Ion Rejection by Nanoporous Membranes in Pressure-Driven Molecular Dynamics Simulations

We perform pressure-driven non-equilibrium molecular dynamics (MD) simulations to drive a 1.0 M NaCl electrolyte through a dipole-lined smooth nanopore of diameter 12 Å penetrating a model membrane. We show that partial, about 70-80%, Cl(-) rejection is achieved at a ~68 atmosphere pressure. At the high water flux achieved in these model nanopores, which are particularly pertinent to atomistically smooth carbon nanotube membranes that permit fast water transport, the ion rejection ratio decreases with increasing water flux. The computed potential of mean force of Cl(-) frozen inside the nanopore reveals a barrier of 6.4 kcal/mol in 1.0 M NaCl solution. The Cl(-) permeation occurs despite the barrier, and this is identified as a dynamical effect, with ions carried along by the water flux. Na(+)-Cl(-) ion-pairing or aggregation near the pore entrance and inside the pore, where the dielectric screening is weaker than in bulk water, is critical to Cl(-) permeation. We also consider negative charges decorating the rim and the interior of the pore instead of dipoles, and find that, with sufficient pressure, Cl(-) from a 1.0 M NaCl solution readily passes through such nanopores.