Violeta Claudia

Cervantes-Martinez

Mélanie Emo

Maria-José García-Celma

Marie-José Stébé

Jean-Luc Blin
email: jean-luc.blin@univ-lorraine.fr

Morphosynthesis of porous silica from biocompatible templates

Keywords: Biomimetics, Sol-gel, Silica, Controlled porosity, Emulsions, SAXS

Taking inspiration from morphogenesis, synthetic porous silica materials have been synthesized through rational designs combining the sol-gel process and the surfactant templating method. Materials have been prepared from biocompatible systems using kolliphor as surfactant.

The effect of Miglyol and Myristate solubilization in the Kolliphor/water system was first investigated in detail. The phase diagram proved that only a weak fraction of these oils (≈ 1 wt.%) can be incorporated into the hydrophobic core of the micelles (L1). Results also show that whatever the liquid crystal phases [direct hexagonal (H1) and lamellar (Lα)] the swelling effect take place upon addition of oil. Moreover, at low oil concentration oil-in-water fine emulsions are formulated, while at high oil concentration, oil-in-water concentrated emulsion, which are formulated.

Starting from the various systems, porous silica materials were then synthesized. Results obtained by SAXS and nitrogen adsorption-desorption analysis show that Miglyol and Myristate can expand the mesopore size. The variations of the pore diameter have been related to the solubilization properties of the oil in the surfactant. In addition, macroporous silica could be design by using concentrated emulsions as template.

This work demonstrates that synthetic silica materials can be obtained by mimic of natural processes.

Introduction

The interest of designing inorganic materials having a microscopic or nanoscopic size lies on the importance of their shape and their texture compared to the intrinsic properties of the solid, for phenomena such as diffusion, catalytic activity, efficiency in separation and so on.

The topology of the surface, as well as the manufacture of hollow or cellular structures are essential factors in new processes for the design of synthetic inorganic materials, with precise and defined architectures. Material design at the mesoporous scale can mimic natural processes by which, from minerals, microorganisms build their vital elements such as the carapaces of different marine organisms, bones, teeth, etc., and which are also explained on the basis of the concept of "template" [START_REF] Mann | Synthesis of inorganic materials with complex form[END_REF]. The replication phenomena and the specificities observed in the biogenic crystals may provide the means and the molecules that will be used in morphosynthesis. Taking inspiration from Mother Nature, sol-gel chemistry is indeed the perfect chemical tool to generate sophisticated materials. For example, combining the sol-gel process with the surfactant templating mechanism the morphosynthesis can lead to ordered mesostructured silica. Thanks to their properties, the emergence of these materials provides not only a series of novel materials that possess large uniform pore sizes (1.5-10 nm), highly ordered nanochannels, large surface areas (~1000 m 2 /g) [START_REF] Kresge | Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism[END_REF][START_REF] Beck | A new family of mesoporous molecular sieves prepared with liquid crystal templates[END_REF], but also an idea of the design of periodic arrangements of inorganicorganic composite nanoarrays. To prepare these compounds two strategies can be adopted following the example of nature: synergistic synthesis also named the cooperative templating mechanism (CTM) [START_REF] Wan | On the controllable soft-templating approach to mesoporous silicates[END_REF][START_REF] Martens | Simple synthesis recipes of porous materials[END_REF][START_REF] Kresge | The discovery of mesoporous molecular sieves from the twenty year perspective[END_REF][START_REF] Blin | Mechanism of self-assembly in the synthesis of silica mesoporous materials: in situ studies by X-ray and neutron scattering[END_REF]] and the transcriptive synthesis [START_REF] Attard | Liquid-crystalline phases as templates for the synthesis of mesoporous silica[END_REF][START_REF] El-Safty | Controlled design of ordered and disordered pore architectures, geometries, and dimensions of HOM-type mesostructured monoliths and their hydrothermal stabilities[END_REF]Zimny et al., 2012;[START_REF] Veliscek-Carolan | Effects of Precursor Solution Aging and Other Parameters on Synthesis of Ordered Mesoporous Titania Powders[END_REF]. In any case, the characteristics of the resulting materials is strongly related to the physicochemical properties of surfactant, which can be modified by the incorporation of additive. In particular, the introduction of oil into the surfactant-water system can lead to the formation of swollen micelles or microemulsions [START_REF] Tiddy | Surfactant-water liquid crystal phases[END_REF], depending on whether or not an oily core can exist. If the concentration of oil is widely increased the oil-rich-corner of the ternary phase diagram is reached and depending on the properties of the systems and in particular its phase inversion temperature (PIT), concentrated emulsions can be formulated [START_REF] Bleta | Solubilization of various fluorocarbons in a fluorinated surfactant/water system : Relation with the design of porous materials[END_REF]. Indeed, at this temperature the transition from direct systems to reverse ones, where the continuous media is non-polar, occurs. When the value of the PIT is very high, the curvature radius of the surfactant is not changed and the Bancroft rule is then verified. Thus, the aqueous phase is the external one and oil-in-water (o/w) concentrated emulsions, which can contain high quantity of the dispersed phase (oil) can be formulated. Direct emulsions consist of oil droplets dispersed in the aqueous phase, whereas inverse emulsions refer to the dispersion of water droplets in oil.

