Confinement-Governed Autoionization of Nanolayered Water

Sub-nanometer confinement profoundly alters the chemistry of water. Using density-corrected density functional theory and machine-learned interatomic potentials, we examine quasi-two-dimensional water monolayers in slit pores and find that extreme confinement suppresses autoionization, increasing the effective pK_w by more than two units. This arises from hydroxide destabilization at interfaces due to limited hydrogen bonding, restricted reorientation, and disruption of Grotthuss proton transport. We also explored the role of chemisorption, showing that strong surface interactions can enhance or suppress ion formation depending on local bonding environments. The reactivity of confined water depends on the nature and flexibility of the confining material, with weakly interacting walls favoring subsurface reactions and strong walls stabilizing interfacial species. These findings reveal how nanoscale confinement and surface interactions jointly govern aqueous chemistry and inform the design of catalytic, membrane, and energy-storage systems.