WAXS-INVESTIGATIONS AND SAXS-INVESTIGATIONS OF STRUCTURE FORMATION IN ALCOHOLIC SIO2 SOLUTIONS

Small- and wide-angle X-ray scattering (SAXS and WAXS, respectively) were used to examine in situ formation and evolution of SiO2-structures in alcoholic solutions of tetra-alkoxysilanes (TAOS) as a function of chemical conditions. In acid-catalyzed reactions with a molar ratio of ≤rw ≤ 51 forH 2O against TAOS primary particles with a diameter of ∼ 1 nm were always found. These particles aggregated to form secondary clusters. The resulting aggregated clusters can be described by means of the fractal concept, diffusion-limited or chemical-limited cluster-cluster aggregations being the dominant mechanisms of formation. When using an under-stoichiometric water content (rw = 1) the formation of smaller primary particles with a diameter of ∼ 0.5 nm is observed; these particles preferably aggregated to form linear particle chains. In alkaline-catalyzed reactions the SiO2-particles grew beyond the visual field of the measurement (rmax = 20 nm) and featured nearly smooth surfaces. As a result of WAXS examinations, it was found that the internal structure of the condensates is characterized by a well-defined topology (polyhedrons). The liquid environment exerts a force on the SiO2-particles and the polyhedron size varies with the composition of the solution. © 1990.