Concentration-dependent formation mechanisms in mesophase silica-surfactant films

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 Angstrom. 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.