Molecular dynamics study of coagulation in silica-nanocolloid-water-NaCl systems based on the atomistic model

In the present work, large scale molecular dynamics (MD) simulations of nanocolloidal silica in aqueous NaCl solutions were performed using a fully atomistic model to study the microscopic structures and dynamics of the systems that lead to aggregation or gelation. Our attention is focused on the self-organizations that occur in the structures of the colloidal silica and water for various concentrations of NaCl. As the salt concentration increased, coagulation developed through the direct bonding of SiO4 units. The trend was explained by the systematic changes in the pair correlation functions related to the barrier height in the potential of mean force [J. G. Kirkwood, J. Chem. Phys., 1935, 3, 300]. Network structures of silica were visualised, and their fractal dimensions were examined by computing the running coordination numbers of Si-Si pairs and also by the analysis of two dimensional images. The calculated dimension by the former method was comparable to the experimental observations for the aggregation of silica colloids, and at longer length scales, super-aggregation was evident in the gelation process. The result from the 2D images is found to be insensitive to the differences in the structure. Clear changes in both the structure and mobility of the water were observed as the NaCl concentration increased, suggesting the importance of the solvent structures to these processes, although such a feature is lacking in the conventional models and most simulations of colloids.