Robert Winkler
email: robert.winkler@cea.fr

Elisa Ré

Guilhem Arrachart
email: guilhem.arrachart@cea.fr

Stéphane Pellet-Rostaing

Impact of the solvent structuring in water/tertbutanol mixtures on the assembly of silica nanoparticles to aerogels

Keywords: nanoparticles, aerogel, sol-gel process, tert-butanol/water, structured solvent, surfactant-less microemulsion

Soft matter structure is a useful tool for the preparation of well-structured inorganic materials.

Here, we report a strategy using a structured solvent based on binary mixtures as directing agent for silica nanoparticles in aerogel elaboration. Binary mixtures involving water/EtOH and water/tert-butanol have been respectively chosen as representatives of unstructured and structured solvents. The systems water/alcohol/TEOS were effectively characterized as surfactant free microemulsions. The enhanced solvent structuring, however, disappears upon the reaction of TEOS and assembly is directed by solvent structuring found in the binary. For the first time, the influence of solvent composition on the sol-gel reaction was investigated with respect to the reaction rate and the structuring behavior thanks to dynamic light scattering (DLS), small and wide angle X-ray scattering (SWAXS) and transmission electron microscopy (TEM) experiments. The silica nanoparticles aggregate in a different manner depending on the solvent composition, which allow changing the morphology, degree of interconnection and surface area of the resulting material. Silica nanoparticles with very high surface area up to 2000 m 2 /g can be obtained by this approach.

Introduction

Silica aerogels are industrially widely applied materials [START_REF] Riffat | A Review of State-of-the-Art Aerogel Applications in Buildings[END_REF][START_REF] Plata | Aerogel-Platform Optical Sensors for Oxygen Gas[END_REF][3][START_REF] Gnade | Low Density, High Porosity Material as Gate Dielectric for Field Emission Device[END_REF] due to their unique properties such as low thermal conductivity, low refractive index optical transmission, low density and high specific surface areas [START_REF] Gurav | Silica Aerogel: Synthesis and Applications[END_REF] . Especially their high porosity and high specific surface area make them a useful choice for many applications like thermal insulation [START_REF] Hu | Hollow-Structured Materials for Thermal Insulation[END_REF] , oil-water separation [START_REF] Shi | Superhydrophobic Silica Aerogels Reinforced with Polyacrylonitrile Fibers for Adsorbing Oil from Water and Oil Mixtures[END_REF] , analytics [START_REF] Kim | Large-Scale Nanoporous Metal-Coated Silica Aerogels for High SERS Effect Improvement[END_REF] and depollution [START_REF] Perdigoto | Application of Hydrophobic Silica Based Aerogels and Xerogels for Removal of Toxic Organic Compounds from Aqueous Solutions[END_REF] . Furthermore, their production is nowadays easily scalable. The crucial factor of aerogel synthesis is the conservation of the wet gel structure during drying. To this end, the capillary forces must be minimized by using a low surface tension solvent during drying like supercritical CO2. [START_REF] Brinker | Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing[END_REF] To create wet gels that exhibit the desired properties, well-reported templating approaches can be used. [START_REF] Huo | Generalized Synthesis of Periodic Surfactant/Inorganic Composite Materials[END_REF][START_REF] Kresge | Ordered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Template Mechanism[END_REF][START_REF] Mihaly | A Systematic Methodology to Design Silica Templates: Silica Microemulsion Formulation and Nanodroplet Type and Size Estimation[END_REF][START_REF] Schmidt-Winkel | Microemulsion Templates for Mesoporous Silica[END_REF][START_REF] Narayan | Mesoporous Silica Nanoparticles: A Comprehensive Review on Synthesis and Recent Advances[END_REF] Also, high internal phase emulsions can lead to foams exhibiting interesting properties. [START_REF] Brun | Hybrid Foams, Colloids and beyond: From Design to Applications[END_REF] Conventionally, large, self-assembling molecules known as surfactants are used because their free energy of structure formation is strongly favorable. 17 However, in a recent study [START_REF] Sun | A Surfactant-Free Microemulsion Consisting of Water, Ethanol, and Dichloromethane and Its Template Effect for Silica Synthesis[END_REF] , the effect of water/ethanol/dichloromethane microemulsion on the ammonia catalyzed silica polymerization was investigated. They found a connection between the type of microemulsion (W/O, bicontinuous or O/W) [START_REF] Esquera | No Title Found[END_REF] and the obtained silica morphology, thus, proving the possibility of templating by surfactant-less microemulsions. In this paper, we investigate the fundamental aspects of using structured solvents in silica aerogel elaboration.

Such structures have been observed in mixtures of small amphiphilic molecules like ethanol (EtOH) or -most prominently-tert-butanol (TBA) and a hydrophilic solvent. [START_REF] Hayashi | Small-Angle X-Ray Scattering Study of Fluctuations in 1-Propanol-Water and 2-Propanol-Water Systems[END_REF][START_REF] Takamuku | Structure of 1-Propanol-Water Mixtures Investigated by Large-Angle X-Ray Scattering Technique[END_REF][START_REF] Nishikawa | Small-Angle X-Ray Scattering Study of Fluctuations in Ethanol and Water Mixtures[END_REF][START_REF] Nishikawa | Temperature Dependence of the Concentration Fluctuation, the Kirkwood-Buff Parameters, and the Correlation Length of Tert-Butyl Alcohol and Water Mixtures Studied by Small-Angle x-Ray Scattering[END_REF][START_REF] Misawa | Mesoscale Structure and Fractal Nature of 1-Propanol Aqueous Solution: A Reverse Monte Carlo Analysis of Small Angle Neutron Scattering Intensity[END_REF][START_REF] Kežić | Aqueous Tert -Butanol Mixtures: A Model for Molecular-Emulsions[END_REF] The addition of hydrophobic molecules like 1-octanol amplifies this structuring. [START_REF] Schöttl | Emergence of Surfactant-Free Micelles from Ternary Solutions[END_REF][START_REF] Klossek | Eco-Solvents--Cluster-Formation, Surfactantless Microemulsions and Facilitated Hydrotropy[END_REF][START_REF] Sedlák | On the Origin of Mesoscale Structures in Aqueous Solutions of Tertiary Butyl Alcohol: The Mystery Resolved[END_REF][START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF] Here, subtle balances between hydration forces, entropy and curvature play important roles in the solvent structuring on the mesoscale. [START_REF] Zemb | How to Explain Microemulsions Formed by Solvent Mixtures without Conventional Surfactants[END_REF][START_REF] Shimizu | Statistical Thermodynamic Foundation for Mesoscale Aggregation in Ternary Mixtures[END_REF][START_REF] Shimizu | Unifying Hydrotropy under Gibbs Phase Rule[END_REF] It is more reasonable to speak of fluctuating hydrophobic and hydrophilic domains instead of defined structures. In spite of their shapeless nature, such compartments show an influence on chemical reactivity. [START_REF] Kononov | Chemical Reactivity and Solution Structure: On the Way to a Paradigm Shift?[END_REF][START_REF] Krickl | A Systematic Study of the Influence of Mesoscale Structuring on the Kinetics of a Chemical Reaction[END_REF][START_REF] Krickl | Enzyme Activity of Horseradish Peroxidase in Surfactant-Free Microemulsions[END_REF][START_REF] Hou | Surfactant-Free Microemulsions[END_REF] Among the studies on chemical reactions in such structured solvents, very few deal with the synthesis of polymers [START_REF] Yan | Modular Construction of Single-Component Polymer Nanocapsules through a One-Step Surfactant-Free Microemulsion Templated Synthesis[END_REF] or the sol-gel process [START_REF] Sun | A Surfactant-Free Microemulsion Consisting of Water, Ethanol, and Dichloromethane and Its Template Effect for Silica Synthesis[END_REF] . Another recent study [START_REF] Lee | Alcohol and Water Free Synthesis of Mesoporous Silica Using Deep Eutectic Solvent as a Template and Solvent and Its Application as a Catalyst Support for Formic Acid Dehydrogenation[END_REF] showed that even the presence of binary mixtures of a deep-eutectic solvent and water is able to influence the silica morphology. A surprising result was reported by Wang et a.l [START_REF] Wang | Wet Chemical Synthesis of Silica Nanosheets via Ethyl Acetate-Mediated Hydrolysis of Silica Precursors and Their Applications[END_REF] where a small amount of a cosolvent is responsible for the formation of silica nanosheets.

In this study, the sol-gel process is performed in a system that contains only the essential components: tetraethyl orthosilicate (TEOS) as Si-source, water for hydrolysis, an alcohol to assure solubilization and HCl as catalyst. TEOS content and HCl/TEOS ratio are kept constant, in order to reduce experimental parameters. To investigate the effect of structuring, like mentioned before, TBA is the ideal candidate. [START_REF] Nishikawa | Temperature Dependence of the Concentration Fluctuation, the Kirkwood-Buff Parameters, and the Correlation Length of Tert-Butyl Alcohol and Water Mixtures Studied by Small-Angle x-Ray Scattering[END_REF] . For comparison, EtOH was chosen as nonstructuring counterpart.

