Electric-Field Effects on Selective Transport through Graphene Nanopores.

Selective ion transport through atomically thin membranes is central to next-generation desalination and nanofluidic technologies, yet how electric-field orientation couples with pore-size-dependent confinement to regulate the permeability-selectivity trade-off remains unresolved. Here, equilibrium and nonequilibrium molecular dynamics simulations systematically resolve pressure-driven water and NaCl transport through chemically unfunctionalized, charge-neutral monolayer graphene nanopores with diameters ranging from 0.73 to 1.91 nm under no-field, parallel-field, and perpendicular-field conditions. Pore diameter establishes the baseline water permeability and steric accessibility, whereas field orientation becomes mechanistically important in intermediate pores (∼1.0-1.5 nm), where ion entry remains sensitive to hydration-shell deformation and reorganization of pore-entrance water. Even without an applied field, interfacial water exhibits pronounced dipolar ordering, while individual O-H orientations remain constrained by the local hydrogen-bond network. Parallel and perpendicular fields therefore generate distinct molecular responses, with perpendicular fields more strongly reorganizing entrance-water polarization and ion hydration and thereby maintaining higher salt rejection than parallel fields in the intermediate confinement regime, while pore diameter remains the primary determinant of water transport. By contrast, field-orientation sensitivity is limited in the smallest pores, where steric and dehydration barriers dominate exclusion and weakens in larger pores as hydration-shell accommodation increases. These findings establish a confinement-dependent crossover in which pore size sets the transport baseline and electric-field direction selectively tunes hydration-mediated ion exclusion, providing a molecular framework for optimizing permeability and selectivity in neutral, atomically thin nanoporous membranes.