Correlations between silica chemistry and structural changes in hydrothermally treated hexagonal silica/surfactant composites examined by in situ X-ray diffraction

In this paper, the effect of silica chemistry on hydrothermal restructuring of silica/surfactant composites is investigated. The materials were studied using real time X-ray diffraction to follow structural changes in p6mm hexagonal samples as they were hydrothermally treated in buffers ranging from pH 7 to pH 11. Changes in pore shape, repeat distance, and peak area were found to depend on the treatment conditions. Treatment at pH 11 caused expansion of the lattice, a small amount of pore shape restructuring, and a small increase in diffraction peak area. Treatment in a pH 7 hydrothermal solution, by contrast, resulted in contraction of the lattice, significant pore shape restructuring, and large increases in diffraction peak areas. These changes were correlated with Si-29 MAS NMR, which was used to examine changes in framework polymerization, and with liquid H-1 NMR, which was used to follow loss of surfactant from the composite. It was found that lattice expansion is facilitated by low framework polymerization and little or no surfactant loss while the opposite conditions were necessary for lattice contraction. The maximum amount of pore restructuring occurred under pH 9 conditions. At this pH, both silica condensation and silica hydrolysis have appreciable rates, suggesting that both processes are necessary for optimum restructuring. Finally, correlations between surfactant loss and changes in overall diffraction intensity indicated that these changes resulted primarily from increased electron density contrast in the material caused by loss of surfactant from the organic domains. The conclusions of this work help explain the molecular basis for the modifications that are commonly observed in X-ray diffraction patterns after hydrothermal treatment of periodic silica/surfactant composites.