If the internal phase volume is higher than 74%, the emulsion belongs to the high internal phase emulsion (HIPE) family, also called concentrated emulsions or gel emulsions [START_REF] Princen | Rheology of foams and highly concentrated emulsions. IV. An experimental study of the shear viscosity and yield stress of concentrated emulsions[END_REF][START_REF] Ravey | Water in fluorocarbon gel emulsions : structures and rheology[END_REF][START_REF] Cameron | High internal phase emulsions (HIPEs) structure, properties and use in polymer preparation[END_REF]. As micelles or liquid crystals, these various systems, which appear in the presence of oil, can also be used as templates to design porous materials by morphosynthesis. Depending on the structure of the initial selected surfactant-system either large size mesostructures, macroporous or hierarchical materials are recovered [START_REF] Imhof | Ordered macroporous materials by emulsion templating[END_REF][START_REF] Sayari | Expanding the pore size of MCM-41 silicas: use of amines as expanders in direct and postsynthesis procedures[END_REF][START_REF] Lind | Controlled solubilization of toluene by silicate-catanionic surfactant mesophases as studied by in situ and ex situ XRD[END_REF][START_REF] Zhang | Uniform emulsion-templated silica beads with high pore volume and hierarchical porosity[END_REF][START_REF] Carn | Inorganic monoliths hierarchically textured via concentrated direct emulsion and micellar templates[END_REF][START_REF] Ottaviani | Synthesis of Micelle-Templated Silicas from Cetyltrimethylammonium Bromide/1,3,5-Trimethylbenzene Micelles[END_REF][START_REF] Kruk | Pore size tailoring in large-pore SBA-15 silica synthesized in the presence of hexane[END_REF][START_REF] Zhang | Synthesis of open porous emulsion-templated monoliths using cetyltrimetylammonium bromide[END_REF][START_REF] Zhao | Emulsion-templated porous materials (polyHIPEs) for selective ion and molecular recognition and transport : applications in electrochemical sensing[END_REF]].

Generally, the large pore mesoporous silica are formed from microemulsions and in that case the amount of oil is relatively low. For example, the Meso-Cellular Silica Foams (MCF's) are prepared from oil-in-water microemulsions [START_REF] Schmidt-Winkel | Microemulsion Templating of Siliceous Mesostructured Cellular Foams with Well-Defined Ultralarge Mesopores[END_REF]. Their structure is described as a macroporous network consisting of spherical cells interconnected by windows, whose presence has been evidenced by TEM experiments and theoretical calculations. [START_REF] Lettow | Hexagonal to Mesocellular Foam Phase Transition in Polymer-Templated Mesoporous Silicas[END_REF] Similar structures were obtained by employing the emulsion method, where the role of oil is to act both as a swelling agent for the mesopore size expansion and as a template for the generation of the MCFs [START_REF] Imhof | Ordered macroporous materials by emulsion templating[END_REF][START_REF] Zhang | Synthesis of open porous emulsion-templated monoliths using cetyltrimetylammonium bromide[END_REF][START_REF] Zhao | Emulsion-templated porous materials (polyHIPEs) for selective ion and molecular recognition and transport : applications in electrochemical sensing[END_REF]].

The obtained porous materials have applications in various domains such as electronic, catalysis, adsorption ad so on [START_REF] Park | Mesoporous silica as a natural antimicrobial carrier. Colloids and Surfaces A: Physicochem[END_REF][START_REF] Braganca | Bimetallic Co-Fe nanocrystals deposited on SBA-15 and HMS mesoporous silicas as catalysts for Fischer-Tropsch synthesis[END_REF][START_REF] Perego | Porous materials in catalysis: challenges for mesoporous materials[END_REF]. More recently, interest has been paid to these compounds in drug delivery [START_REF] Arruebo | Drug delivery from structured porous inorganic materials[END_REF][START_REF] Moritz | Mesoporous materials as multifunctional tools in biosciences: Principles and applications[END_REF]. Indeed, they enhance the solubilization of poorly soluble drugs and allow a sustained, controlled or pulsed release [START_REF] Arruebo | Drug delivery from structured porous inorganic materials[END_REF]. In that way, much efforts are devoted to the preparation of these porous silica from biocompatible compounds, i.e. surfactant and oil. However, in the literature this kind of studies is barely reported. In this paper, we have investigated in detail the effect of oil solubilization into Kolliphor EL in relation with the design by morphosynthesis of porous silica. Kolliphor EL is a biocompatible surfactant, which is synthesized from the reaction of castor oil with ethylene oxide in a molar ratio of 1:35.

