Sub-nanometer Confinement Suppresses Autoionization of Water

Water confined within nanometer-scale environments plays a central role in functional materials for nanofluidic and membrane-based applications, where acid-base equilibria and proton transport govern essential processes such as ion conduction, energy conversion, and chemical separations. Similar mechanisms are also fundamental to biological systems, including enzyme catalysis and cellular signaling. At sub-nanometer scales, confinement and interfacial interactions dramatically reshape the molecular landscape, challenging conventional assumptions about pH and chemical reactivity. Here, we combine density-corrected density functional theory with machine-learned interatomic potentials to investigate the autoionization of water confined to quasi-two-dimensional monolayers within sub-nanometer slit pores. We find that extreme confinement markedly suppresses water autoionization, raising the effective pK_w by more than two units. This suppression originates from hydroxide ion destabilization at interfaces, driven by restricted hydrogen bonding, hindered molecular reorientation, and a breakdown of Grotthuss proton transport caused by topological frustration in the hydrogen-bond network. These findings offer a molecular-level understanding of how confinement modulates fundamental aqueous chemistry and establish guiding principles for tuning aqueous phase reactivity in nanoscale environments.