Chemical control of phase transformation kinetics in periodic silica/surfactant composites.

Abstract

Control of phase stability is investigated through control of silica chemistry in ordered silica/surfactant composites under hydrothermal conditions. The composites were hydrothermally treated in pH 9 through pH 11 buffers while using in situ real time X-ray diffraction to follow a p6mm hexagonal-to-lamellar structural transition. The data were analyzed using both isothermal and nonisothermal (temperature-ramped) kinetics to determine activation energies. It was found that the most mildly basic conditions utilized (pH 9), which favor silica condensation, best inhibit the phase transition and thus produce the most kinetically stable composites. High-pH treatment, conversely, allows for the most facile rearrangements. Condensation occurring during composite synthesis rather than during hydrothermal treatment has a much smaller effect on phase stability, probably because much of the condensation that occurs during synthesis is random and not optimally coupled to the nanoscale architecture. Materials that start out poorly condensed, by contrast, can be extensively hydrothermally modified so that the final material has an inorganic framework with a highly uniform silica density; this provides the maximum resistance to transformation and the highest kinetic stability. In all cases, very good agreement is found between the results of isothermal and nonisothermal kinetic methods. The trends across pHs indicate that both isothermal and nonisothermal measurements are accurate and that differences between them are meaningful and represent physical differences in the transforming materials resulting from the different heating processes.