Formation of mesostructured silica in nonionic fluorinated surfactant systems

Abstract

The aqueous phase behavior and preparation of mesoporous silica by new fluorinated surfactants C8F17SO2(C3H7)N(C2H4O)nH (abbreviated C8F17(EO)n) is reported here. C8F17(EO)n forms elongated micelles and liquid crystals in water. Mesostructured silica was prepared by the cooperative self-assembly precipitation method and a systematic study was carried out, investigating the influence of surfactant and silica precursor (TEOS) concentrations, pH and the effect of poly(ethylene oxide) chain lengths. The resulting materials were characterized by SAXS, nitrogen sorption and TEM. The pore inner diameters are small (35 Å) and the pore walls are thick (>20 Å). The materials possess high specific surface areas (≈1000 m2/g), which are achieved at very small surfactant concentrations (2 wt.%), producing robust thick walls with no significant microporosity. The specific surface area is preserved during calcination despite a small shrinkage attributed to silica cross-linking. The d-spacing appeared invariable over a wide range of surfactant/SiO2 ratios, between 0.006 and 1 molar ratios. Hexagonal ordered (p6mm) mesopores were formed at HCl concentrations higher than 0.1 M, while disordered worm-like mesopores were obtained at HCl concentrations lower than 0.1 M. The optimum ethylene oxide chain length to obtain well-ordered mesoporous hexagonal silica corresponded to 10 ethylene oxide units.