Materials and methods

Kolliphor EL and isopropyl Myristate (IM) were purchased from Sigma Aldrich and used as received, without any further purification. Miglyol® 812 N (Miglyol), which is a mediumchain triglyceride composed mainly of caprilic and capric fatty acids, was provided by IOI Oleo GmbH-IOI Oleochemical company. Deionized water was obtained using a Milli-Q water purification system. Tetramethoxysilane (TMOS), used as the silica source, was purchased from Sigma Aldrich.

Phase diagrams determination:

Samples were prepared, by weighting the needed amounts of surfactant, water and oil in hermetically sealed glass tubes. The homogenization of the samples was done by mixing with a vortex stirrer. To improve the homogeneity of the samples, they were also centrifuged several times.

The phase diagrams were established by preparing samples over the whole range of surfactant/water and surfactant/water/oil concentrations, and placing them in a water bath at the desired temperature until reaching equilibrium (1 week). Visual observations allowed to determine the mono-, bi-or multiphasic domains and polarized light optical microscopy (Olympus BX 50) was used to identify the different liquid crystals phases. Additional small angle X-ray scattering measurements were performed to establish precisely the phase boundaries, to confirm the nature of the different phases and to determine the space group of the cubic phase.

Emulsion formation process

The water-rich emulsions were formed either by the Phase Inversion Composition (PIC) or the Phase Inversion Temperature (PIT) [START_REF] Solans | Nano-emulsions: Formation by low-energy methods[END_REF]. Those two techniques are based on the capacity of the surfactants to change their curvature, thus modifying the continuous medium. Using the PIT method, emulsions are obtained by increasing the temperature quickly to pass through the PIT. With the PIC method, the mixtures of the oil and surfactant with different oil / surfactant ratios were first prepared. Then, water was added dropwise by using an automatic peristaltic pump which injects the right amount of water in a constant settled rate of 250 µL.s -1 ; the mixture was constantly magnetically stirred at 1250 rpm at 25 °C.

For the oil-rich emulsions, mixtures of water and surfactant were prepared with ratios water/surfactant (R) between 1.9 to 9. Then, the oil is added dropwise to the mixture under continuous stirring. The emulsions with the ratio R equal to 5.7 containing 70, 80, 90 et 95 wt. % of oil were characterized.

Silica materials preparation

To synthesized the mesostructured materials, first the oil and the surfactant were mixed at 1250 rpm at 25°C, then water (pH = 7) was added at a fixed rate of 250 µL.s -1 . The surfactant concentration in water was fixed to 10 wt. % and samples containing 1, 3, 5, 7, 10 and 15 wt. % of Miglyol or Myristate were studied. Then, TMOS was added dropwise to the solutions and the mixtures were stirred at 300 rpm for 1 h. The surfactant / silica molar ratio was fixed to 0.024 Macroporous materials were obtained by the emulsions templating mechanism. Before the incorporation of oil, a micellar solution of Kolliphor containing 15wt.% of surfactant was prepared. The concentration of oil, added to the micellar solution, was made to vary from 70 to 95 wt.%. The pH value of the solutions and the surfactant / silica molar ratio were fixed to 7 and 0.024, respectively. In all cases after preparation the obtained mixtures were placed into sealed Teflon autoclaves at 40°C for 24 h, then at 100°C for 24 h. After the hydrothermal treatment, the materials were transferred into cellulose extraction cartridges to remove the template by Soxhlet ethanol extraction for 48 h. After drying at room temperature during 24 h, samples were thermally treated under synthetic air as follow. A first temperature increase was applied at 2 °C/min until 150 °C with a 1 h plateau, followed by a second temperature ramp at 2 °C/min to reach 350 °C. Temperature was held for 1 h, then, a final temperature ramp at 2 °C/min was imposed to reach 550 °C with a 1 h plateau. The cooling process was uncontrolled and directed by the oven inertia.

Characterization

Small angle X-Ray scattering (SAXS) measurements were carried out on a "SAXSess mc²" instrument (Anton Paar), using line-collimation system. This instrument is attached to a ID 3003 laboratory X-Ray generator (General Electric) equipped with a sealed X-Ray tube (PANalytical, λ Cu, Kα = 0.1542 nm) operating at 40 kV and 50 mA. Each sample was introduced in a "Special Glass" capillary, with a diameter equal to 1.5 mm and 2.0 mm for micellar solutions and liquid crystals, respectively, or between two sheets of Kapton® for materials, then placed inside an evacuated sample chamber and exposed to X-Ray beam.

