Molecular Dynamics Simulations of Reverse Osmosis in Silica Nanopores

Selective transport of water, salt, and ionic current across water-filled nanopores is a key feature in a variety of natural and engineered systems including geologic media, biological channels, and water treatment membranes. This feature has been extensively observed at the macroscale, but its relation to nanoscale material properties (pore shape, surface charge density, and the behavior of water and ions in individual nanopores) remains incompletely understood. Here, we use nonequilibrium molecular dynamics simulations to examine reverse osmosis during pressure-driven flow of NaCl aqueous solutions across individual silica nanopores separating two bulk-liquid-like water reservoirs as a function of salinity (0.07 or 0.7 M) and pore wall surface charge density (0 or -0.1 C m^-2). We quantify the fluxes of water, salt, and ionic current both within the pore and through the pore as well as the thermodynamic gradients (pressure, salt chemical potential, and electrostatic potential) that develop between the pore entrance and exit. Finally, we characterize the selective transport properties of the nanopores. We find that fluid flow in our nanopores is consistent with classical theories but with a hydraulic permeability that decreases as the streaming potential increases as expected based on nonequilibrium thermodynamics. Furthermore, we predict a preferential transport of anions (vs cations) through both uncharged and negatively charged pores, which contrasts with the behavior expected based on classical representations of the electrical double layer. Our results support emerging efforts to incorporate molecular-scale phenomena and coupled fluxes into continuum scale models of transport in nanoporous materials in the geosciences, engineering, and biology.