understanding the details of the mechanisms of the formation The formation of colloidal silica particles and the dynamics of of these systems, especially the formation of nanosize, stable the nanostructure of the particles are investigated by small-angle particles derived by the Stober method (5-18). The primary X-ray scattering (SAXS) technique. Solute concentrations of 0.5 goal of these new studies is to test available models of M tetraethylorthosilicate (TEOS), 1.1 or 2.2 M water (H 2 O), and growth and to determine the essential chemical (i.e., reac-0.04 or 0.1 M ammonia base (NH 3 ) in ethanol were used to obtain tion) and/or physical (i.e., aggregation) processes behind reaction conditions as close to those of the Stober method as possithe growth. For this purpose several techniques have been ble and to have reaction kinetics that were slow enough to probe applied to monitor the chemistry of the hydrolysis and conthe changes in the nanostructure of the growing particles and to densation reactions and the formation of silica particles unobtain good statistics from the SAXS measurements. We measured the changes in the radius of gyration and the fractal dimension der different thermodynamic (i.e., temperature) and chemias a function of time during growth. Remarkably, we find that, cal (i.e., pH and ionic strength) conditions: gas chromatograafter an induction period, the first particles to appear in the soluphy (18), light-scattering techniques (5, 7, 8,17,18), tion have a radius of gyration of approximately 10 nm and are nuclear magnetic resonance (NMR) spectroscopy (12, 14, mass fractals characterized by their polymeric, open structure. 17), Raman spectroscopy (5), transmission electron micros-This stage is followed by an intraparticle densification process and copy (TEM) (14, 18), conductivity techniques (10), and smoothing of the interface, leading to the usual compact nonfracsmall angle X-ray scattering technique (SAXS) (16). tal, stable structures. The growth models proposed so far cannot Despite these extensive investigations a clear and comaccount for the observed continuous changes of stages during the plete picture for the formation of the particles that spans the formation and growth of the particles. ᭧ 1997 Academic Press full time range, from initiation of synthesis to the final stage of stable particles, has not yet emerged. Noticeably, there are still open questions regarding the growth mechanism Recently, some investigators have shifted their attention to nanostructure as well as measure the size and size distribution of the scattering particles.