Monte Carlo Simulations of Silica Polymerization and Network Formation

We present a new model and method for the Monte Carlo simulation of silica polymerization in aqueous solution. We focus on the idea that silica structures are built from corner sharing tetrahedra and these tetrahedra are the basic units of the model. Rather than use a reactive force field, the assembly of tetrahedral units is accomplished via Monte Carlo simulation in the reaction ensemble. The simplicity of the model and the use of the reaction ensemble make it possible to study silica polymerization for quite large system sizes, reaching a high degree of condensation under ambient conditions. We find that the reaction ensemble Monte Carlo simulation protocol can provide a description of the overall polymerization kinetics, after making some key assumptions. Very good agreement is obtained when comparing simulated and experimental evolutions of the Q(n) distribution as a function of both time and degree of condensation, indicating an approximately linear relationship between physical time and number of Monte Carlo steps up to about 5 h. Analyses of cluster-size and ring-size distributions reveal that polymerization proceeds in the following main stages: oligomerization forming small units (0-1 h), ring formation (1-2.6 h), cluster aggregation (2.6-5.6 h), and finally cross-linking of the aging gel at later times.