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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.
Brennan et al. (Thu,) studied this question.