Nonionic Fluorinated Surfactant Removal from Mesoporous Film

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Nonionic fluorinated surfactant removal from mesoporous film using sc-CO2 Elvia Annabella Chavez Panduro, Karine Assaker, Thomas Beuvier, Jean-Luc Blin, Marie Jose Stebe, Oleg V. Konovalov, and Alain Gibaud ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12936 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on January 3, 2017

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ACS Applied Materials & Interfaces

Nonionic fluorinated surfactant removal from mesoporous film using sc-CO2 Elvia A. Chavez Panduroa,b,d, Karine Assakerc, Thomas Beuvierb, Jean-Luc Blinc, Marie–José Stébéc, Oleg Konovalovd, Alain Gibaud b* a

Deparment of Physics, Norwegian University of Science and Technology,Høgskoleringen 5,

7491 Trondheim, Norway. b

c

IMMM, UMR CNRS 6087, Université du Maine, 72085 Le Mans cedex 09, France.

Université de Lorraine/CNRS, SRSMC, UMR7565, F-54506 Vandoeuvre-lès-Nancy cedex,

France. d

ESRF, 6 Jules Horowitz, 38000 Grenoble cedex, France.

KEYWORDS Silica mesoporous, nonionic fluorinated surfactant thin film, sc-CO2, XRR, GISAXS.

ABSTRACT Surfactant templated silica thin films were self-assembled on solid substrates by dip-coating using a partially fluorinated surfactant RF8(EO)9 as the liquid crystal template. The aim was twofold: first we checked which composition in the phase diagram was corresponding to a 2D rectangular highly ordered crystalline phase and second we exposed the films to sc-CO2 to foster the removal of the surfactant. The films were characterized by in-situ X-Ray Reflectivity

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(XRR) and Grazing Incidence Small Angle X-ray Scattering (GISAXS) under CO2 pressure from 0 to 100 bars at 34°C. GISAXS patterns reveal the formation of a 2–D rectangular structure at a molar ratio RF8(EO)9/Si equal to 0.1. RF8(EO)9 micelle have a cylindrical shape, which have a core/shell structure ordered in a hexagonal system. The core contains the RF8 part and the shell is a mixture of (EO)9 embedded in the silica matrix. We further evidence that the extraction of the template using supercritical carbon dioxide can be successfully achieved. This can be attributed to both the low solubility parameter of the surfactants and the fluorine and ethylene oxide CO2philic groups. The initial 2D rectangular structure was well preserved after depressurization of the cell and removal of the surfactant. We attribute the very high stability of the rinsed film to the large value of the wall thickness relatively to the small pore size.

INTRODUCTION

The synthesis of nanostructured hybrid oxide materials using the so-called, ‘surfactant templating’ route has attracted a great deal of attention over the past decade1. Such materials constitute an ideal playground to material engineers aiming at producing porous objects of controlled size, shape and orientation inside the silica backbone2–4. The large diversity of observed structures is made possible by the wealth of templating surfactants available on the market. Indeed the surfactant is the key component which dictates after removal the porosity and the structure of the silica backbone5,6. If in the early stages, powdered samples were the sole materials that were synthesized, very rapidly many researchers tried to design such materials in thin films7–10. Pioneered by J. Brinker8 the most common technique to make highly organized thin films is the EISA (Evaporation Induced Self Assembly) process which consists in fostering the self-assembly of the surfactant during the fast evaporation of a volatile solvent such as


