Dynamics of oligomerization of silicate solution studied by Molecular Dynamics

Abstract

The main route of silica powder production is the precipitation from sodium silicate solution in the 8–10 pH range. It was earlier established that the oligomerization of the silicate species is an important stage in silica nucleation. We performed Molecular Dynamics simulations of unsteady silicate solutions at T > 1000 K to overcome the difficulties encountered in oligomerization experiments (e.g. reliability of the obtained kinetic data). We studied the dynamics of oligomerization for various basic pH, temperature, and silicate concentration values. Based on the Becker–Döring theory of nucleation we now propose a simple reaction mechanism and a quantitative model explaining the evolution of the smaller oligomers concentration with time. The model requires fitting two physical parameters: the kinetic constants of oligomerization and hydrolysis; they depend on the temperature, but not on the size of the oligomer reacting with the monomer. Using the Arrhenius law, we deduce the activation energies for condensation and hydrolysis; we show that the pre-exponential term in the oligomerization rate follows Smoluchowski’s theory for Brownian aggregation. Applying the model to the silicate oligomerization at room temperature, we show that our model agrees with previously published modeling based on energetics considerations. The role of the counter-ions is also confirmed.