Results and discussion

Phase behavior of the ternary systems

In order to determine the solubility of TEOS in the binary mixtures of alcohol/water and to choose the most suitable sol compositions, first, the phase diagrams of the systems TEOS/alcohol/water were determined. Since TEOS hydrolyzes very fast at low pH, the phase diagram was prepared without catalyst addition. The phase diagrams are presented in Figure 1.

Sample compositions are also displayed. "A" to "G" denote both AC2 and AC4 to GC2 and GC4.

For presentability in Figure 1, the water and the HCl weight fraction in the prepared samples (see Table 2 for compositions) were added to give "wwater" in the diagram. 2 for compositions).

"A" to "G" denote both AC2 and AC4 to GC2 and GC4. For presentability, the water and the HCl weight fraction in the prepared samples (see Table 2 for compositions) were added to give "wwater" in the diagram.

Both phase diagrams show the classical behavior of systems containing two immiscible components, in this case water and TEOS, and a co-solvent, which is completely miscible with the other two. When adding a certain quantity of co-solvent (here: EtOH or TBA) the system can be mixed. The necessary amount to reach the monophasic region depends on the ratio of the immiscible components and the nature of the co-solvent. In addition, for considerations on the solubilization capacity of the co-solvent, a ternary phase diagram in molar fractions is illustrated in the Figure S1 in the Supporting Information. The biphasic area is slightly smaller with TBA than with EtOH. Several reasons for this difference are identified: (i) the nature of the co-solvent, i.e. its polarity or its molecular structure 17,[START_REF] Zemb | How to Explain Microemulsions Formed by Solvent Mixtures without Conventional Surfactants[END_REF][START_REF] Klossek | Eco-Solvents -Cluster-Formation, Surfactantless Microemulsions and Facilitated Hydrotropy[END_REF] , and (ii) the co-solvent-co-solvent and co-solvent-solute interactions. [START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF][START_REF] Shimizu | Hydrotropy: Monomer-Micelle Equilibrium and Minimum Hydrotrope Concentration[END_REF] According to earlier work [START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF] , the high concentration of a solute with weak co-solvent-solute interactions favors solubilization by the integration of the solute in a hydrophobic pseudophase. In our case, the presence of a sufficient amount of TBA leads to a pre-structuring where the hydrophobic TEOS can be solubilized. In the case of EtOH, this pre-structuring does not exist and the solubilization of strongly hydrophobic compounds is less efficient.

Impact of the TEOS addition to the sol structuring

To gain further information on the samples, their structuring on the mesoscale was investigated by SWAXS. The scattering patterns of the samples without HCl just after the addition of the 7.5 w% TEOS and of their associated binary mixtures are shown in Figure 2.

The data in Figure 2 c) were already published earlier. [START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF] The SAXS spectra of samples Fcx to HREF are not displayed in Figure 2 because they are biphasic and, therefore, not comparable to the systems without TEOS. The weight fractions of water of samples ACX were adjusted to 0.1 in binary mixtures and 0.0925 in ternary mixtures to be compared with the data from the literature. 3.

These SWAXS patterns display several results which depend on the solvent. First, at high q (q > 3 nm -1 ) several peaks attributed to the various pair correlations are visible. First, the peak at 20 nm -1 assigned to O-O pair correlations in the H-bonding network of water are visible in all patterns. It shifts slightly to smaller q with increasing alcohol contents. This corresponds to an increase of the distances in real space and may indicate the partial disruption of the H-bonded network. [START_REF] Clark | The Structure of Ambient Water[END_REF] Second, the peaks at 15.5 nm -1 for the samples with EtOH and at 13 nm -1 for the samples with TBA corresponds to characteristics distances of approximately 0.4 nm and 0.5 nm respectively and are assigned to the pair correlations between the alkyl chains. [START_REF] Tomšič | Structural Properties of Pure Simple Alcohols from Ethanol, Propanol, Butanol, Pentanol, to Hexanol: Comparing Monte Carlo Simulations with Experimental SAXS Data[END_REF] Whereas their intensity depends on the alcohol content, their position does not shift significantly with the concentrations. The apparent shift of this peak to higher q in EtOH containing samples is due to the superposition with the peak assigned to O-O pair correlations. The apparent broadening upon addition of TEOS is due to the apparition of Si-Si pair correlations at 9 nm -1 and 16 nm -1 .

In these mixtures, the peaks of TEOS are not clearly visible due to its low concentration. Thus, the positions of these peaks were determined using the SWAXS spectrum of pure TEOS. (see Figure S2 in the Supporting Information). Third, the peaks at 8 nm -1 (0.8 nm in real space) for EtOH and at 7 nm -1 (0.9 nm in real space) for TBA are attributed to the O-O correlation distance of the alcohol -OH groups. [START_REF] Tomšič | Structural Properties of Pure Simple Alcohols from Ethanol, Propanol, Butanol, Pentanol, to Hexanol: Comparing Monte Carlo Simulations with Experimental SAXS Data[END_REF] Accordingly, the peak intensity decreases with lower alcohol contents.

In the low q region of the pattern (q < 3 nm -1 ), significant differences between the two cosolvents can be noticed. In the samples with EtOH, the addition of TEOS, leads to a significant scattering increase presenting various slopes depending on the mixture: For the samples AC2 and BC2 a constant increase (q -1 for AC2 to q -2.4 for BC2) and no plateau are visible. For sample CC2, DC2 and EC2 a plateau is reached. The plateau shifts to lower q values with increasing water contents. In the samples with TBA without TEOS, the scattering intensity increases with the water content from xwater > 0.638. A plateau is visible that shifts to lower q with increasing water content. After the TEOS addition, this scattering intensity increases noticeably whereas the shape remains the same (around one order of magnitude for sample EC4) and the plateau shifts to lower q.

This intensity increase at low q indicates a structuring at the mesoscale (According to IUPAC 2 to 50 nm). 44 In the case of EtOH two different behaviors are observed. For lower water content (AC2 and BC2), the maximum intensity is not inside the observation window indicating organization on a very large scale. The higher slope for BC2 compared to AC2 indicates a structuring change. For CC2 and the samples with higher water contents, the intensity limit is in the observed q-range. This may be explained by the formation of large structures with trace amounts of highly hydrophobic compounds. [START_REF] Sedlák | On the Origin of Mesoscale Structures in Aqueous Solutions of Tertiary Butyl Alcohol: The Mystery Resolved[END_REF] In this case, the highly hydrophobic compounds may be the polycondensed oligomers of TEOS which are formed in the commercial product after the first opening due to air humidity. [START_REF] Pelster | Monitoring Temporal Evolution of Silicate Species during Hydrolysis and Condensation of Silicates Using Mass Spectrometry[END_REF] Thus, the excess scattering at low-q for samples AC2 and BC2is assumed to be caused by large shapeless aggregates. When adding more water, these aggregates get increasingly constrained by the H-bonding network of water formed around them (BC2) until reaching objects of less than 30 nm in size (CC2 to EC2). In the binary system TBA/water, organization is due to rapidly fluctuating water-or TBA rich aggregates. [START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF] This is typical for short chain alcohol/water mixtures. [START_REF] Hayashi | Small-Angle X-Ray Scattering Study of Fluctuations in 1-Propanol-Water and 2-Propanol-Water Systems[END_REF][START_REF] Takamuku | Structure of 1-Propanol-Water Mixtures Investigated by Large-Angle X-Ray Scattering Technique[END_REF][START_REF] Nishikawa | Small-Angle X-Ray Scattering Study of Fluctuations in Ethanol and Water Mixtures[END_REF][START_REF] Nishikawa | Temperature Dependence of the Concentration Fluctuation, the Kirkwood-Buff Parameters, and the Correlation Length of Tert-Butyl Alcohol and Water Mixtures Studied by Small-Angle x-Ray Scattering[END_REF][START_REF] Misawa | Mesoscale Structure and Fractal Nature of 1-Propanol Aqueous Solution: A Reverse Monte Carlo Analysis of Small Angle Neutron Scattering Intensity[END_REF] The visibility of these aggregates on SAXS patterns depends on their size and on their electronic density contrast to the rest of the solution. In EtOH/water mixtures, for example, the aggregates are too small and do not have a sufficiently high contrast to their surroundings to be visible. [START_REF] Nishikawa | Small-Angle X-Ray Scattering Study of Fluctuations in Ethanol and Water Mixtures[END_REF] When adding the hydrophobic TEOS a transition to a "pre-Ouzo" system is observed. Depending on the type and the weight percentage of co-solvent this leads to an enhanced structuring. [START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF][START_REF] Zemb | How to Explain Microemulsions Formed by Solvent Mixtures without Conventional Surfactants[END_REF][START_REF] Diat | Octanol-Rich and Water-Rich Domains in Dynamic Equilibrium in the Pre-Ouzo Region of Ternary Systems Containing a Hydrotrope[END_REF] In the case of TBA, the preformed structures are swollen by TEOS and this is visible on SAXS pattern by an increase of the intensity and the plateau shift to smaller q. This is due to a higher solubility of TEOS in the TBA-rich domains than in the water-rich domains Samples which do not present any scattering intensity at low q in the binary mixtures remain unchanged upon addition of TEOS.