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ethanol together with the concomitant condensation of the silica network in acidic condition under controlled relative humidity. Highly ordered crystalline thin films have thus be synthesized with the ultimate goal to produce mesoporous materials by removing the surfactant either by calcination, solvent extraction or UV-light exposure11–13. Accessing the porosity is indeed of paramount importance in many applications such as dye sensitive solar cells14 (low-k) dielectric materials15, for functionalizing the pore surface or to address the condensation of fluids inside the pores16. So far hydrocarbon surfactants (cationic17, nonionic18 and anionic) were mostly used as templates because they are the most common ones available on the market at a cheap price. On the opposite, even though they are of peculiar interest, fluorinated templates have been barely used. Indeed, due to the presence of fluorine atoms, fluorinated surfactants have unique properties and their chemical and thermal properties19 allow applications under conditions that would be too severe for hydrocarbon amphiphiles. They are used in many practical applications such as additives for paints, coatings, fire extinguisher foams, levelling and wetting agents and so on20. For example, Keller et al. have demonstrated that fluorinated surfactants can aid the functional refolding of a membrane protein under conditions where gentle membrane interactions prove superior to the inertness of lipophobic fluorinated surfactants21.. In addition, these surfactants allow co-solubilization of water and perfluoroalkanes22,23 and the specific property of fluorocarbon to dissolve high quantities of oxygen and carbon dioxide make them very attractive for biomedical applications as oxygen vectorization for instance24,25. More recently they have been used for the synthesis of mesoporous materials26–28. For this application, the main benefit of fluorinated surfactant is their high thermal stability. Thanks to this property, the hydrothermal treatment can be performed at high temperature, involving a higher level of silica condensation and as a consequence the recovered mesoporous material exhibits a better


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hydrothermal stability than when synthesized from a hydrogenated surfactant29. Nevertheless, it should be outlined that these surfactants have not yet been extensively used for making thin films. Among such templates, the phase behavior of F(CF2)8C2H4(OC2H4)9OH used in this study (hereafter abbreviated as RF8(EO)9) was investigated in aqueous solution and subsequently applied to the preparation of mesoporous silica powder. Based on the biphasic phase diagram of this surfactant in water, mesoporous materials made with RF8(EO)9 and TMOS (Tetramethyl orthosilicate) were then reported by Blin & Stébé group26, using either micelles or liquid crystals as templates. In the latter case, they used a molar ratio of RF8(EO)9/TMOS in the range 0.119 to 0.175 to produce a 2D hexagonal structure. They observed that the pore diameter could be tuned from 2.8 nm up to 4.3 nm by modifying the surfactant/water concentration in weight. This variation in the pore size was attributed to a change in the degree of hydration of the hydrophilic part of the surfactant. For a surfactant/water concentration higher than 55% the pore diameter fits with the diameter of the hydrophobic part of the surfactant and for lower concentrations the pore diameter increases and the wall thickness decreases. Yet to the best of our knowledge the preparation of mesoporous silica thin films templated by RF8(EO)9 has not been reported in the literature. We thus present in this paper the experimental study of the synthesis of a hybrid mesophase formed by RF8(EO)9 and TEOS (Tetraethoxysilane) in thin films. In order to identify the structure of these films, we have performed GISAXS measurements since XRR (classical θ-2θ measurements in the incidence plane) only provides information in the direction normal to the surface of the film. The crystal structure of the films was therefore determined using the in-house GISAXS facility of the IMMM (Le Mans, France) before making additional measurements at the ESRF. As discussed in the following, we evidence that a 2D rectangular phase could be


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successfully obtained for a molar ratio RF8(EO)9/TEOS equal to 0.1. With our ability to design a highly ordered structure, we then focused on the removal of the surfactant using carbon dioxide in its supercritical state (sc-CO2) as the solvent.

Supercritical fluids are indeed providing in some cases a wonderful non-intrusive alternative to classical heat treatment for surfactant removal30–34. In order to identify the most suitable solvent of an organic molecule it is important to compare to the solubility parameter of the solvent to the one of the molecule. If the difference between the solubility parameters is less than 4 MPa1/2, it is possible that the molecule will be soluble in the solvent35. Note that this rule cannot be taken as a strict one since there are evidences that it does not work in some cases. When working with liquid solvents, their solubility parameter does not change much with pressure and experiments are generally done at ambient pressure as for the extraction of CTAB by ethanol in mesoporous silica. In the case of CO2 under pressure, on the contrary to conventional liquid solvents, the solubility parameter of CO2 can be easily tuned by adjusting the temperature and pressure of the solvent. It can be calculated at any temperature and pressure provided that its density is known. As reported by Giddings36, the solubility parameter of CO2 can be obtained from the following expression ߜ(‫)ܽܲܯ‬ଵ/ଶ = 17.53 × ߩ஼ைమ

