Water Flows Collectively in Simulated Ion Exchange Membranes

Two conflicting models have been advanced to describe water transport in membranes. The solution diffusion model postulates that water molecules diffuse independently down chemical potential gradients; the pore flow model envisions water as flowing collectively through pores, driven by a pressure gradient. To resolve this conflict, we conduct nonequilibrium molecular dynamics simulations of water transport in membranes with different water contents. Unlike previous works, we simulate a wet membrane in periodic boundary conditions, and drive the flow with a constant force per water molecule. For the same force per water molecule, water flows faster in wetter membranes, consistent with collective transport. To relate the transport to the pore flow picture, we measure the pore dimensions both structurally, by quantifying the ratios between “surface” and “bulk” water, and dynamically, by measuring spatial correlations of the drift velocity. The two measures are consistent, increase with membrane wetness, and generally support the pore flow picture.