Beyond Pairwise Interactions: How Interfacial Polarization Modulates Water Flow in Graphene Nanochannels

Although incorporating many-body polarization effects is essential for accurately modeling water transport under nanoconfinement, the use of conventional pairwise additive force fields fails to account for polarization-induced charge redistribution at the interface. Using Grand Canonical Molecular Dynamics (GCMD) simulations, we demonstrate that explicitly incorporating many-body polarization effects to model graphene-water interactions as well as graphene flexibility is essential for accurately predicting water interfacial structure and dynamics under nanoconfinement. For narrow channels of 8 Å spacing, graphene polarization is shown to disrupt the confinement-induced ordering of water predicted using Lennard-Jones-based nonpolarizable force fields, resulting in (i) increased out-of-plane orientations of the water molecules, (ii) weakening of hydrogen bonding and hindering of water crystallization, and (iii) faster force decorrelation, resulting in lower interfacial friction. Further, the water densities obtained from the GCMD simulations and accounting for polarization effects increased monotonically with an increase in the channel spacing, whereas the interfacial friction was found to reach a minimum value at 10 Å spacing before attaining a constant value at larger channel spacings. Decomposing friction into static and dynamic contributions reveals that molecular memory effects enhance friction under extreme confinement, while bulk-like interfacial behavior emerges at larger spacings. Importantly, we predict a slip length of ∼200 Å using the polarizable model at a larger channel spacing, in close agreement with previous experimental measurements. These findings demonstrate that many-body polarization, graphene flexibility, and accurate water density predictions are all essential for capturing nanoscale water transport. By providing a predictive framework that links interfacial structure and dynamics, this study advances our fundamental understanding of confined fluid behavior, which is essential for membrane-based applications, including seawater desalination, molecular separation, and energy harvesting.