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The validity of the fractal versions of the FHH and BET theories for describing the adsorption of butane and nitrogen on a variety of partially dehydroxylated silica surfaces has been tested. The fractal dimensions obtained from adsorption data have been compared with those obtained completely independently using SAXS. It was found that the fractal dimensions obtained from butane adsorption isotherms, using both the fractal FHH and fractal BET theories, agreed well with the corresponding values obtained from SAXS over overlapping length scales. However, in general, a systematic deviation between the fractal dimension obtained from nitrogen adsorption and that obtained from SAXS was observed. The fractal dimensions obtained from nitrogen adsorption were consistently larger than those obtained from SAXS, which is the opposite of what has often been found in the literature. It has been suggested that the differences in the suitability of the adsorption theories tested to describe butane and nitrogen adsorption is due to the significant difference between the interaction strengths of these two different molecules with silica surfaces. A modified theory that can account for the discrepancy between the fractal dimensions obtained from nitrogen adsorption and SAXS has been proposed. The implications of the new theory for the accuracy of nitrogen adsorption BET surface areas for silicas are discussed.

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.

Grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity have been used in situ to study the structure of surfactant-templated silica films grown at the air/water interface at different depths in the film. The results confirm that cylindrical silica-encased surfactant micelles are predominantly organized into a two-dimensional hexagonal structure, with the long axis parallel to the surface of the film. The arrangement of the micelles is well orientated near to the air/film interface but becomes disordered deeper into the sample. GIXD also reveals the existence of vertical channels extending down from the bottom of the film. This suggests a transition from in-plane to unconstrained growth.

The development of surfactant-templated mesoporous films grown at the air/water interface was investigated using specular and off-specular X-ray reflectivity techniques. The samples were prepared in acidic conditions using cetyltrimethylammonium bromide (CTABr, 0.075 M) and different concentrations (0.27−0.88 M) of tetramethyloxysilane (TMOS). At CTABr/TMOS molar ratio between 0.277 and 0.093, the final films exhibited at least three orders of diffraction, corresponding to a real space repeat distance of 46 Å. This long-range order was not observed when the TMOS content was increased to 0.88 M at constant CTABr concentration. In each case, the induction period prior to film formation was monitored using diffuse X-ray scattering. The intensity of the specular reflectivity peak, first and second-order diffraction peaks, and Yoneda wing were recorded as a function of time. The reflected intensity of the specular peak oscillated during the lengthy induction period, indicating that at some concentrations the films nucleate at the surface through a phase separation mechanism followed by growth of ordered arrays. This oscillation has been modeled as a surface layer having increasing thickness and roughness with time. The evolution of the first-order diffraction peak displays different characteristics depending on the TMOS content. For the shortest induction times (corresponding to a molar ratio of 0.139) the film formation mechanism appears to be bulk driven, but for longer development periods the film forms through assembly of ordered arrays at the surface.

We describe a detailed synthesis of a silicified inorganic/organic nanoporous monolithic composite conforming to the lyotropic liquid crystalline L3 phase. The pore dimensions of the silicified L3 phase scale with the solvent volume fraction in the synthesis reaction mixture. Changing the solvent fraction in the initial solution changes the ultimate pore diameter in the silicate, providing a simple method for tuning the diameter of the pores in the matrix. The resulting monolith is optically isotropic and transparent with a nonperiodic network. Accessible pores (which permeate the entire structure) in the silicified materials correlate with the solvent domain of the original liquid crystalline phase and therefore negate the need to remove the surfactant in order to gain access to the pore network. Measured characteristic dimensions are from 6 to well over 35 nm. X-ray scattering studies indicate a low polydispersity in the pore diameters for a given solvent fraction. Transmission electron and atomic force microscope images are consistent with a random morphology and measured surface areas exceed 960 m² g^-1 in extracted materials.