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

Understanding the behaviour of water confined in atomic scale channels is crucial for advancing nanoscience and nanofluidics. Unlike graphene, the isostructural hexagonal boron nitride (hBN) spontaneously gains electric charges in liquid water, a phenomenon attributed to the adsorption of hydroxide ions OH-. It is however unclear whether this reactivity persists under atomic scale confinement. Using ab initio and machine-learning molecular dynamics, we study neutral, acidic, and alkaline water confined between hBN sheets, considering two subnanometer interlayer distances and two stacking symmetries (AA and AB). Given the tendency of hBN to covalently bind hydroxide, we compare the stability of OHeither chemisorbed on the sheet or remaining solvated. We show that AA versus AB stacking strongly impacts the narrower slit, where water forms a quasi-twodimensional monolayer: AA walls tightly constrain the structure and dynamics of water and its self-ions (hydroxide OHand hydronium H 3 O +), whereas AB stacking allows greater structural and dynamical freedom. As a result, monolayer water between AB-stacked hBN walls exhibits a 30% reduction in interfacial water friction, while hydroxide chemisorption leads to a twofold friction increase. At the larger interlayer distance, water forms a bilayer with ion solvation and diffusion properties distinct from the monolayer yet largely insensitive to stacking. Collectively, these results highlight the interplay between the stacking of hBN walls and their Janus affinity for hydroxide ions in governing collective water transport via friction.