Development of a ReaxFF Reactive Force Field for NaSiO _x/Water Systems and Its Application to Sodium and Proton Self-Diffusion

We present the development of a ReaxFF reactive force field for Na/Si/O/H interactions, which enables reactive molecular dynamics simulation of the sodium silicate-water interfaces. The force field parameters were fitted against various quantum mechanical calculations, including equations of state of different NaSiO_x crystalline phases, energy barriers of a sodium cation's transport within the sodium silicate crystal structure, interactions between the hydroxylated silica surface and sodium cation-water systems, and dissociation energies of [NaOH-n(H₂O)] (n = 1-6) clusters. After the optimization process, we validated the force field capability through calculating the structures of sodium silicate crystals and glasses and transport properties of sodium ions and protons within the amorphous structures. The force field was also applied to validate the dissociation behavior of sodium hydroxides within the bulk water. Our results with the developed force field are relevant to detailed chemical dissolution mechanisms, which involve (a) the interdiffusion process of sodium ions from glasses and protons from water, (b) subsequent ionic self-diffusion of sodium ions from the subsurface region to vacancy sites at the glass-water interface, and (c) sodium ions interaction with water after leaching from the amorphous sodium silicate system.