Molecular dynamics of electric field enhanced water permeation through N-doped graphene.

Abstract

<h4>Context</h4>Nanoporous graphene has emerged as a promising platform for desalination because its atomic thickness and tunable pore chemistry can, in principle, overcome the conventional permeability-selectivity trade-off of polymeric reverse osmosis membranes. Here, we investigate water and ion transport through a single-layer pyridinic-N-doped nanoporous graphene membrane containing sub-nanometer pores with an effective diameter of ~ 2.75 Å, separating a saline feed from a pure permeate reservoir under an axial electric field of 0.05 V Å⁻¹. Non-equilibrium molecular dynamics simulations reveal that this functionalized membrane supports very high-water throughput while maintaining complete Na⁺/Cl⁻ rejection over multi-nanosecond trajectories. The inferred water permeance is on the hundreds of LMH bar⁻¹, and potential of mean force calculations show ion translocation barriers, dominated by steric confinement and dehydration. Structural analysis indicates strong hydrogen bonding between water and pyridinic N at the pore rim, together with field-induced alignment of water dipoles along the transport direction. These synergistic effects enable rapid, selective water transport through sub-nanometer pores, highlighting pyridinic N-doping and modest electric fields as a viable design strategy for next-generation high-flux desalination membranes.<h4>Methods</h4>We performed non‑equilibrium classical molecular dynamics simulations of a slit‑pore desalination cell containing a pyridinic‑N‑doped nanoporous graphene membrane, explicit water, and NaCl electrolyte. Water was represented by a rigid four‑site TIP4P‑family model, and Na⁺/Cl⁻ parameters were chosen to be compatible with this water model. The N‑doped graphene sheet, patterned with ~ 2.75 Å pores and decorated at the rims by pyridinic-N, was modeled using an AIREBO/REBO‑type potential for C-C and C-N bonding. Electrostatics were treated with a particle-particle particle-mesh (PPPM) method under three‑dimensional periodic boundary conditions and van der Waals interactions were described by Lennard-Jones potentials with Lorentz-Berthelot mixing and an appropriate real‑space cutoff. A uniform external electric field of 0.05 V Å⁻¹ was applied along the membrane normal, and a pressure‑like driving force was imposed via rigid carbon pistons at the cell ends. Water geometry was constrained using SHAKE/SETTLE, and the equations of motion were integrated with a velocity-Verlet scheme in the NVT ensemble at 300 K, controlled by a Nosé-Hoover thermostat. Trajectories on the order of a few nanoseconds were generated using the LAMMPS package and analyzed with in‑house Python tools to obtain fluxes, permeances, z‑resolved densities, mean‑squared displacements, radial distribution functions, orientational order parameters, and accessible‑area‑corrected potentials of mean force.