Molecular Dynamics Simulations of Water Uptake into a Silica Nanopore

Canonical ensemble molecular dynamics (NVT-MD) simulations of water uptake in a silica nanopore were performed to investigate water transport mechanisms. A silica nanopore was modeled as a finite-length cylindrical nanopore normal to the surface of a thin film of alpha-quartz crystal. The surface of the silica was fully hydroxylated, and the pore radius and film thickness were 1.38 and 7.93 nm, respectively. Initially, thin water films were located on both surfaces of the thin film, and the relaxation process of water uptake into the pore was simulated up to the equilibrium state. The results show two different water transport mechanisms: the transport of water films along the pore surface and the transport of water columns. In both mechanisms, for the short initial period, condensed water flows advectively and diffusively. The water columns reached equilibrium quickly, while the water films gradually approached equilibrium through relaxation of the nonuniform water film after quick water flow. Furthermore, the activation energy of water transport was calculated by the temperature dependence of the relaxation rate. The relationship between the activation energy and transport mechanism was clarified.