Electronic Polarization Governs Structure-Transport Coupling of Angstrom-Scale Confined Water.

Abstract

Angstrom-scale confinement fundamentally controls the structure and dynamics of water, leading to behaviors that are very different from those observed in the bulk. Using many-body polarizable force fields combined with Grand Canonical Molecular Dynamics (GCMD) simulations, we investigate the behavior of water confined between multilayer graphene channels spanning 5.5 to 20 Å. We find that explicit modeling of the electronic polarization of graphene is essential: it suppresses the artificially solid-like ordering that arises with conventional nonpolarizable pairwise-additive descriptions based on Lennard-Jones interactions and restores liquid-like behavior even at the smallest channel spacings. The equilibrium densities obtained using GCMD simulations exhibit pronounced oscillations with channel spacing, reflecting discrete monolayer, bilayer, and trilayer packing regimes, which in turn give rise to strongly nonmonotonic variations in lateral diffusion. Structure factor analysis reveals that diffusion minima coincide with ordered, highly correlated configurations, whereas diffusion maxima arise from sparsely populated water configurations with lower density, where lateral correlations are also weaker. By integrating the hydration pressure, we construct the confinement-free-energy landscape and show that these oscillations emerge from a balance between energetic stabilization within compact layers and entropic penalties associated with restricted configurational freedom. As additional molecular layers form, confinement-induced structural, dynamical, and thermodynamic signatures progressively weaken and converge smoothly toward bulk-like behavior. Together, these results provide a unified microscopic picture of the behavior of water under extreme confinement, with direct implications for the design of ultrathin membranes, selective transport devices, and next-generation 2D nanofluidic platforms.