Probing the dynamics of the silica nanostructure formation and growth by SAXS

The formation of colloidal silica particles from tetraethyl orthosilicate in several low molecular weight alcohols under base-catalyzed conditions is investigated by small-angle X-ray scattering technique. We measured the changes of the radius of gyration and the fractal dimension as a function of time during growth. We find that, after an induction period, the first particles to form in the solution are mass fractals characterized by their polymeric, open structure. Interestingly, these mass fractals are sizable and could be considered the primary particles. Indeed by undergoing continuous intraparticle densification and further smoothing of the interface, they turn later into the usual compact, nonfractal, stable structures. The stages of growth and transformation, though continuous, cannot be accounted for by the growth models proposed so far.