Stacking-Controlled Reactivity and Friction of Water in Atomic-Scale hBN Channels from First Principles

Abstract

Understanding the behavior of water confined in atomic-scale channels is a cornerstone of nanoscience and nanofluidics. Unlike graphene, isostructural hexagonal boron nitride (hBN) spontaneously acquires electric charges in liquid water due to hydroxide ion (OH–) adsorption. Yet, whether this reactivity persists under monolayer and bilayer confinement remains an open question. Here, we use first-principles modeling to uncover the role of stacking symmetry (AA vs AA’) between hBN walls─a factor that has so far received little attention in boron nitride systems─on the structural and dynamical properties of Å-scale water. We systematically probe neutral, alkaline, and acidic environments to elucidate how water self-ions (H3O+ and OH–) interact with hBN interfaces. Combining ab initio molecular dynamics (AIMD) simulations with machine learning molecular dynamics (MLMD) for extended time scales, we obtain a consistent description, at the revPBE-D3 level of theory, of water’s structural and transport properties under extreme confinement. Our results demonstrate that stacking symmetry critically impacts monolayer water systems and modulates the energetics of chemisorbed hydroxide and interfacial friction. In alkaline conditions, chemisorbed hydroxide doubles friction, yet its effect on water diffusion is minimal. Strikingly, monolayer water exhibits a twofold diffusion enhancement compared to bilayer water, reaching bulk-like values─revealing that diffusion is an intrinsic property of the water layer, independent of wall chemistry. This work delivers quantitative estimates of friction and diffusion coefficients, conditional on the chosen functional and water loadings, that provide a reference for future experiments on Å-scale water transport.