AGGREGATION AND AGING IN SILICA-GEL

Aggregation and aging of silica gels, prepared by controlled addition of water glass to hydrochloric acid, and the transformation of an aged gel to a crystalline phase have been studied in-situ using high-brill iance synchrotron radiation. Small angle X-ray scattering (SAXS) has p roved to be very informative in readily detecting transformations in t he silica gels which can be described using the concepts of mass and s urface fractality. The interpretation of SAXS spectra of aged silica g els has been accompanied by computer simulations of aggregation and ag ing based on a model for the aging mechanism and the calculation of th e corresponding structure-factor patterns. Comparison with experimenta l spectra of aged silica highlights the important role of particle gro wth on the fractal dimension. Gel transformations during the transitio n from an amorphous gel to the crystalline phase of silicalite have be en successfully monitored, in-situ, both for heterogeneous and for hom ogeneous preparations, using the combination of small and wide angle X -ray scattering (SAXS-WAXS).