Kinetic and thermodynamic study of the hydrolysis of silicon alkoxides in acidic alcohol solutions

Though the relevant literature offers little consistency in the kinetic data of the acid-catalyzed hydrolysis of silicon alkoxides, reliable rate constants are essential for the development of kinetic models for sol-gel processing. Si-29 NMR was used in conjunction with numerical simulations to measure hydrolysis rate constants for tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), and hexaethoxydisiloxane. Unlike previous efforts, we have used conditions where the effects of hydrolysis and condensation reactions can be decoupled. We have verified our rate constants using a range of solution compositions. Implications regarding the influence of the synthesis protocol on gel homogeneity are discussed. We have also estimated the enthalpies, entropies, and activation energies for the hydrolysis of TEOS. We find that each subsequent hydrolysis reaction has a higher rate constant, confirming some earlier studies. However, we also find that each hydrolysis step becomes thermodynamically less favorable. These opposing kinetic and thermodynamic trends explain why acid-catalyzed hydrolysis produces a distribution of hydrolyzed intermediates rather than just fully hydrolyzed products. They also suggest that complete and immediate hydrolysis would be difficult to achieve except at very high water concentration.