A size analysis of the aggregates in the structured systems gives insight on its morphology. Characteristic sizes were evaluated by the fitting of SAXS data. Classically, the SAXS patterns of surfactant-free microemulsions can be fitted using then Ornstein-Zernicke (OZ) function.

The samples DC4 and EC4 and their corresponding binary mixtures show good agreement with the fit. In the samples without TEOS correlation lengths of 0.65 nm for the binary mixture corresponding to DC4 and 0.77 nm for the binary mixture corresponding to EC4 are obtained. This is in agreement with literature. [START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF] In the samples with TEOS, correlation lengths of 0.88 nm for DC4 and 2.82 nm for EC4 have been found. Supposing spherical aggregates, the respective aggregate dimensions are 3.52 nm and 11.23 nm. The samples CC2, DC2 and EC2 are fitted increasingly worse using the OZ function because the slopes at low-q are too steep. However, approximatively, correlation lengths between 1.8 nm (CC2) and 2.5 (EC2) are obtained. The results are in agreement with an O/W microemulsion. The addition of TEOS enhances structuring by (I) swelling the existing structure in the case of TBA or (II) inciting the aggregation of the co-solvent around it in the case of EtOH. When decreasing the alcohol/water ratio (i.e. DC4→EC4), the system will minimize the interphase formed by the alcohol. This is done by increasing the aggregate size. The results agree with the literature for similar systems.

A swollen network of H-bonded groups is probably at the origin of the structuring. [START_REF] Schöttl | Combined Molecular Dynamics (MD) and Small Angle Scattering (SAS) Analysis of Organization on a Nanometer-Scale in Ternary Solvent Solutions Containing a Hydrotrope[END_REF] For ternary mixtures with low hydrophobic phase content close to the demixing line, structures resembling O/W aggregates were proposed. [START_REF] Lopian | Morphologies Observed in Ultraflexible Microemulsions with and without the Presence of a Strong Acid[END_REF] To summarize, ternary mixture TEOS/alcohol/water (where "alcohol" can be EtOH or TBA) are typical surfactant-free microemulsions. The components take their roles as hydrophilic component (water), hydrophobic component (TEOS) and co-solvent or -more generallyhydrotrope (EtOH or TBA). The organization of components at the mesoscale increases when approaching the phase boundary. [START_REF] Klossek | Eco-Solvents--Cluster-Formation, Surfactantless Microemulsions and Facilitated Hydrotropy[END_REF][START_REF] Klossek | Structure and Solubility in Surfactant-Free Microemulsions[END_REF][START_REF] Fischer | Toward Surfactant-Free and Water-Free Microemulsions[END_REF] The type and the extend of structuring depends on the co-solvent. When using TBA, the aggregation is heavily influenced by structuring which is already present in the binary mixture TBA/water. They will allow the TEOS solubilization.

With EtOH, structures which allow solubilization of TEOS are formed only in the ternary system. [START_REF] Klossek | Eco-Solvents--Cluster-Formation, Surfactantless Microemulsions and Facilitated Hydrotropy[END_REF][START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF]

Study of the sol-gel transition

After the addition of HCl used as a catalyzer to initiate the TEOS hydrolysis, the durations before gelification at 50°C were determined by naked eye for both series. To confirm the results obtained by these observations for the TBA series and to reduce the error margins, gel time determination was also performed by DLS. Upon TEOS addition, a slight heating was noticed that indicates the start of hydrolysis. The results are presented in Figure 3. Comparing the results obtained by observation and DLS, overall shorter times were observed with the DLS method. However, all trends remain consistent. First, the gel times of the sols with EtOH at xwater < 0.7 are three times longer than the ones with TBA. This difference is reduced when the water content increases. Second, whatever the solvent, the gel time globally decreases with increasing water content. This evolution follows three stages. Up to xwater = 0.5, the gel time decreases almost linearly with the water content. For xwater higher than 0.5, the gel time decrease follows two different behaviors depending on the solvent. For sols containing EtOH, the gel time decreases linearly from xwater = 0.5 to xwater = 0.8 with a lower slope than at lower water contents. For sols containing TBA, the gel time evolution between xwater = 0.5 and xwater = 0.8 presents a minimum for the sample DC4 (xwater = 0.677). Finally, at high water contents (xwater > 0.9 for TBA and xwater > 0.8 for EtOH), a strong decrease of gel time is observed. Gel-time depends on the volume fraction occupied by the growing aggregate.

Consequently, the aggregate structure has an important influence.

To investigate the morphology of the growing structures with the gel time, the structure evolution was followed by SAXS. The SAXS pattern evolution of the two sols CC4 and CC2 were chosen to illustrate the behavior and are presented in As discussed earlier, immediately after TEOS addition, the observed scattering originates from the mesoscale organization of unreacted components (t0 in Figure 4 a)). During the polymerization, the low q scattering develops the same way for all sols. Three aspects should be highlighted. First, all SAXS patterns present a slope proportional to ~ q -2.1 (Figure 4 a)). A plateau marks the end of this slope at low q. The evolution rate of this plateau is proportional to the gel time as highlighted by the scattering patterns of CC4 8h and CC2 24h recorded at similar points during the polymerization ( 𝑡 𝑡 𝐺 ≈ 0.12). During polymerization the plateau shifts to increasingly low q and higher intensities. Finally, after 𝑡 𝑡 𝐺 ≈ 0.3 no plateau is observable.

Second, the slope is proportional to ~ q -2.1 giving evidence of the nature of polymerization.

Third, the initial organization of the water and the alcohol is not visible anymore after TEOS addition probably because its scattering is hidden under that of the developing silica polymer.

The evolution of the low-q scattering pattern is characteristic for objects that primarily increase in size and not in number. [START_REF] Brinker | Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing[END_REF] Logically, the disappearance of the plateau after 𝑡 𝑡 𝐺 ≈ 0.3 indicates that the correlation lengths become too large for the observation window. In Figure 4 b), the scattering intensities of selected samples at q = 0.3 nm -1 as a function of time are plotted. For all samples, like mentioned before, an increasing intensity is observed at longer times. The intensity increase is accelerated at higher water contents. The acceleration is directly correlated to the gel time of the samples. The shorter the gel times the faster the intensity increases. This is also observed for the development of Si-O-Si bonds as observed in the FTIR spectra (see Figure S3 in the Supporting Information).

The constant slope of ~ q -2.1 is characteristic of the formation of fractal objects via a reaction limited cluster-cluster growth (RLCA) or a diffusion limited cluster-cluster growth (DLCA). [START_REF] Brinker | Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing[END_REF][START_REF] Meakin | The Effects of Restructuring on the Geometry of Clusters Formed by Diffusion-limited, Ballistic, and Reaction-limited Cluster-cluster Aggregation[END_REF][START_REF] Ralph | The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica[END_REF] Evidence has shown that rearrangements can lead to the same fractal dimensions. [START_REF] Aubert | Restructuring of Colloidal Silica Aggregates[END_REF] From the SAXS spectra we conclude that the polymerization rate is proportional to the gel time observed for the sols. Polymerization occurs via cluster-cluster aggregation of primary particles that are formed early in the process. The structural evolution is identical for all samples.

With the obtained structural information, the two different behaviors with EtOH and TBA in the region of samples ACX to FCX can be explained by several experimental factors. In order to simplify the explanation, it is assumed that hydrolysis was very advanced in all samples after several minutes and, thus, does not contribute to the gel time. This last assumption is reported in literature [START_REF] Kelts | Sol-Gel Chemistry Studied by 1H and 29Si Nuclear Magnetic Resonance[END_REF][START_REF] Devreux | Sol-Gel Condensation of Rapidly Hydrolyzed Silicon Alkoxides: A Joint Si29 NMR and Small-Angle x-Ray Scattering Study[END_REF] and supported by the fact that under acidic conditions, the hydrolysis is generally faster than condensation and the high [H2O]/[TEOS] ratio ([H2O]/[TEOS] = 15.7 for samples ACX) increases that effect. [START_REF] Brinker | Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing[END_REF] This is also visible in the FTIR spectra of all samples which display a peak at 950 cm -1 corresponding to the ≡Si-O stretching of non-hydrolyzed TEOS only just after the TEOS addition. Moreover, the initially biphasic sample HREF becomes monophasic after 3 min of vortexing due to the increasing hydrophilicity of hydrolyzed species and the production of solubilizing EtOH during the reaction.

