Structural evolution of surfactant-silica film-forming solutions, investigated using small-angle neutron scattering

Time-resolved small-angle neutron scattering has been used to investigate the evolution of micelles in the subphase of surfactant-templated silica film-forming solutions. Two samples have been prepared using cetyltrimethylammonium bromide (CTABr), as the structure-directing agent, and different amounts of tetramethoxysilane (TMOS) (CTABr/TMOS molar ratio 0.093, 0.139). The solutions have been reproduced at three different H2O/D2O contrasts and allowed to develop until a precipitate formed in the bulk. The surfactant initially forms micelles in solution while the hydrolyzed TMOS is dispersed uniformly through the aqueous phase. The micelles appear to have an ellipsoidal structure immediately after mixing, composed of a surfactant core surrounded by a thin, low-density silica film. The ellipsoidal micelles are typically 32 Angstrom in length and with time elongate and become "rodlike" silica-coated cylinders with length in excess of 150 Angstrom. This value is significantly larger in the composition containing more TMOS (approximate to1100 Angstrom). In contrast, the radius of the core exhibits limited growth, swelling from 15.6 to approximate to19 Angstrom during the induction period. At long times the CTABr/TMOS = 0.139 sample exhibits a first-order diffraction peak at Q = 0.133 Angstrom(-1), corresponding to cooperative scattering from an ordered mesostructure. An analogous peak is not observed in the CTABr/TMOS = 0.093 solution. This is discussed in terms of different film formation mechanisms and compared with previous data measured by X-ray reflection techniques at the air/water interface.