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

Small-angle and wide-angle X-ray scattering (SAXS and WAXS, respective ly) were used to examine in-situ formation and evolution of the SiO2 s tructure 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. I t differs from the network structure of vitreous SiO2 or of the gel ne twork produced in alcoholic solution. After the ion exchange the prima ry particles aggregated to form secondary clusters. The resulting clus ters can be described by means of the fractal concept. Diffusion-limit ed or chemical-limited cluster-cluster aggregation were the dominant m echanisms of formation, depending on the pH value. At the same time an Ostwald ripening of the primary particles is observed. When the hydro gels were dried, the fractal clusters distorted and a conversion of th e SiO2 network took place. A structure was formed which is typical for dispersed SiO2.