Compositional effects and hydrothermal reorganization of mesoporous silicates synthesized in surfactant solutions

Surfactant aggregates play a key role in aqueous condensation polymerization reactions of silicate species to form mesoporous siliceous solids. The effects of surfactant (cetyltrimethylammonium chloride, CTAC) concentration and silicate/surfactant ratio on the synthesis of mesoporous silicates were studied. The subsequent hydrothermal reorganization of the surfactant-silicate mesophases during drying was also investigated. At low CTAC concentration(<10 wt %) and low Si/CTAC molar ratio(<2.6), the CTAC micellar aggregates and bound silicate counterions have sufficient mobility to form hexagonal arrangements through the intermicellar silicate condensation. At higher CTAC concentration and higher Si/CTAC ratio, the hexagonal arrangement is considerably hindered due to the increased contour length of the micelles and the reduced intermicellar distance, resulting in cross-linking of micelles that disrupts formation of hexagonal pore structures. During drying, hydrothermal reorganizations of lamellar silicate mesophases into hexagonal structures and of cubic mesophases into lamellar structures were observed. These transitions provide insight into the role of bilayer assemblies as precursors for the formation of cubic and hexagonal geometries.