Humidity and Temperature Effects on CTAB-Templated Mesophase Silicate Films at the Air-Liquid Interface Cristina Fernandez-Martin, Karen J. Edler,* and Stephen J. Roser Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K. Received June 25, 2004. In Final Form: September 1, 2004 Off-specular X-ray reflectivity measurements were carried out to follow the in situ development of surfactant-templated silica thin films at the air-water interface under conditions of controlled relative humidity and temperature, using an enclosed sample cell designed for this purpose. The results suggest a strong dependence of formation time and growth mechanism on ambient conditions. Thin films were synthesized at the air-water interface using cetyltrimethylammonium bromide (CTAB, 0.075 M) and a silica precursor, tetramethoxysilane (TMOS, 0.29-0.80 M) in an acidic medium. The studied humidity range was from 50 to 100%, the temperature was between 25 and 40 °C, and the TMOS/CTAB molar ratio was between 3.3 and 10.7. We observed that high humidity slows down the growth process due to lack of evaporation. However, increasing the temperature results in a decrease in the film-formation time. We proposed a formation mechanism for film growth as a consequence of phase separation, organic array assembly, and silica polymerization. Introduction Mesoporous materials became a field of highly active research interest when, in 1992, scientists at Mobil Research reported the synthesis of surfactant-templated mesoporous inorganic materials. 1-3 Such materials have since been developed for applications in catalysis, mem- brane and separation technology, biomedicine, and op- toelectronic devices. 1-4 Mesoporous silica thin films can be grown from an acidic medium at solid-liquid interfaces using graphite, mica, and silica 5-9 as substrates and also at the air-liquid interface. 10-14 The surfactants, which act as structure-directing agents in the synthesis of mesoporous silicas, are able to self-assemble in aqueous media into aggregates with different shapes, such as micelles, vesicles, and bilayers, 15,16 according to a great number of different parameters. The addition of an inorganic precursor, which condenses and polymerizes around the micellar assembly, forms a mesoporous silica network under appropriate pH conditions. 15 However, a full understanding of the formation mechanisms in the self-assembling system of surfactant-templated meso- porous silica is yet to be achieved. In this work, studying the formation of mesoporous silica thin films from acidic solutions, the inorganic silica network is formed via a so-called counterion-mediated interaction (S + X - I + ) at low pH where both the surfactant and inorganic precursor species are positively charged. 17 Work by others, based on reflectivity measurements, described the main stages in the mechanism, with an induction period and transitional growth phase observed prior to establishment of the final film structure. 18 The addition of extra counterions accelerates the film formation and causes a shift in the phase diagram, changing the mesostructure of the films. 19-21 Previously, we investigated the concentration dependence of the self-assembly process. We proposed a mechanism to explain the spontaneous self-assembly observed during the formation of thin films at the air-water interface using cetyltrimethylammonium bromide (CTAB) as the surfactant and tetramethoxysilane (TMOS) as the silicate precursor. This mechanism is based on the observed horseshoe-shaped variation in the film- formation time with changing TMOS concentration and studies of the development of mesostructure at the air solution interface, 12,14 which suggest two self-assembly regimes in the formation of these films. The mechanism is surface-driven at high and low silica concentrations where cylindrical silica-coated micelles reach the surface and reorder to form the oriented hexagonal mesophase * Author to whom correspondence should be addressed. E-mail: K.Edler@bath.ac.uk. (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Vartuli, J. C.; Schmitt, K. D.; Kresge, C. T.; Roth, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellring, S. D.; Beck, J. S.; Schlenker, J. L.; Olson, D. H.; Sheppard, E. W. Chem. Mater. 1994, 6, 2317. (3) Kresge, C. 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