The first factor which affects the condensation rate is the concentration of catalytic species.

A higher concentration increases the hydrolysis/condensation rate. Here, the catalysis occurs via the protonation of silica species and is proportional to the H3O + concentration. [START_REF] Ralph | The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica[END_REF] The qualitative inspection of FTIR spectra (see Figure S4 in the Supporting Information) recorded along the polymerization showed that, indeed, the concentration of H3O + increases with increasing water content. This is accordance with the observed trends.

Second, the re-esterification of hydrolyzed species may also impact the gel time. Indeed, partial re-esterification (RE) of hydrolyzed species is possible due to the reversibility of the hydrolysis reaction and can occur with reactive alkoxy species. RE is also favored under acidic conditions. [START_REF] Brinker | Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing[END_REF] TBA is too unreactive whereas EtOH is sufficiently reactive for this process to be considered. [START_REF] Lim | Precise Size-Control of Silica Nanoparticles via Alkoxy Exchange Equilibrium of Tetraethyl Orthosilicate (TEOS) in the Mixed Alcohol Solution[END_REF][START_REF] Chen | Sol-Gel Processing of Silica[END_REF] For our two systems, due to the high concentrations of EtOH compared to the strong dilution of silicate species, it is probable that RE leads to an overall decrease of silicate species reactivity. This effect is more pronounced at higher alcohol concentrations. In mixtures with high water content xwater > 0.8 this effect vanishes.

Third, the reaction rate is influenced by the repulsive forces between the charged primary particles and their diffusion. This is described by the aggregation rate constant k, as described in the Supporting Information. The values were calculated from the primary particle radii from the SAXS measurements (Table 1) and the dielectric constants (see Figure S5 in the Supporting Information) and the viscosities (see Figure S6 in the Supporting Information) from literature.

Overall, the estimated reaction rate increases at increasing water concentration (see Figure S7 in the Supporting Information). For one, the viscosity decreases at increasing water concentration. Thus, the diffusion increases and objects potentially encounter each other more frequently. The encounters are moderated by repulsive forces between the objects. The increasing dielectric constant at increasing water concentrations attenuates the repulsive forces and the reaction rate accelerates. While most of the gel time tendencies can be explained by these factors, the minimum of gel time for the sample DC4 cannot be explained by any of the mentioned factors because they describe continuous trends. A hypothesis allowing the explanation of this behavior will be proposed in the discussion part.

Influence of the sol composition on material structure

In order to obtain aerogels, first, a solvent exchange with EtOH was performed to induce the re-esterification of the materials surface making it more hydrophobic. [START_REF] He | Modified Aging Process for Silica Aerogel[END_REF][START_REF] Chou | Solvent Effect on Ageing of Silica Gels[END_REF] This re-esterification reduces the capillary forces during solvent removal and, thus, minimizes the material modification during drying. Second, another solvent exchange with acetone and SC CO2 was Except for the samples DC4 and EC4, and whatever the solvent, aerogels have a similar SBET of as around 1000 m 2 /g. For aerogels obtained from sols DC4 and EC4 SBET of 1400 m 2 /g and 2100 m 2 /g are obtained respectively. This result can be related to the particular behavior in the sol-gel process already observed. The solvent of the samples DC4 and EC4 showed the most pronounced scattering.(see Figure 2 a)) Furthermore, the samples DC4 and EC4 displayed unexpectedly low gel times.(see Figure 3 a))

To investigate the morphology at a scale of 1 to 30 nm SAXS was performed (see Figure S8 in the Supporting Information). All samples showed the same scattering intensity evolution characteristic for aerogels produced from one-step acid catalysis. [START_REF] Marlière | Two Fractal Structures in Aerogel[END_REF][START_REF] Schaefer | Fractal Aspects of Ceramic Synthesis[END_REF] The fractal dimension increases from -2.15 (see Figure 4 a)) to values around -2.7 (see Table S1 in the Supporting Information). From this, a typical porosity shrinkage at small size scales can be concluded. [START_REF] Brinker | Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing[END_REF][START_REF] Hench | The Sol-Gel Process[END_REF][START_REF] Davis | Pore Structure Evolution in Silica Gel during Aging/Drying II. Effect of Pore Fluids[END_REF] The radii of the primary particles (r0) that form the network were determined by fitting the SAXS patterns (see SI for complete results). The results are shown in Table 1. The sizes of primary particles can be separated in two domains: In the first, at higher water contents, values between 0.23 and 0.3 nm are obtained. In EtOH containing samples, this is the case at xwater > 0.58 (DC2). Whereas in TBA containing samples, this is the case at xwater > 0.757 (EC4). In the second, at lower water contents, the particle size evolution depends on the alcohol.

In aerogels produced using EtOH as co-solvent, the radii of primary particles decrease continuously until DC2. When using TBA as a co-solvent, r0 increases up to a maximum at sample DC4. The evolution of the primary particle size when using EtOH as a co-solvent can be explained by the experimental factors mentioned above. Since primary particles are formed in the first instants of the reaction, their size is largely controlled by the hydrolysis reaction. At higher alcohol content, the catalytic activity is lowered. Lower catalytic activity leads to slower hydrolysis. When the hydrolysis is slower, the nucleation is slower and the nuclei formed can grow to larger sizes before the TEOS is completely consumed. At xwater > 0.58, the critical size for the nuclei under these conditions is obtained. When using TBA as co-solvent, the water content where the largest primary particles are found (DC4) can be correlated to the structuring in the binary water/TBA system. [START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF][START_REF] Krickl | A Systematic Study of the Influence of Mesoscale Structuring on the Kinetics of a Chemical Reaction[END_REF] Here, an inverse structuring with water-rich domains in a TBA-rich phase was proposed. Since the primary particles grow in the aqueous phase, these water-rich domains can serve as microreactors. Inside, the particle growth is controlled by a lower number of nuclei. Containing the same amount of TEOS, this leads to larger particles.

When entering the bicontinious region at higher water contents, a change to nuclei size-limited behavior is observed.

The samples were also characterized by TEM. (Figure 6) The enlarged images can be found in the SI. The TEM images show the typical granular structure of aerogels consisting in nanoparticles with radii of 0.2 to 0.8 nm. This agrees well with the sizes obtained by SAXS where radii of 0.23 and 0.36 have been obtained. The structure at the scale of tens of nanometers varies between the samples. Samples where the solvent shows no excess scattering at low q are densely aggregated clusters (see Figure 6 a) -e) and h). Aerogels that were produced in solvents where excess scattering at low q has been observed show an open network at the mesoscale (see Figure 6 f) and g)). This coincides with the samples that display increased specific surface areas. It is important to distinguish the porosity observed in samples DC4 and EC4 from the openings observable in the other samples. In DC4 and EC4 the surroundings of pores are like elongated bridges and flat surfaces. The openings observed in the other samples show the roughness at the size scale of primary particles and, therefore, merely represent holes due to sample preparation. Thus, TEM images indicate that the particular behavior observed for the sample DC4 and EC4 originates from structural differences.

Discussion

Binary mixtures of water and alcohol show interesting properties for the application as solvents in the sol-gel process. In the present comparison of EtOH and TBA, the typical formation of a surfactant-free microemulsion in the ternary mixture of water/alcohol/TEOS was observed.

This mesoscale structuring, induced by the addition of hydrophobic TEOS, however, can be considered unimportant for the sol gel process at low pH. The reason is the rapid hydrolysis of TEOS in the examined conditions which leads to the elimination of the hydrophobic compound.

Then, the resulting hydrolyzed silicate species react to form sub-nanometer sized primary particles. By further Si-O-Si bond formation between the particles, an open silica network with fractal character is formed. The ratio of water/alcohol or the choice of alcohol does not change the mechanism of polymerization allowing the direct comparison. The network formation proceeds at different rates depending on the water/alcohol ratio which was observed by SAXS, FTIR and gel time measurements. As discussed, several experimental factors are responsible for the different rates. Investigations on the aerogels obtained after drying by SC CO2, allow the conclusion that, indeed, a network of primary particles was formed during the sol gel process. The aerogels displayed typical properties concerning specific surface areas and SAXS patterns and a typical morphology. However, two samples from the TBA-series display exceptional features that will be discussed in the following. The hypothesis on the mechanism is depicted in Figure 7 in which the water content increases along the axis. The images illustrate the solvent behavior around the silica polymer. Light blue is water-rich phase, dark blue the hydration layer around silica and green the TBA-rich phase. In the picture illustrating pure water and the mixture 25 w% water in TBA (or TBA in water) the solution is considered as an homogeneous mixture. At the 50/50 mixture the solvent is considered heterogeneous. Here, it creates voids in the silica matrix after the sol-gel process.

