New insights into the sol-gel condensation of silica by reactive molecular dynamics simulations.

The sol-gel method is an attractive technique to synthesize homogeneous silicate glasses with high purity while relying on a lower synthesis temperature than in the melt-quench method. However, the mechanism and kinetics of the condensation of the silicate network in aqueous solution remain unclear. Here, based on reactive molecular dynamics simulations (ReaxFF), we investigate the sol-gel condensation kinetics of a silica glass. The influence of the potential parametrization and system size is assessed. Our simulation methodology is found to offer good agreement with experiments. We show that the aqueous concentration of the Si(OH) precursors and the local degree of polymerization of the Si atoms play a crucial role in controlling the condensation activation energy. Based on our simulations, we demonstrate that the gelation reaction is driven by the existence of some local atomic stress that gets released upon condensation.