WAXS AND SAXS INVESTIGATION OF STRUCTURE FORMATION OF GELS FROM SODIUM-WATER GLASS

Small-angle and wide-angle X-ray scattering (SAXS and WAXS, respectively) were used to examine in-situ formation and evolution of the SiO2 structure in aqueous solution as a function of the chemical conditions. In contrast to the sol-gel process in alcoholic solution the starting solution (sodium waterglass solution) is not a monomer. The existence of quite definite SiO2 primary particles having average diameters of 1 nm and with sodium ions attached to their surfaces was established. The inner structure of these particles is determined by the solvent. It differs from the network structure of vitreous SiO2 or of the gel network produced in alcoholic solution. After the ion exchange the primary particles aggregated to form secondary clusters. The resulting clusters can be described by means of the fractal concept. Diffusion-limited or chemical-limited cluster-cluster aggregation were the dominant mechanisms of formation, depending on the pH value. At the same time an Ostwald ripening of the primary particles is observed. When the hydrogels were dried, the fractal clusters distorted and a conversion of the SiO2 network took place. A structure was formed which is typical for dispersed SiO2.