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 t ime dependences of the intermediate species formed in the hydrolysis a nd first condensation steps in the sol-gel formation of silica gel fro m 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 p rocess to consider the last hydrolysis step as an equilibrium process, yielding k(4f) and k(4b) For the first time, from measurements made a t different pH values, catalyst independent rate constants have been o btained. For the hydrolysis and condensation reactions, these follow t he general trend k(1) < k(2) < k(3) > k(5) with the associated errors reflecting the concentrations of the intermediate species.