Scattering of X-Ray beam was recorded on a CCD detector (Princeton Instruments, 2084 x 2084 pixels array with 24 x 24 µm² pixel size, sample-detector distance = 309 mm). Using SAXSQuant software (Anton Paar), the two-dimensional image was integrated into onedimensional scattering intensities I(q) as a function of the magnitude of the scattering vector q = (4π/λ) sin(θ), where 2θ is the total scattering angle. Thanks to a translucent beamstop allowing the measurement of an attenuated primary beam at q = 0, all measured intensities can be calibrated by normalizing the attenuated primary intensity. The data were then corrected for the background scattering from the cell and for slit-smearing effects by a desmearing procedure from SAXSQuant software, using Lake method. For micellar solutions, after correction, obtained intensities are scaled into absolute units using water as a reference material. Nitrogen sorption isotherms were determined on a Micromeritics TRISTAR 3000 equipment at -196 °C over a wide relative pressure range from 0.01 to 0.995. The pore diameter and the pore size distribution were determined by applying the BJH (Barret, Joyner, Halenda) method [START_REF] Barrett | The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms[END_REF] to the adsorption branch of the isotherm. Samples were prepared for Scanning electron microscopy (SEM) by dispersing some material in ethanol.

The experiments were done on HITACHI S-2500 at 15 keV. Intrusion/extrusion mercury porosimetry were measured with a Micromeritics Autopore IV 9500 porosimeter. Dynamic Light Scattering (DLS) experiments were performed with a Malvern 300HSA Zetasizer instrument.

RESULTS AND DISCUSSION

Solubilization of Miglyol or Myristate in the Kolliphor/water system

Ternary phase diagrams

The phase diagram of Kolliphor/Miglyol/water and Kolliphor/Myristate/water at 25°C are reported on Figure 1. A micellar solution is observed for surfactant concentrations until 45 wt.

% but only a few quantities (1wt.%) of Miglyol (Fig. 1A) or Myristate (Fig. 1B) can be solubilized in the micelles of Kolliphor. The cubic phase can incorporate some amounts of oil, up to around 5 wt. % of Miglyol and 8 wt. %, of Myristate. A more significant amount of oil can be accommodated in the hexagonal liquid crystal, up to 20 and 15 wt.% of Miglyol and Myristate, respectively. For water/surfactant (R) ratios between 0.18 and 0.33, a lamellar phase (Lα) appears, but only after the addition of 5 wt. % of Miglyol (Fig. 1A). This lamellar phase is highly stable and it can incorporate until 55 wt. % of Miglyol. In the Kolliphor Myristate/water system (Fig. 1B) this lamellar phase appears after the addition of 4 wt.% of oil and for R ratio between 0.14 and 0.4. In that case up to 70 wt.% of oil can be incorporated in Lα. Two types of direct emulsions can be prepared. On the one hand, by the PIC method, emulsions stable for few days were formed in the water-rich domain (Em) and can incorporated until 15 wt. % of oil (Fig. 1). Depending on the kind of oil and on its quantity the size of the emulsions varies between 20 and 45 nm (Table 1). Thus these emulsions have been labeled as fine emulsions. On the other hand, in the oil-rich domain, high concentrated emulsions (between 70 and 95 wt. % of oil) were highlighted for 1.9 < R < 9. (CE in Fig. 1).

The size of the concentrated emulsions obtained by optical microscopic is around 15 µm and to 12µm for the emulsions containing 90 and 95 wt.% of Miglyol, respectively. These systems are stable for several months.

Effect of oil solubilization on the liquid crystals

SAXS measurements were performed to determine the evolution of the structural parameters of the liquid crystals in function of the addition of oil .

The structural parameters were determined in accordance with the equations reported in the literature [START_REF] Sakya | Micellar cubic phases and their structural relationships: The nonionic surfactant system C12EO12/water[END_REF][START_REF] Alibrahim | Effect of an ionic surfactant on the phase behavior of a nonionic surfactant-based system[END_REF]. For the lamellar phase, the cross sectional area S can be obtained from the the following equation :

S = ( )
The hydrophobic thickness dB and the hydrophilic thickness dA can be deduced from the following equations : Plots of the structural parameters in function of the quantity of incorporated oil for Lα are represented on Figures 2A and2B for Miglyol and Myristate, respectively. Whatever the oil used, the spacing parameter d and the hydrophobic thickness dB increase with the addition of oil in the system, while the area of the polar head S and the hydrophilic thickness dA are constant and equal to 1.1 nm² and 6.5 nm, respectively. This means that the oil probably does not penetrate between the hydrophobic chains, but rather forms an oily film which becomes thicker with the addition of Miglyol or Myristate. Interestingly, by extrapolating the value for β = 0, the hydrophobic thickness for the lamellar phase is comprised between 2.5 and 2.0 nm, depending on the oil used. Given that the dimension of a hydrophobic chain is estimated at 2.5 nm, we can deduce that the alkyl chains in the lamellar phase are folded. It is noted that the thickness of the hydrophobic part is much thicker with isopropyl myristate, about 30 nm against 10 nm with miglyol since the incorporation of this latter can reach 75 wt.%, while the incorporation of Miglyol is limited to 55 wt.%

For the hexagonal phase, the hydrophobic radius RH is related to the distance d associated to the first peak by the following relation :

∝ = √ ²
The cross sectional area S can be deduced as :

S = ( )
The spacing parameter d and the hydrophobic radius RH increase with the addition of oil in the system (Fig. 3), whereas the area of the polar head S equal at 1.1 nm² (Fig. 3) remains constant. This indicates that the oil is located essentially in the hydrophobic core of the cylinders. The value of S is the same than the one obtained for the lamellar phase.

