verified for CTAB/Si ratios ranging from 0.15 to 0.25 with an apex in the organisation degree located at 0.18 as illustrated in Figure 05. This optimal system will then be used to study the influence of the processing conditions on the organisation degree.

In order to create larger pores, we have used Pluronics as structuring agents and a tri-dimensional micellar Cubic structure was found, as shown by the corresponding TEM image in Figure 06 (e). The N2-BET adsorption-desorption isotherm of the P123 sample, shown in Figure 07 (c), is of type IV showing the presence of a large hysteresis loop of type H1 28 , indicating that capillary condensation and evaporation do not take place at the same pressure, and that bottlenecks between pore arrays are present above the critical point of 0,43 P/P0 (where N2 critical liquefaction can occur from). The isotherm idicates a high pore size uniformity centred at 9,2 nm and a microporous interconnectivity. The specific pore volume was found being 0,69 cm 3 .g -1 the specific surface area was 540 m 2 .g -1 .

Intermediate Brij 58 (pore diameter -dp= 44Å) template gives similar structures with the following porosity characteristics: 0,75 cm 3 .g -1 mesoporosity, and 700 m 2 .g -1 specific surface area. The corresponding isotherm is shown in Figure 07 (b).

Mesostructure formation (influence of processing parameters).

The processing conditions, that is to say the successive residence time of the particles in a certain environment (temperature, humidity) before been collected on the filter, does influence the final morphology and the mesostructural organisation of the micelles. The diffraction diagrams corresponding to various studied conditions are given in Figure 08.

Furnace temperature. The role of the furnace is to allow for the particle to solidify by condensation. As a result of the short period of time in the furnace (typically less than 10s), the heat is mainly located at the particle surface since it is diffusion limited inside the particle. A peripherical condensed layer (outer shell) is then formed by condensation at the periphery. This shell is of outmost importance since it prevents the particles to coalesce when they found themselves in contact on the filter, and allows thus the particles to keep the spherical shape.

Such a coalescence phenomenon occurred for temperatures below 200°C. In terms of micellar organisation, a too high temperature leads to less organised particles. The best organisation (highest and narrowest diffraction peaks) was found by allowing the particles to pass inside a first furnace at 150°C before entering the second furnace at 350°C, suggesting that a progressive heat treatment is favourable to the mesoorganisation. A too high temperature tends to reduce the degree of organisation (see Figure 08), which is attributed to a too extended condensation of the inorganic phase. In view of these results, one may say that the heat treatment is necessary to obtain discrete particles with well organized mesoporosity (low dispersion in pore size distribution). The optimal temperature conditions are the following: a progressive temperature up to 350°C leads to the formation of the hard shell around the particles leaving the inside flexible enough to complete the organisation. The silica condensation inside the particle is naturally achieved by ageing. Humidity. The versatility of the technique allows to change the composition of the carrying gas in the system. Recent studies on similar system as thin films demonstrated the critical role of the water relative pressure in the environment during evaporation 19 . At high humidity, water diffuses inside the system, promoting self assembly of the micelles by adjusting the micellar interfacial interaction with the inorganic matrix. This later wet condition can be applied to the aerosol generation by filling part of the drying chamber with water. In the present conditions of production, a relative humidity of 70 % was measured in the chamber. The corresponding XRD diagram is given in Figure 08 and confirmed the positive role of water on the organisation during EISA, since it provided the most intense diffraction peak.

Conclusion

Preparation of mesoporous silica sub-micro spheres exhibiting high surface area, narrow pore size distribution, and hexagonal organization of cylindrical pores with CTAB or compact body centered cubic organization of spherical pores with copolymer blocks is possible by spray drying. Control on the pore size and organization is achieved by selecting the appropriate surfactant and by adjusting its composition. Also, careful control on the chemical parameters (initial sol composition, and pH) and on the processing parameters (temperature and humidity) is required to promote self-assembly and to retain the spherical shape. The optimal conditions involved pH = 1,2, CTAB/TEOS = 0.18, the presence of water and ethanol in the solution, a progressive heat treatment up to 350°C and a high relative pressure of water in the carrying gas.

Fig. 02

 02 Fig. 02 Scheme of the aerosol apparatus generator; a) N2 gas injection at constant pressure and flow rate; b) initial solution container; c) liquid N2 ethanol trap; d) particles drying chamber; e) furnace at 350°C; f) pparticles membrane filter paper

Fig. 03

 03 Fig.03IR Spectra compared between CTAB 0,18 as made and calcined at 400°C in the region 1500 -4000cm -1 . The high adsorption peaks (calcined sample) in the 1600 cm -1 area are due to water condensation in the empty mesopores.

Fig. 05

 05 Fig. 05 Diffraction pattern of sample mole ratios (a) CTAB/TEOS = 0,10 (worm-like structure) and (b) CTAB/TEOS = 0,18 (P6mm structure). The hexagonal and worm-like micelle models are shown. The interplanar distance d10 is given as well as the cell parameters a and b. Undeneath, diffraction pattern intensities and Full Width at Half Maximum (FWHM) of samples shown in Table2show that the highest organization is reached in the central region of the graph, when the intensity is maximized and the FWHM is minimized.

Fig. 04 Fig. 06

 0406 Fig. 04 Experimental lognormal shaped particle size distribution. The particle diameter count has been manually made on some SEM picture like the one shown in the up right corner. The experimental average particle mean diameter (dp) and the standard deviation (σgn) are given in the box.

Fig. 07

 07 Fig. 07 BET isotherms for a) CTAB sample (type IVc), b) Brij 58 sample (type IV) and for c) P123 sample.

Fig. 08 X

 08 Fig.08X-ray pattern showing the influence of the aerosol pathway temperature and humidity. In the legend, the first temperature refers to the drying chamber, the second one refers to the furnace and the third value refers to the relative humidity percentage in the drying chamber.