Specific case of the CC4 and DC4 samples: impact of the solvent structuring

All the results obtained for the samples, n(TBA)/n(water) of 0.404 (DC4) and 0.264 (EC4), suggest that compartmentation into water-and TBA-rich domains in mixed solvents is responsible for the gel time and the high specific surface area through the modification of the immediate environment of the growing silica polymer. Those ratios n(TBA)/n(water) are similar to the ratios in the TBA/water binary mixtures for which structuring has been observed. [START_REF] Kežić | Aqueous Tert -Butanol Mixtures: A Model for Molecular-Emulsions[END_REF][START_REF] Krickl | A Systematic Study of the Influence of Mesoscale Structuring on the Kinetics of a Chemical Reaction[END_REF] However, this structuring is different from the effect of structuring due to the templating formed by the aggregation of large amphiphilic molecules. The free energy gain of structure formation in the binary mixtures is very little. Therefore, the introduction of highly hydrophilic silica will influence the morphology of separated TBA-rich and water-rich domains instead of forming silica around existing domains. In this case, water-rich domains cover the surface of the growing silica polymer while TBA-rich domains are not in contact with silica.

(see Figure 7) The reason is the positive surface charge on the silica surface that exists at the high acid concentrations (≈ 1mol/L) in our experiments [START_REF] Duval | Evidence of the Existence of Three Types of Species at the Quartz-Aqueous Solution Interface at PH 0-10: XPS Surface Group Quantification and Surface Complexation Modeling[END_REF] . The resulting counter-ion layer of solvated chloride ions supports the hydrophilicity. Thus, the immediate environment of the silica polymer can be considered as a bulk water phase above xwater ≈ 0.5. As visible by the highest reaction rate observed for HREF, a solution containing more water has an accelerating effect. The accelerating effect is most pronounced at sample DC4 because the catalyst concentration in the water-rich domain is the highest. When adding more water, the catalyst in the water-rich domain gets diluted. This modification of the immediate environment appears to have an important effect on the specific surface area. The TBA-rich domains, although strongly fluctuating, are volumes that are excluded for the growing silica polymer. The excluded volumes will lead to pores in the aerogel (see Figure 7) which ultimately lead to unique structural features like observed in the TEM images. The size discrepancy between the solvent structuring in the binary case water/TBA and the observed morphological features is well documented in literature. [START_REF] Pileni | The Role of Soft Colloidal Templates in Controlling the Size and Shape of Inorganic Nanocrystals[END_REF][START_REF] Richard | Nanoparticle Size Control in Microemulsion Synthesis[END_REF] These excluded volume pores connect the typical microporosity.

In the case of unstructured solvents, the microporosity is inaccessible to N2 for measurement.

When mesoscale structuring is present, however, microporosity is accessible and leads to a significant increase of specific surface area.

Conclusion

In this work, the influence of a structured solvent on the kinetics and the resulting morphology of the sol-gel process was investigated. The results are promising for applications in several domains where high specific surface areas are important. [START_REF] Hu | Hollow-Structured Materials for Thermal Insulation[END_REF][START_REF] Shi | Superhydrophobic Silica Aerogels Reinforced with Polyacrylonitrile Fibers for Adsorbing Oil from Water and Oil Mixtures[END_REF][START_REF] Kim | Large-Scale Nanoporous Metal-Coated Silica Aerogels for High SERS Effect Improvement[END_REF][START_REF] Perdigoto | Application of Hydrophobic Silica Based Aerogels and Xerogels for Removal of Toxic Organic Compounds from Aqueous Solutions[END_REF] For the investigation, binary mixtures of water/EtOH and water/tert-butanol were chosen as representatives of unstructured (EtOH containing) and structured (tert-butanol containing) solvents. Upon the addition of hydrophobic TEOS, enhanced structuring was observed for several samples, which agrees with literature. [START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF] After the rapid hydrolysis of TEOS (< 3 min) which was at the origin of the enhanced solvent structuration the solvents are assumed to return to their state in the binary mixtures. Here, the reaction conditions and structuring on the mesoscale determine the size of primary particles. Small silica particles (radii between 0.2 and 0.5 nm°) then react with each other to form a fractal network. Discontinuity of the reaction rate between the samples was observed in sols containing the structured solvent (tert-butanol) by observation, DLS, FTIR and SAXS. Furthermore, the aerogels obtained by drying with SC CO2 display noticeable morphological features and specific surface areas when resulting from structured solvents.

These criteria lead to the conclusion that the solvent structuring in the immediate environment of the silica particles is responsible for the behavior observed. A hypothesis of the influence of the solvent structuring on the polymerization mechanism is proposed where the silica particles aggregate in a different manner depending on the solvent composition in their close surrounding. In structured solvents water-rich domains cover the silica particles. This leads to reaction conditions that are similar to those in pure water while tert-butanol-rich domains form regions were polymerization does not take place.

Experimental

Chemicals

Ethanol absolute anhydrous (p.a.), acetone (technical grade) and 37 w% HCL solution (p.a.)

were purchased from Carlo Erba Reagents (Val de Reuil, France). Tert-butanol (p.a., ≥ 98 %) was purchased from Sigma-Aldrich (Darmstadt, Germany). Tetraethyl orthosilicate (TEOS) (≥ 98 %) was purchased from Acros Organics (Geel, Belgium). Chemicals were used without further purification. Aqueous solutions were prepared using Milli-Q water.

Ternary phase diagrams

Ternary phase diagrams were determined at room temperature using a static and dynamic process as described in literature. [START_REF] Buchecker | The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilisation of a Third Hydrophobic Component[END_REF] For this purpose, binary mixtures of water/alcohol and alcohol/TEOS were prepared. The third component was added dropwise and the tube was stirred after every addition. The composition at the moment of demixing was noted as the phase boundary. After every addition, the tube was closed and then stirred. To limit the TEOS hydrolysis, ultrapure water was used.

Material preparation

From the ternary diagrams, several water/alcohol ratios were selected and are referred Acx to Gcx with X=2 for ethanol and X=4 for tert-butanol (Table 2). For the sample HREF no alcohol was added. The materials were prepared using an acid catalyzed sol-gel process. Ethanol or TBA, water and 37 w% HCl were mixed in the appropriate ratios indicated in Table 2. After stirring, TEOS was added and the container was closed. . Biphasic samples were vortexed until complete mixing. The longest time required to obtain a monophasic sample was 3 min for HREF (Table 2). Afterwards, the sols were kept in an oven at 50°C until two days after gelification.

Then, the gels were transferred into 100 mL of ethanol and aged for one day at room temperature. Next, ethanol was decanted from the transparent gels and 100 mL of acetone were added. After ageing for 24h, the decanted gels were dried using supercritical (SC) CO2 process.

This was performed with a SEPAREX supercritical fluid extractor using a SC CO2 flow of 30 g/min, at 100 bar and 60°C during 3 hours. The obtained white powders were kept in the glove box to prevent degradation. The sample references and compositions are presented in the Table 2.

Characterization techniques

Gel times were determined by the naked eye and using dynamic light scattering (DLS) at 50°C on a Zetasizer Nano ZS (Malvern Panalytical Ltd., United Kingdom) with automatic attenuation in silicone glue sealed 10 mm polystyrene cuvettes. Acquisition was performed at 173° for 5 runs of 20 s each every hour and data were treated using a method adapted from literature. [START_REF] Matsunaga | Gel Point Determination of Gelatin Hydrogels by Dynamic Light Scattering and Rheological Measurements[END_REF] The time of the last correlation function consisting of a single monomodal exponential function was chosen as gel-time.

Sols were analyzed using Fourier Transformed Infrared spectroscopy (FTIR) with a Perkin Elmer Spectrum 100 spectrometer in Attenuated Total Reflection (ATR) mode equipped with a DTGS/KBR detector. The samples were placed at the surface of the diamond in a PTFE setup to avoid solvent evaporation. The spectra were recorded from 380 to 4000 cm -1 adding 4 scans with a 4 cm -1 of resolution correcting from the background spectrum for each substrate.

Baseline adjustments and peak decomposition were performed using the Origin software.

Small and Wide Angle X-ray Scattering (SWAXS) analysis were carried out in transmission geometry with a Xenocs setup equipped with a Mo anode (λ = 0.71 Å) using a MAR345 2D imaging plate detector. Such short wavelength allows to cover a scattering range for standard SWAXS that extends to large wave vectors in order to obtain information at a small scale down to few angstroms typically a wavenumber q ranged from 0.2 nm -1 to 30 nm -1 . The collimation was ensured by a Fox2D multilayer mirror and by a set of scatterless slits that delimitated the beam to a square section (0.8 mm side length at the sample position). The distance from the sample to the detector was about 750 mm and was calibrated using silver behenate powder.