Micellar structure

The effect of the addition of oil on the micelles structure was examined by comparing the SAXS spectrum of the micelles containing 5 wt. % of Kolliphor in water (R = 19) with the one of the swollen micelles with 1 wt. % of Miglyol or Myristate (same R). The appearance of the spectra shown in Figure 4 are different. Indeed, while the micellar solution presents a weak interaction peak at q = 0.78 nm -1 , the spectra of the swollen micelles show a broad peak at q = 0.50 (Fig. 4Aa) and 0.53 nm -1 (Fig. 4Ba) in the presence of Miglyol and Myristate, respectively, which are characteristic of the micellar structure. The data were analyzed by using the Generalized Indirect Fourier Transform (GIFT) method [START_REF] Glatter | Small Angle X-Ray Scattering[END_REF]Fritz and Glatter, 1982]. The obtained pair-distance distribution functions (PDDFs) were represented on Figures 4Ab and4Bb. For all samples, the PDDFs exhibit a pronounced dip which goes to negative values, suggesting that the micelles are inhomogeneous ("core-shell" type particles).

The curve presents also a bell-like shape, characteristic of spherical micelles, with a maximum dimension of about 12.5-13 nm for the micelles and 17.5-18 nm for the swollen micelles with Myristate (Fig. 4Bb). For the swollen micelles with Miglyol (Fig. 4Ab), a slightly elongated tail with a size at about 19-20 nm is observed. In any cases the results are in good agreement with the hydrodynamic diameter found by DLS measurements : 12-15 nm for the micelles, depending on the Kolliphor concentration and ≈ 16 nm for the swollen micelles with 1 wt.% of oil. Moreover, looking at the excess-electron profile on Figures 4Ac and4Bc, the core-shell type is confirmed for all the samples. The hydrophobic radius RH can be determined when Δρ(r) = 0. It is equal to 3.3 nm for the micelles, and around 5.3 nm for the swollen micelles. The micelle size increases of about 2.5 nm when adding oil. This suggests that with the incorporation of small quantities of Miglyol or Myristate, a core of oil is created.

Additionally, the dimension of the alkyl chains in the swollen micelles, corresponding to the lowest value of Δρ(r), can be estimated at about 2.6 (Fig. 4Ac) and 2.3 nm (Fig. 4Bc) in the presence of Miglyol and Myristate, respectively. The value of the alkyl chain in the presence of oil slightly increases from the value found for the sample without oil (1.9-2.0 nm). Given that the dimension of the alkyl chains in Kolliphor is 2.5 nm, the hydrophobic chains are folded in the micelles and the addition of oil induces a slight unfolding of the alkyl chains, which characterizes the existence of a real oily environment in the core of the micelles.

Concentrated emulsions

Highly direct concentrated emulsions were prepared for a ratio R = 5.7 and quantities of oil varying from 70 to 95 wt. %. The systems are stable for several months. These emulsions were observed by optical microscopy and the oil droplets are polydisperse. SAXS experiments were performed and for all the samples, the SAXS spectra present the same behavior (Fig. 5)

At low q-values, the intensity is proportional to q -4 and characterizes the presence of big oil droplets. At higher q-values, a broad peak (q ≈ 0.7nm -1 for Miglyol and q ≈ 0.5nm -1 for Myristate) which becomes less visible for higher concentrations of oil is observed. The position of this broad peak corresponds to the one of the micelles. When miglyol is used as oil (Fig. 5A), an additional very broad peak at 3.5 nm -1 is observed. This peak, which is associated to a repeated distance of 1.8 nm, has the same appearance and the same position than the one on the SAXS spectrum of pure Miglyol. Given that Miglyol is mostly a mixture of triglycerides of capric (C10 :0) and caprylic acids (C8 :0), the dimension of the backbone of the triglycerides can be estimated around 1.7-1.8 nm, which fits to the position of this peak.

