SMALL-ANGLE X-RAY-SCATTERING STUDY OF THE FORMATION OF COLLOIDAL SILICA PARTICLES FROM ALKOXIDES - PRIMARY PARTICLES OR NOT

The formation of colloidal silica particles and the dynamics of the na nostructure of the particles are investigated by small-angle X-ray sca ttering (SAXS) technique. Solute concentrations of 0.5 M tetraethylort hosilicate (TEOS), 1.1 or 2.2 M water (H2O), and 0.04 or 0.1 M ammonia base (NH3) in ethanol were used to obtain reaction conditions as clos e to those of the Stober method as possible and to have reaction kinet ics that were slow enough to probe the changes in the nanostructure of the growing particles and to obtain good statistics from the SAXS mea surements. We measured the changes in the radius of gyration and the f ractal dimension as a function of time during growth. Remarkably, we f ind that, after an induction period, the first particles to appear in the solution have a radius of gyration of approximately 10 nm and are mass fractals characterized by their polymeric, open structure. This s tage is followed by an intraparticle densification process and smoothi ng of the interface, leading to the usual compact nonfractal, stable s tructures. The growth models proposed so far cannot account for the ob served continuous changes of stages during the formation and growth of the particles. (C) 1997 Academic Press.