Condensation-Induced Decrease of Small-Angle X-ray Scattering Intensity in Gelling Silica Solutions

We propose a mathematical modeling of the total small-angle X-ray scattering (SAXS) intensity in silica sol gel processes in terms of hydrolysis and condensation reactions as well as of microsyneresis. The results are used to rationalize previously published SAXS data of tetraethoxysilane solutions reacting with organically modified trialkoxysilanes. We notably show that the decrease in SAXS intensity reported for these samples at the end of gelation is a consequence of condensation reactions. The release of water by the latter reactions contributes to reduce the electron density difference between the silica and the solvent phases or the gels.