Papadimitriou et al. observed the same phenomenon with veiled virgin olive oils, showing a broad peak on SAXS spectra corresponding to the structural organization of triglycerides in the oils [START_REF] Papadimitriou | Structure and Dynamics of Veiled Virgin Olive Oil: Influence of Production Conditions and Relation to its Antioxidant Capacity[END_REF]. Thus, the SAXS spectra of the concentrated emulsions correspond to the sum of the scattering of the big droplets of Miglyol and the scattering of the swollen micelles, completed by the diffraction peak of Miglyol due to the molecular structure of the triglycerides. To summarize, looking at the ternary diagram (Fig. 1A and1B), the structure of the concentrated emulsions are thus big droplets of Miglyol or Myristate stabilized by molecules of Kolliphor and the continuous phase is constituted of swollen micelles of Kolliphor in water. The scattered intensity I(q) of such concentrated emulsions has already been studied before and can be expressed by the following equation [START_REF] Rocca | Mixed concentrated Water/Oil emulsions (Fluorinated/Hydrogenated): Formulation, Properties, and Structural Studies[END_REF] :

( ) = •! " •# $ (%) % & + (1 -* + ) , ( ) (1)
where the first term represents the diffusion due to large droplets and is approximated by Porod-Auvray's law, with Sv the specific surface and Is is a function of the scattering length density Q profile at the water /oil interface and can be noted as :

! ( ) = (-./0 --120 )²

where Qext and Qint represent the scattering length densities of the external and internal phase of the emulsion respectively.

The second term Im can be defined as the contribution of the continuous phase.

According to equation ( 1), the specific area SV can be calculated from the slope of the portion of the curve with a scattering proportional to q -4 . Then, the corresponding radius of oil droplets r can be estimated as :

3 = 4 " ! "
where φ V is the volume fraction of oil.

The results are given in Table 2 for the two higher concentrations of oil. Given that the number of points in the portion of the curve with a scattering proportional to q -4 is limited, the error on the calculation of the radius of the droplets can be important. Consequently, the calculated values given in Table 2 have to be taken as an estimation.

Porous materials

After the determination of the phase behavior, starting from the ternary systems, porous materials have been synthesized. Compounds have been prepared with a 10 wt.% of surfactant in the aqueous mixture, and in the presence of Miglyol and, we can see that mesostructured silica materials are recovered until the addition of 1 wt.% of oil (Figure 6A);

i.e. using the swollen micelles as template. If the Miglyol concentration is increased, that means using the fine emulsions as template, no secondary reflections are detected any longer.

Only a broad peak is observed on the SAXS spectra. The material synthesized with an oil quantity higher than 1% thus adopt a wormhole-like structure. The position of this peak remains around 12.0 nm when the oil concentration is raised from 3 to 7 wt.% (Fig. 6Ac-e).

Beyond 7 wt.% of Miglyol meaning that the channel arrangement is completely random.

Thus, the regular mesopores array is completely lost and in this case, the addition of oil disfavors the formation of mesostructure.

Nitrogen adsorption-desorption isotherms and the corresponding BJH pore size distributions (obtained from an analysis of the adsorption branch of the isotherm) are shown in Figure 7.

According to the IUPAC classification [START_REF] Sing | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984)[END_REF] a type IV isotherm, characteristic of mesoporous materials is obtained for the material prepared with a concentration of oil lower than 7 wt% (Fig. 7Aa-d). When the loading of oil is raised the shape of the isotherm changes and it becomes type II (Fig. 7Ae,f). In addition, with increasing the Miglyol concentration, the relative pressure for which capillary condensation takes place is shifted toward higher values.

Since the p/p0 position of the inflection point is related to the pore diameter, it can be inferred that an enlargement of the mean pore diameter occurs when the loading of oil is raised. This increase in pore diameter is further confirmed by the pore size distribution whose maximum is shifted from 5.7 to 11.1 nm when the content of Miglyol is varied from 0 to 3 wt.% (insert of Fig. 7Ab,c). However, the mesopore size distribution becomes broader. This indicates that the mesopores become less homogeneous. If the Miglyol loading is further increased, the dV/dD values decreases and no significant pore size distribution in the mesopore range is obtained for oil amount higher than 7 wt.% (insert Fig. 7A d-f) Materials obtained from the Kolliphor/Myristate/water system exhibit the same characteristics than the ones synthesized from the Kolliphor/Miglyol/water one. (Fig. 6B and Fig. 7B). The main differences between the two systems are that the increase of the d-spacing with the addition of oil is more significant (10.6 to 16.0 nm when the Myristate amount is varied from 1 to 5 wt.% (Fig. 6Bb-d)] and the complete disorganization of the channel array occurs at a lower oil concentration (after 5 wt.% of oil). In addition, beyond the addition of 1 wt.% of oil, materials prepared in the presence of Myristate have a higher pore diameter (insert of Fig. 7) than the ones obtained using Miglyol as oil. For example, in the presence of 5 wt.% of Myristate the mesopore diameter is 14.4 nm against 9.8 nm when the same amount of miglyol is used to prepare the porous silica.

The specific surface area drops from 930 to around 650 and 600 m²/g when 1 wt.% of Miglyol and Myristate are added, respectively (Fig. 8). Taking into account the error on the measurement, beyond 1 wt.%, higher amount of oil has no effect on the specific surface area, indeed from Figure 8 it can be seen that for concentration of Miglyol or Myristate in the range of 1-15 wt.%, its value remains almost constant upon addition of oil.

