Molecular Dynamics Simulations of the Polymerization of Aqueous Silicic Acid and Analysis of the Effects of Concentration on Silica Polymorph Distributions, Growth Mechanisms, and Reaction Kinetics

We have performed large-scale molecular dynamics simulations of the polymerization of silicic acid in aqueous solution using the potential developed by Fueston and Garofalini [ J. Phys. Chem. 1990, 94, 5351]. Seventeen simulations, with different water-to-silicon ratios and silicic acid concentrations, were each run for between 1.6 and 12.5 ns, at temperatures of 1500, 2000, and 2500 K. Water clearly acts as a catalyst in these simulations. When the water-to-silicon ratio is large, we find that the initial stages of the polymerization process are dominated by the conversion of monomers to dimers and addition of monomers to small clusters, while at longer times cluster−cluster aggregation is observed. Using data from simulations at different temperatures, the activation energies of condensation between silicic acid monomers were calculated at different water-to-silicon ratios and found to compare favorably with experimental results; an extrapolation (at constant density) of simulated reaction rates to ambient conditions (a temperature difference of more than 1200 K) agrees with experimental rates to within one order of magnitude.