Morphology and pore structure of silica xerogels made at low pH

The pore structure and morphology of sol-gel derived silica xerogels made with high concentrations of acid catalyst are reported. In particular, a range of acid concentrations between 0.01 M, corresponding to a pH near the point-of-zero-charge (PZC) of silica and 1.03 M were investigated. Consistent with previous studies, the gelation time of the sols decreased with increasing acidity in this range. Nitrogen physisorption measurements of the dried xerogels revealed that the pore volumes and surface areas increased dramatically with increasing acid concentration. The microstructure of the samples made under high acid conditions, analyzed by atomic force microscopy (AFM), showed globular structures of similar to 20 nm diameter which pack in such a way as to create mesoporous regions between them. This morphology is quite similar to that observed for materials with similar gelation times, but made at pH values higher than the PZC. This supports the generally held view that the pore structure and morphology are dictated by the relative rates of condensation vs. hydrolysis. (C) 1999 Elsevier Science B.V. All rights reserved.