At high concentrations of Miglyol (beyond 70 wt.%) the emulsions can be used as templates to prepare macroporous materials. Indeed, as evidenced in Figure 9a-c Consequently, the skeleton density varies from 0.7 to 3.7 g/cm 3 when the Miglyol amount used to prepare the concentrate emulsion is changed from 70 to 95 wt.%. By nitrogen adsorption-desorption analysis, the obtained macroporous silica also present type II isotherm characteristic of macroporous materials, according to the IUPAC classification. Their specific surface area is around 300 m²/g and no pore size distribution is detected in the mesopore range.

Discussion

To explain the variation of pore diameter with the incorporation of oil, we should have a look at the phase diagram. From Figures 1A and1B, it can be seen that very low amount of oil, about 1 wt.% of Miglyol or Myristate can be solubilized in the micelles involving an increase of their size. As a consequence of the swelling of the micelles core, when the porous silica are prepared from the micelles, according to the cooperative templating mechanism (CTM) the mesopore size is enlarged from 5.7 to about 8.5 nm. This mechanism is usually considered in order to explain the pore size expansion of mesoporous materials by adding organic auxiliaries as swelling agents. [START_REF] Boissière | Mechanisms of Pore Size Control in MSU-X Mesoporous Silica[END_REF][START_REF] Jana | Pore size control of mesoporous molecular sieves using different organic auxiliary chemicals[END_REF][START_REF] Ottaviani | Synthesis of Micelle-Templated Silicas from Cetyltrimethylammonium Bromide/1,3,5-Trimethylbenzene Micelles[END_REF][START_REF] Cao | Synthesis of Ultra-Large-Pore SBA-15 Silica with Two-Dimensional Hexagonal Structure Using Triisopropylbenzene As Micelle Expander[END_REF]. The determination of the micellar structure has shown that the size of the micelles is not affected by the type of oil and remains around 17.5-20 nm. This can explain why similar mesopore size is obtained in the presence of Miglyol and Myristate. Spherical micelles are formed with Myristate whereas the latter are slightly elongated when Miglyol is used a soil. As a consequence, the mesopore pore size distribution is slightly broader when this oil is used as expander.

If the oil amount incorporated in the Kolliphor/water system is increased, in the water rich part of the diagram, the fine emulsions are formed until around 10 wt.% of oil for the considered Kolliphor quantity. The oil droplets are stabilized by the surfactant molecules and when the silica precursor is added, hydrogen-bonds between hydrogen atoms of hydrolyzed TMOS and of the oxygen atoms of the oxyethylene group of the surfactant are formed, leading to the formation of a hybrid-silica. After the hydrothermal treatment and surfactant removal the large mesopore silica materials are recovered. Since the size, obtained from DLS measurements, of the oil droplets formed with Myristate (43.2 nm for a solution containing 7 wt.% of Kolliphor) are larger than the ones obtained with Miglyol (35.4 nm for a solution containing 7 wt.% of Kolliphor), the mesoporous materials recovered in the presence of Myristate have a higher diameter than the silica synthesized in the presence of Miglyol.

If more and more oil is added, the oil-rich part of the diagram is reached and the concentrated emulsions with oil droplets having a size is the micrometer range are obtained. The oil droplets stabilized by the surfactant are dispersed in the continuous medium. The mineralization of the concentrated emulsion gives rise to the macroporous silica where the oil droplet is the imprint of the macropore.

Conclusion

In the present paper silica materials with well-defined porosity have been prepared by biomimetics by combining the sol-gel process, which is a powerful chemical tool to design complex materials taking inspiration from Mother Nature, and the surfactant templating method.

First, we have investigated the ability of the Kolliphor/Miglyol/water and Kolliphor/Myristate/water surfactant-based system to be used for the preparation of porous silica by morphosynthesis. For each system, we have delimited the domains of the various phases and determined the structural parameters of the liquid crystals. We have shown that only a low quantity of oil can be incorporated in micelles (L1). Fine emulsions particularly stable which may contain up to 15% of oil have been formulated. Highly direct concentrated emulsions stable for several months were also prepared with a water/Kolliphor ratio = 5.7 and for oil concentrations between 70 and 95 wt. %.

As regards the porous materials syntheses, the incorporation of low fraction of Miglyol or of Myristate leads to the formation of large mesoporous silica templated by swollen micelles or fine emulsions. Results also show that macropores templated by the oil droplets appear if the amount of oil concentration (> 70 wt.%), these macropores are the imprint of the oil droplets of the concentrated emulsions.

This study shows that synthetic mesoporous or macroporous silicas can be prepared from biocompatible compounds by mimic of natural processes. The approach described herein will open a new way to prepare hierarchical meso-macroporous silica or hybrid organic-inorganic materials and straightforward applications toward catalysis, particularly as support where molecular hindrance of big active confined entities as enzymes can be minimized. In addition, regarding the interest of hybrid materials for drug delivery we can assume that without removing organic the hybrid porous silica are excellent candidates for this application. This work is under progress.

