Beyond Force Field Mixing Rules to Model Silica–Water Interfaces

Abstract

The silica–water interface is well studied given its ubiquity in geochemical environments. Many force fields have been developed for both silica and water independently, however, little attention has been given to interaction parameters developed specifically for the interface. As a consequence, simulations continue to use traditional “mixing rules” to calculate silica–water Lennard–Jones interaction parameters. This study bridges this gap by developing a force field explicitly optimized for silica–water interfaces beyond mixing rules. Silica-DDEC, a recently developed force field with electrostatics matched to density functional theory (DFT) is used as the starting point. New Lennard–Jones parameters are developed by benchmarking against DFT-derived interaction energies for crystalline (β-cristobalite) and amorphous silica. The results reveal that traditional mixing rules overestimate the binding strength of water molecules to silica compared to DFT, particularly for crystalline phases. The new parameters correct this overbinding error, and they are also shown to be transferable to another silica interface, α-quartz. The improved parameters result in faster water dynamics due to elimination of the overbinding effect. We also show that these parameters improve the accuracy of silica–water interactions in other force fields based on mixing rules.