Equation 1

where ρ is the density of CO2 in g/cm3. The solubility parameter for CO2 has been calculated according to Eq. 1 and is plotted in Figure S2 with the density of CO2 taken from the NIST database37. One can see that at T=34 °C (T>Tc=31.9 °C), the solubility parameter δ(CO2) of pure CO2 changes from 0 to about 14 MPa1/2 when the pressure is raised from 1 to 150 bars. This tunability of sc-CO2 solubility parameter is


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of great interest to figure out its ability to extract some surfactants. To accredit the use of CO2 for surfactant extraction, we have also shown in Figure S2 the values of δ for several surfactants. Surfactants having low solubility parameters (δ < 16 MPa1/2 as PDMS) and in addition CO2philic groups as carbonyls, ethers and fluorocarbon groups are appropriate to be miscible in scCO238–41.

Thus sc-CO2 is clearly a very good choice to extract RF8(EO)9 since in addition to the low solubility parameter of the surfactant (δ ~ 16 MPa1/2), the surfactant has CO2-philic groups (ether and fluorocarbon). Figure S2 shows that the extraction of the surfactant is expected to occur around 90 bars at 34°C. In addition, the specific interactions of CO2 with EO and CF3 can reduce the solubility parameter of the surfactant, i.e. increase the solubility of CO2 and therefore decrease the pressure at which the surfactant can be removed. In the case of hydrocarbon surfactants (e.g. CTAB, F127), it is clear that CO2 is not adapted since its solubility parameter is much lower than those of the surfactants in particular CTAB. As shown in Figure S2, the good solvent that can be used to remove CTAB by solvent extraction is ethanol. Yet CO2 could be used if water, methanol or other solvents are added to CO2 to match the solubility parameter of the CO2/co-solvent mixture to the one of the hydrocarbon surfactant30,42,43. These surfactants can thus be removed from the host matrix after depressurization of the vessel. In the following, we confirm that the approach based on the calculation of the solubility parameters is perfectly appropriate to identify the CO2 pressure range for removing the surfactant. By performing in-situ XRR and GISAXS experiments, we evidence that CO2 under pressure becomes particularly efficient to remove RF8(EO)9 in its supercritical state. Under P=75 bars, the fluorinated surfactant remains in the silica matrix and disappears above this value. It is also worth noting that the 2D structure is remarkably preserved after surfactant removal. In


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addition we also prove that in the case of CTAB templated films, sc- CO2 remains inefficient for removing the surfactant. Before reporting these in-situ results, we first describe the sample preparation and the analysis of the initial structure.

EXPERIMENTAL SECTION Materials Tetraethyl-orthosilicate (TEOS) used as a silica precursor was purchased from Sigma-Aldrich and the fluorinated surfactant was provided by Dupont. The molecular average formula of the surfactant RF8(EO)9 used in this study is F(CF2)8C2H4(OC2H4)9OH with a molar mass of 861 g/mol. The molecule is composed of a fluorinated chain C8F17 with a length of ~ 1.1 nm, a spacer chain C2H4 of ~ 0.25 nm and a chain containing 9 EO groups with a length ~ 3.1 nm. These length values were estimated using a geometry optimized single molecule of surfactant assuming a fully elongated molecule. The results are in good agreement with those reported by Schmitt et al.44

Sample Preparation Trial and errors experiments varying the molar ratio R’=( RF8(EO)9/TEOS) in the range of 0.04

Nonionic Fluorinated Surfactant Removal from Mesoporous Film

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