Figure caption

  where α stands for the number of molecules of water per molecule of surfactant, β stands for the number of molecules of oil per molecule of surfactant, and VW, VO ,VB, VS, respectively stand for the molar volumes of water, oil, the hydrophobic part of the surfactant, and the surfactant, (VW = 18, VO = 524 (Miglyol® 812N) or 318 (isopropyl myristate), VB = 834 and VS = 2354 cm 3 /mol).

  , macropores having a size in the micrometer range are visible. The presence of macropores in the samples was further confirmed by mercury intrusion experiments (Fig.9d). The macropore size distribution exhibits two main components at 4.1 and 105 µm. The first component corresponds approximately to the size of the pores observed by SEM. It is compatible with the size of the oil droplets of the emulsions determined by SAXS likely arises from their mineralization. We believe that the second peak at around 105 µm due to the interparticle porosity induced by the aggregation of particles. Depending on the oil quantity, the percentage of porosity varies from 84 (70 wt.% of Miglyol) to 96% (95 wt.% of Miglyol).

Figure 1 :

 1 Figure 1 : Ternary phase diagram, at 25 °C, of the Kolliphor EL / Miglyol / water (A)and Kolliphor EL / Myristate / water (B) systems. L1 and L2 denote direct and reverse micellar phase, respectively. I1, H1 and Lα correspond to direct micellar cubic phase, direct hexagonal phase and lamellar phase, respectively.Em and CE correspond to fine emulsion and concentrated emulsion, respectively.

Figure 2 :

 2 Figure 2 : Lamellar liquid crystal : Evolution of the structural parameters in function of β at 25 °C for R = 0.25 for the Kolliphor EL / Miglyol / water (A) and Kolliphor EL / Myristate / water (B) systems. β is the number of molecules of oil per molecule of surfactant.

Figure 3 :

 3 Figure 3 : Hexagonal liquid crystal : Evolution of the structural parameters in function of β at 25 °C for R = 0.43 for the Kolliphor EL / Miglyol / water (A) and Kolliphor EL / Myristate / water (B) systems. β is the number of molecules of oil per molecule of surfactant.

Figure 4 :

 4 Figure 4 : Micellar structure : Experimental (dotted line) and approximated (GIFT) (solid line) SAXS spectra of the samples at R = 19 without oil and with 1wt. % of oil at 25 °C (a); corresponding pair-distance distribution functions (PDDFs) (b) and corresponding excess-electron density profiles (c). Micelles have been obtained from the Kolliphor EL / Miglyol / water (A) and Kolliphor EL / Myristate / water (B) systems. IM corresponds to isopropyl Myristate.

Figure 5 :

 5 Figure 5 : Concentrated emulsions containing various amount of oil : SAXS spectra of the concentrated emulsions at R = 5.7 in log-log representation in absolute units (a); SAXS spectra of the concentrated emulsions and the micelles at R =

Figure 6 :

 6 Figure 6 : Porous materials : SAXS patterns of samples synthesized with a: 0, b : 1, c : 3, d : 5 and e: 7 wt.% of Miglyol (A) or Myristate (B).

Figure 7 :

 7 Figure 7 : Porous materials : adsorption-desorption isotherms and pore size distribution of compounds synthesized with a: 0, b : 1, c : 3, d : 5, e : 7 and f: 10 wt.% of Miglyol (A) or Myristate (B).

Figure 8 :

 8 Figure 8 : Porous materials : Evolution of the specific surface area as a function of the oil amount (wt.%) from 0 to 15 wt.% Figure 9 : Porous materials :SEM images of the macroporous materials prepared with concentrated emulsions containing 80 (a, b) and 95 wt.% (c) of Miglyol with the corresponding macropore size distribution (d) obtained by mercury porosity.

  Figure 1

Table 1 :

 1 Size (nm) of the fine emulsions from DLS measurements. The kolliphor concentration in aqueous solution is 10 wt.%.

	Wt.% of oil	Miglyol	Myristate
	3	21.4	28.7
	5	30.5	39.5
	7	35.4	43.2
	10	41.5	45.8

Table 2 :

 2 Specific area (SV) and radii of oil droplets (r) estimated from SAXS measurements for the direct concentrated emulsions at R = 5.7 at 25°C

			Miglyol			Myristate
	wt. % oil	S V (cm	-1 )	r (µm)	S V (cm	-1 )	r (µm)
	90	2605	10	6300	4.4
	95	3985	7	4300	6.7

Acknowledgements :

Authors would like to thank the IOI Oleo GmbH-IOI Oleochemical company which kindly provides Miglyol® 812 N. Claudia Violeta Cervantes-Martinez also thanks the CONACYT for the financial support of her PhD.