Sols were analyzed in glass capillaries of 2 mm of diameter. Azimuthal averaging of 2D-data recorded by a MAR345 imaging plate detector was performed using the FIT2D software taking into account the electronic background of the detector, the empty cell subtraction and an intensity calibration. The scattered intensity in absolute scale (in cm -1 ) was expressed versus the magnitude of the scattering vector q = (4πsinθ)/λ, where θ was the scattering angle.

Experimental resolution was Δq/q = 0.02. Data was fitted using the SASfit software.

Nitrogen adsorption-desorption analyses were carried out using a Micromeritics apparatus (ASAP 2020). Before analysis, all samples were outgassed at 60 °C during 24 h under high vacuum (10 -5 Pa). Specific surface area was obtained using the Brunauer-Emmett-Teller (BET) model.

For transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) measurement, one or more drops of the dilute solution of dried aerogels in ethanol are deposited on the amorphous carbon film. High-resolution transmission electron microscopic measurements were carried out using a JEOL 2200FS microscope that operates at 200 kV. Transmission electron microscopy observations were then carried out using a FEI Tecnaï G2 TEM microscope equipped with a LaB6 filament that operates at 200 kV.

Figure 1 .

 1 Figure 1. Ternary phase diagrams of the systems water/TBA/TEOS (-) and water/EtOH/TEOS

Figure 2 .

 2 Figure 2. a) and c) SWAXS patterns of binary alcohol/water mixtures. b) and d) SWAXS

Figure 3 .

 3 Figure 3. Gel times of the samples as a function of the molar fraction of water xwater as

  Figure 4 a).

Figure 4 .

 4 Figure 4. a) Evolution of the SAXS patterns of the sols CC4 and CC2 during the sol-gel transition.

  performed. Acetone was used for its high solubility in SC CO2.The obtained materials were analyzed by nitrogen adsorption-desorption experiments. The specific surface areas SBET obtained as a function of xwater are shown in Figure 5.

Figure 5 .

 5 Figure 5. Specific surface areas (SBET) of aerogels obtained by the BET method as a function

Figure 6 .

 6 Figure 6. TEM images of aerogels produced from different sols: a) BC2, b) EC2, c) GC2, d) HREF,

Figure 7 .

 7 Figure 7. Simplified illustration of the structuring at the mesoscale during the sol-gel process

Table 1 .

 1 The radii (r0) of primary particles as obtained by fitting the SAXS patterns of the aerogels.

	Co-solvent	Reference	r0 /nm
		CC2	0.35±0.04
		DC2	0.30±0.04
	Ethanol	EC2	0.27±0.03
		FC2	0.28±0.03
		GC2	0.29±0.03
		AC4	0.23±0.03
		BC4	0.32±0.04
		CC4	0.32±0.04
	Tert-butanol	DC4	0.36±0.04
		EC4	0.28±0.03
		FC4	0.26±0.03
		GC4	0.23±0.03
	Ref. without
		HREF	0.27±0.03
	alcohol

Table 2 .

 2 References, weight and molar fractions of alcohol and water in the samples containing 7.5 w% TEOS and 3.6 w% HCl (n(TEOS)/n(HCl) = 2.78). Their localization in a ternary phase diagram is shown in Figure 1.

			Weight fractions	Molar fractions
	Co-solvent	Reference
			wwater	walcohol	xwater	xalcohol
		AC2	0.1	0.79	0.24	0.71
		BC2	0.15	0.74	0.32	0.63
		CC2	0.24	0.65	0.47	0.49
	Ethanol	DC2	0.33	0.56	0.58	0.38
		EC2	0.43	0.46	0.68	0.29
		FC2	0.61	0.28	0.82	0.15
		GC2	0.8	0.09	0.93	0.04
		AC4	0.1	0.79	0.32	0.6
		BC4	0.15	0.74	0.42	0.51
		CC4	0.24	0.65	0.57	0.37
	Tert-butanol	DC4	0.33	0.56	0.68	0.27
		EC4	0.43	0.46	0.76	0.2
		FC4	0.61	0.28	0.87	0.1
		GC4	0.8	0.09	0.94	0.03
	Ref. without
		HREF	0.89	0	0.97	0
	alcohol

Table 3 .

 3 Samples used for the SAXS measurements (see Figure 2 b) and d)) All sample contain 7.5 w% TEOS.

			Weight fractions	Molar fractions
	Co-solvent	Reference
			wwater	walcohol	xwater	xalcohol
		AC2*	0.14	0.79	0.28	0.71
		BC2*	0.19	0.74	0.36	0.63
	Ethanol
		CC2*	0.28	0.65	0.50	0.49
	(SAXS at t0)
		DC2*	0.37	0.56	0.61	0.38
		EC2*	0.46	0.46	0.70	0.29
		AC4*	0.14	0.79	0.38	0.6
		BC4*	0.19	0.74	0.47	0.51
	Tert-butanol (SAXS at t0)	CC4*	0.28	0.65	0.61	0.37
		DC4*	0.37	0.56	0.71	0.27
		EC4*	0.46	0.46	0.79	0.2

Acknowledgement

The authors would like to thank Cyrielle Rey and Bruno Corso for the technical support for Nitrogen adsorption-desorption and SAXS analysis, Jingxian Wang and Martiane Cabié for the TEM images and Olivier Diat, Pierre Bauduin, Jeremy Causse, Diane Rebiscoul and Sandrine Dourdain for many helpful and fruitful discussions.

Associated content

Supporting Information

The Supporting Information is available and contain the Figures S1-S9 and Table S1 which are related to additional information regarding the phase diagrams, SAXS, FT-IR, TEM analysis and physico-chemical properties of binary systems.


Karim Keddadouche

Regis Braucher

Didier Bourles

Mélanie Baroni

Valery Guillou

Laettitia Léanni

Georges Aumaitre

Régis Braucher

Didier L Bourlès

Valéry Guillou

Laëtitia Léanni

Georges Aumaître

Design and performance of an automated chemical extraction bench for the preparation of 10Be and 26Al targets to be analyzed by accelerator mass spectrometry

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Since its development in the late 1970's [START_REF] Raisbeck | Beryllium-10 mass spectrometry with a cyclotron[END_REF], the AMS technique continuously improved, notably it sensitivity gained orders of magnitude. This made possible in the mid-1980's [START_REF] Klein | Revealing histories of exposure using in situ produced 26 Al and 10 Be in Libyan Desert Glass[END_REF] to measure the minute concentrations of cosmogenic nuclides produced within the pertinent mineral lattices of rocks exposed at the Earth's surface. This possibility of measuring concentrations of the so-called « in situproduced » cosmogenic nuclides revolutionized the study of landscape evolution through accurate quantifications of exposure durations or rates of denudation of surfaces exposed to cosmic radiations. However, until the physical parameters governing the production of the cosmogenic nuclides as a function of depth were not well-constrained, the scope of the studies was limited by the need to make strong assumptions about either duration of exposure to determine denudation rates (maximum denudation rate) or denudation rates to determine durations of exposure (minimum exposure duration) [START_REF] Lal | In situ-produced cosmogenic isotopes in terrestrial rocks[END_REF]. This dramatically changed in the early 2000's when both the attenuation lengths of the cosmic-rays derived particles (neutrons and muons) responsible for the production of the cosmogenic nuclides at the surface and in the upper few meters of rock below it were accurately constrained [START_REF] Braucher | Colin -In Situ -Produced 10 Be Measurements at Great Depths: Implications for Production Rates by Fast Muons -Earth and[END_REF][START_REF] Braucher | Keddadouche -Determination of Muon Attenuation Lengths in Depth Profiles From In Situ Produced Cosmogenic Nuclides -Nuclear Instruments and Methods in[END_REF] and a depth-profiling technique developed [START_REF] Siame | Yiou -Local Erosion Rates Versus Active Tectonics: Cosmic Ray Exposure Modelling in Provence (South-East France)[END_REF]. The methodology based on these improvements indeed allows quantifying both the denudation rates and the exposure durations of surface deposits through modelling of the measured decrease with depth of in situ-produced cosmogenic nuclides concentrations using an inversion method. As a consequence of all these innovative refinements, the number of samples to process exponentially increased due to fact that not only the depth profiles must be as constrained as possible but also because in many cases both the 10 Be and the 26 Al concentrations have to be measured all along the profiles to decrypt the history of the exposure to the cosmic-ray particles.

Therefore, the necessity to conceive and develop an apparatus able to handle reproducibly all the repetitive steps of the sample preparation procedure, appears essential in order to respond to the increasing demand of analyses.

2.

