QUANTITATIVE KINETIC-ANALYSIS BY HIGH-RESOLUTION SI-29 NMR-SPECTROSCOPY OF THE INITIAL-STAGES IN THE SOL-GEL FORMATION OF SILICA-GEL FROM TETRAETHOXYSILANE

Silicon-29 NMR spectroscopy has been used to determine concentration time dependences of the intermediate species formed in the hydrolysis and first condensation steps in the sol-gel formation of silica gel from tetraethoxysilane. A detailed kinetic analysis of these data yields the rate constants k(1) to k(4) for the hydrolysis reactions and k(5) for the first condensation reaction. It is necessary for the fitting process to consider the last hydrolysis step as an equilibrium process, yielding k(4f) and k(4b) For the first time, from measurements made at different pH values, catalyst independent rate constants have been obtained. For the hydrolysis and condensation reactions, these follow the general trend k(1) < k(2) < k(3) > k(5) with the associated errors reflecting the concentrations of the intermediate species.