Extreme Nanoconfinement Reshapes the Self-Dissociation of Water

TL;DR

Extreme nanoconfinement drastically alters water's self-dissociation process, lowering energy barriers and restructuring hydrogen bonds, which could lead to superdielectric and superionic behaviors relevant to nanofluidic and biological systems.

Contribution

This study uses ab-initio and machine-learning molecular dynamics to reveal how monolayer confinement reduces dissociation barriers and restructures water's hydrogen-bond network.

Findings

Lower dissociation barrier in confined water

Restructured hydrogen-bond network under confinement

Potential for superdielectric and superionic behavior

Abstract

Water's ability to self-dissociate into H$_3$O$^+$ and OH$^-$ ions is central to acid-base chemistry and bioenergetics. Recent experimental advances have enabled the confinement of water down to the nanometre scale, even to the single-molecule limit, yet how this process is altered at the extreme nanoconfinement remains unclear. Using \emph{ab-initio} calculations and enhanced-sampling machine-learning potential molecular dynamics, we show that monolayer-confined water exhibits a markedly lower barrier to auto-dissociation than bulk water. Confinement restructures both intramolecular bonding and the intermolecular hydrogen-bond network, while enforcing quasi-2D dipolar correlations that amplify dielectric fluctuations. Our results imply that two-dimensional confined water could act as a \emph{superdielectric} medium and may exhibit \emph{superionic} behavior, as observed in recent experiments. These findings reveal confinement as a powerful route to enhanced proton activity, shedding light on geochemical niches, biomolecular environments, and nanofluidic systems where water's chemistry is fundamentally reshaped.