Automated bench principles and conception

Process description

All matrices whose concentration in cosmogenic nuclide(s) intended to be measured by the AMS technique undergo physico-chemical processing to produce the chemical species selected either because they lead to the best ionization efficiency in the ion source (BeO -) or to eliminate the isobar in the ion source (Al -). The final steps of the 10 Be and 26 Al chemical extraction rely on the column chromatography separation technique to separate these cosmogenic nuclides. The difference in affinity of the solutes with respect to the stationary phase contained in the column causes, by repeated sorption and desorption during percolation, a gradual separation of the elements. The stationary phase used in this protocol is a cation exchange resin.

Manual chemical extraction protocol ( 10 Be and 26 Al)

The experimental set-up includes, for each sample, a column filled with ~ 12 ml of cationic Dowex 50WX8 type resin surmounted by a funnel maintained on a rack and three beakers collecting the successive selected fractions that contain Boron, Beryllium and Aluminum.

Following the sequences presented Figure 1, after having washed, first with water and then with 1N hydrochloric acid, the resin contained in each column, the operator i/ injects the solution resulting from the sample dissolution or leaching; ii/ successively pours different volumes of hydrochloric acid 1N to eluate Be and 4.5N to eluate Al into each column; and, finally, iii/ recovers the different fractions (Boron, Beryllium, Aluminum) in beakers. The different elution volumes pass through the column under the combined effect of gravity and atmospheric pressure. This induces a more or less slow drop-by-drop extraction according to the type of sample. This step may sometimes require more than three hours, which is always tedious and demands a great attention of the operator in order to strictly respect the sequence and to fully recover the fractions of interest. In addition, the manipulation of concentrated hydrochloric acid could be dangerous for the operator and damage the material.

Issues

• This operation is long, time consuming and tedious. In addition, it requires a high concentration of the operator in order to strictly follow the sequence and recover the fractions of interest at the right time for each treated sample.

• Each volume (especially the one corresponding to the "sample" solution) poured into the column (1 to 2 mm above the resin level) must be eluted almost completely before the next volume can be added.

• The injection volume of the "sample" solution, usually 2ml, may vary from 1.5 to 10 milliliters, depending both on the mass of sample processed and on the solubility of the sample.

• Stationary phase (resin) is for single use only.

•

The repeated handling of hydrochloric acid may be perilous for the operator and damaging for the equipment.

•

The different elution volumes which pass through the column under the effect of gravity and atmospheric pressure induce a more or less slow drip extraction depending on the type of sample, the settling of the resin and the volumes poured.

Proposed resolution

Because it follows a simple and repetitive protocol, the chromatographic separation and purification of the cosmogenic radionuclides of interest, namely 10 Be and 26 Al, is automatable. Therefore, to significantly increase the number of samples that can be processed in a given period of time and thus meet the growing demand for analyzes, an automated system able to reproducibly process simultaneously up to eight samples of different volumes in a short run time period was designed and developed, freeing in addition the operator from these tedious tasks.

Automated chemical extraction protocol ( 10 Be and 26 Al).

The proposed solution (Fig. 2) retains the initial process. Once the columns loaded with resin and equipped with a reservoir, a peristaltic pump distributes the type of eluent selected (water or hydrochloric acid of different molarities) from a three-way solenoid valve through an eight-way solenoid valve to each funnel. After the resin rinsing and conditioning, eight peristaltic pumps perform the simultaneous injection of eight samples to their dedicated funnel. These pumps distribute at a constant rate, the different sample volumes according to a fixed pumping time. After the injection of the sample, the first fraction eluted using the pertinent HCl 1N volume which elutes 10 B, the interfering isobar of 10 Be, is directed to the beaker "trash" which already collected the rinsing and conditioning solutions. Eight three-way valves ensure the collection of the fractions of interest containing first the 10 Be and then the 26 Al in two separate beakers, according to the HCl 1N and then 4N volumes injected. In this prototype one peristaltic pump is in operation to distribute the needed volume of elution solution.

All beakers are contained in a sealed rack maintained under a slight depression by a diaphragm pump in order to accelerate the drop-by-drop and therefore the transfer to the column outlet.

A level sensor (Fig. 3) located within the funnel five centimeters above the zero level sets as the upper bound of the resin in the column, validates the elution of each volume and conditions the transition to the next step. The remaining volume is eluted according to a fixed time transition until complete elution.

The control of the system is managed by a C / rio machine from National Instrument. Finally, the sequential and whole execution of the determined process is supervised by a LABVIEW program (Fig. 4).

Selected components

The use of hydrochloric acid requires that the various parts in direct contact with the eluent (pump, tubing, sensors, solenoid valves ...) are chemically compatible materials (PMMA, PTFE, PP).

Given the surrounding atmosphere (chemical laboratory), the bench prototype is designed in a workable and chemically resistant (PP) material. The electrical box accommodating all the electrical equipment and the automat located at the rear of the bench is sealed and maintained under slight pressure of inert gas (nitrogen) to prevent the intrusion of acidic vapors.

Peristaltic pumps drive the liquid by the action of a roller rotating at a constant speed and exerting pressure on a flexible tube. The advantage of this technique is, on the one hand, the wide variety of material type and diameter of the tubes to achieve this operation and, on the other hand, these pumps delivering a constant flow, minimal maintenance (change worn tubes, ...) is required. The structure of the multi-way solenoid valves (three and eight) is designed in such a way that the fluid does not come into contact with the metal parts of the solenoid valve, thanks to the isolation of the valve and the membrane mechanism. Highly inert materials such as PTFE, PEEK, PPS, etc., are used for the parts in contact with the fluid that make these solenoid valves compatible with a wide range of liquid. Each of them has a common port and as much output as necessary. Level sensors are screwed onto each funnel to detect the presence of liquid five centimeters above the resin bed. The immersed part of the sensor is made of chemically resistant materials (Polysulphone). The fraction collector is a PVC rack, consisting of twenty-four compartments (three per sample) and equipped with a vacuum gauge to control the vacuum produced by the diaphragm vacuum pump.

3. Automated bench performances.

Experiment 1: Automatic Process Verification

This experiment aims to determine if the automated extraction bench: i/ strictly follows the planned procedure, ii/ allows performing separations identical to the manual ones in a reduced time. It consists first in injecting a synthetic solution containing 1000 µl of a boron solution (1000 ppm), 300

µl of a beryllium solution (1000 ppm) and 1000 µl of an aluminum solution (1000 ppm) in a column loaded with 12 ml of a Dowex 50WX8 cation exchange resin, then to regularly collect each fraction of 10 ml at the outlet of the column to be analyzed on an Thermo ICAP6500, Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) to reconstitute an overall chromatogram (the efficiency of the separation of the elements of interest depending on the type, concentration and volume of the selected eluent (Fig. 6).

The results obtained demonstrate that the automated process allows obtaining a separation of the chemical species identical to those obtained performing the manual protocol, the collected volumes and concentrations of the different elements of interest being in addition comparable, but with a significantly reduced execution time. Currently, the entire process (rinsing and activation of the resin, boron, beryllium and aluminum elutions) is performed in 40 minutes with this prototype instead of the 2 hours usually needed by an operator.

Experiment 2: Extraction yields.

Despite the fact that 10 Be and 26 Al concentrations are determined from isotopic ratios measured by AMS and thus the yield of the chemical preparation will not affect the measured ratios, it is however important to estimate how much amount of nuclide, initially present in a natural sample is recovered through the entire preparation process. In the case of the automated bench, does the depression forcing the liquid phase to pass through the resin, affect the yield of extraction? To answer this question, a large amount of snow coming from the Talos Dome station in Antarctica, has been melted and separated in 10 subsamples of ~150g each. Each subsample received a weighted amount of a commercial 9 Be carrier. Five of these subsamples plus a blank sample underwent a beryllium extraction in normal conditions i.e. with an operator following the classical method of Baroni et al.

(2011) [START_REF] Baroni | Volcanic and solar activity and atmospheric circulation influences on cosmogenic 10 Be fallout at Vostok and Concordia (Antarctica) over the last 60 years[END_REF] and five of them were prepared using the automated bench with an associated blank sample.

All samples have been eluted with 18 mL of HCl (3M). From these 18ml and for each subsample, an aliquot has been taken, diluted 9 times and 9 Be concentration were determined by ICP-OES. The extraction yields from both the manual and automated extractions were determined by dividing the initial beryllium content ( 9 Be carrier) and the final one determined by ICP-OES. Results are presented in table 1.

The mean extraction yield obtained with the automated bench is 93.9 ± 0.9 %, in agreement with the one calculated for the classical method, 95.6 ± 4.1 %.

One of the yields of the classical protocol is higher than 100% (sample 3) but within the variability, it may simply indicate a total recovery of the 9 Be in that case.

At this stage, based on the extraction yield and because 9 Be and 10 Be have strictly identical chemical properties, one can conclude that the automated extraction bench is efficient and reproducible. The efficiency values obtained from the automated extraction are also less dispersed than those obtained by the conventional method.

Experiment 3: Validation by Accelerator Mass Spectrometry

In order to verify the reproducibility of the automated process, the isotopic ratio of the cosmogenic radionuclide (in our case, 10 Be) over that of its stable isotope (in our case, 9 Be) was measured by AMS following two separation experiments performed using the automated chemical extraction bench on two solutions.

The first solution to be injected was a calibrated standard (KNSTD13), usually used in AMS and whose calibrated isotopic 10 Be/ 9 Be ratio is 5.24 x 10 -13 . After collection, the final beryllium fractions were evaporated, the residues dissolved in 1N HCl, the solubilized beryllium precipitated as Be(OH) 2 before to be oxidized to BeO at 800°C during 1 hour.

In a second time, all solutions prepared for the tests of the extraction yields (section 3.2) plus two new (samples 11 and 12) were analyzed by AMS. The results are presented in table 2 and related to the description in table 1: the subsamples 1 to 5, 11, the Blanks 1 and 2, were prepared classically [START_REF] Baroni | Volcanic and solar activity and atmospheric circulation influences on cosmogenic 10 Be fallout at Vostok and Concordia (Antarctica) over the last 60 years[END_REF] and the subsamples 6 to 10, 12, the Blanks 3 and 4 were prepared with the automated bench. All beryllium oxides (BeO) were finally mixed with Niobium powder and pressed in copper cathodes.

The AMS analyses were performed at French national facility ASTER (Aix-en-Provence) using NIST SRM4325 standard. The results are presented in table 2.

The three measured values for the KNSTD13 solutions as well as all concentrations excepted that of sample 7, measured from snow samples (figure 7) being identical considering the associated uncertainties, one can conclude that the automated bench is reliable for the sample preparation aiming at measuring 10 Be/ 9 Be ratios. Blank ratios obtained from the automated bench are identical to the one prepared manually by an operator. Despite the same extraction yield obtained from the automated extraction, the AMS measurement performed on sample 7 highlights a 10 Be concentration significantly lower than those measured for all samples (Fig. 7). As already observed for similar cases, this discrepancy results from an error of the manipulator during the last stages of the preparation of the sample (precipitation) and its sintering in the cathode following the separation and the purification carried out by the automated bench and which leads to a too small amount of BeO to be able to perform an AMS measurement that we have nevertheless attempted.

Experiment 4:

Influence of the state of the stationary phase on the separation.

During manual preparation, the stationary phase (resin) is kept submerged without contact with air.

This experiment measured the influence of the state of the stationary phase on the separation of the elements. At each step, from the detection threshold imposed by the sensor, the collector sucks the remaining volume according to a time parameterized in the sequence. Because of the imperfections of the connecting tubing and, more importantly, because of the viscosity specific to each sample, the elution rate through the eight treatment channels may differ slightly. This can cause fluctuations in the level of residual eluent over the resin or its partial or total drying. In the first case, this would generate difficulties at the time of the injection because the residual eluent would significantly increase (double) the volume of the sample (1.5 ml), changing the time necessary for its total elution from the column and, therefore, a partial loss of the sample. In the second case, the dewatering of the resin can create preferential paths within the resin, modifying the interaction between the eluent and the resin and, thus, influencing the separation of the compounds.

During this experiment, the collection time has been modulated to submit the stationary phase to the following three states:

1. Eluent completely sucked up to the raz of the resin; 2. Dry resin;

3. 2 millimeters of residual eluent above the resin.

The results are presented in table 3.

In order to benefit from a good sensitivity to investigate the effect of the state of the stationary phase on the separation, the processed sample was a chemical blank (containing only 9 Be). The measurements are statistically comparable considering the associated uncertainties. This demonstrates that the state of the stationary phase does not significantly disturb the separation.

However, for a better control of the process, it would be preferable to implant a technical solution allowing detection the level of the eluent flush relative to the level of the resin.

Driven by Labview software, an automated bench has been designed to perform the tedious and repetitive tasks in the final steps of 10 Be and 26 Al cosmogenic sample preparation. In addition to freeing the operator, this system allows 9 Be yields as high as those obtained manually (>93 %), it is reproducible and permits processing simultaneously up to eight samples of different volumes in roughly 40 minutes to be compared to the three hours when processed manually.

In a near future it is planned to improve the system adding one elution pump per sample which will allow a significant time reduction of the process and manufacture a new instrumented column designed with level sensor detection the closest as possible of the top of the resin. Furthermore, a resin regeneration step will be added at the end of the extraction protocol to reduce both the time dedicated to the machine starting set-up and the costs of preparation. A commercial version of this automated will be soon available patented under the following designations and references:

Apparatus for separating chemical compounds present in a sample with a corrosive eluent. K.

Figure 1 :

 1 Figure 1: Chronology of the different steps involved in the sequential elution of boron, beryllium and aluminum though the cationic resin.

Figure 2 :

 2 Figure 2: Scheme of the automated chemical extraction bench.

Figure 3 :

 3 Figure 3: Instrumented funnel and column.

Figure 4 :

 4 Figure 4: Control interface LABVIEW.

Figure 5 :

 5 Figure 5: The patented automated chemical extraction bench prototype [7].

Figure 6 :

 6 Figure 6: Elution diagram of boron and beryllium using 1N HCl and of aluminum with 4.5N HCl monitored by ICP/OES measurements.

Figure 7 :

 7 Figure 7: 10 Be concentrations of snow samples prepared manually (1,2,3,4,5,12; black dots) and with the automated bench (6,7,8,9,10,11, triangles); sample 7 is considered as outlier (black triangle). Horizontal black and dashed lines are the mean values with their 1 sigma uncertainties.

Table 2 :

 2 Table1: Concentration of beryllium before and after extractions (classical and automated) and corresponding yields in 10 snow samples and two blanks. AMS measurements of snow samples and KNSTD13

	Keddadouche, D. Bourlès, R Braucher. Patents No.: FR2016/050505; EP16 712,961.8 and
	WO2016142609 (A1).
	5 Acknowledgments.

ASTER AMS national facility (CEREGE, Aix-en-Provence) is supported by the INSU/CNRS, the ANR through the "Projets thématiques d'excellence" program for the "Equipements d'excellence" ASTER-CEREGE action, IRD. The authors thank the NERYS industry, C. Couvert and T. Pugliesi and INOVAPLAST Industry, F.Aniel and J-B. Richemont for their valuable help in the assemblage of the automated bench

Table 3 :

 3 Measurements of three blank samples prepared using the automated bench under three different conditions (see text).

  : Blank 1 is associated to samples 1 to 5; Blank 2 to sample 11, Blank 3 to samples 6-to10 and Blank 4 to sample 12. ** 10 Be concentrations are blank corrected. Blank concentrations are

	Method Method	Sample Sample Name Measured 10 Be/ 9 Be Uncertainty (%) 9 Be added (10 -5 mol) 9 Be measured (10 -5 mol) Yield (%) Average (%)	Std Deviation (%) 10 Be ** kat/g
		Blank 1 Blank 1*	2.79	2.63 7.46E-15	94.2 21.5		< 1.1
		1		1	2.79	2.64 8.21E-14	94.7 4.5		8.8 ± 0.4
	Classical Classical	2 3 4		2 3 4	2.76 2.78 2.78	2.53 8.741E-14 2.87 2.68 9.96E-14 9.23E-14	91.4 5.9 103.2 96.5 4.7 5.1	95.6	8.7 ± 0.5 4.1 9.9 ± 0.5 9.5 ± 0.5
		5		5	2.78	2.60 9.14E-14	93.5 5.3		9.8 ± 0.5
		Blank 2	11	2.78	2.64 9.29E-14	94.9 5.0		10.2 ± 0.5
		6	Blank 2	2.78	2.64 4.38E-15	94.8 20.7		< 0.66
	Automated bench	7 8	Blank 3* 6	2.78 2.78	2.61 4.31E-15 2.60 9.47E-14	94.1 14.3 93.6 9.9	93.9	<0.57 10.1 ± 1 0.9
		9		7	2.78	2.57 6.31E-14	92.5 13.6		7.0 ± 1
		10		8	2.78	2.60 9.07E-14	93.7 4.5		9.4 ± 0.4
				9		8.51E-14	4.12		9.4 ± 0.4
	Automated			10		1.07E-13	3.9		10.5 ± 0.4
	bench			12		9.02E-14	4.1		9.4 ± 0.4
			Blank 4*		5.60E-15	16.7		< 0.74
			KUNI 1		5.21x10 -13	3.0		n.a.
			KUNI 2		5.12x10 -13	3.1		n.a.
			KUNI 3		5.28x10 -13	3.1		n.a.
	upper limits.

*