Water Self-Dissociation is Insensitive to Nanoscale Environments

TL;DR

This study uses ab-initio simulations to show that water self-dissociation energetics are largely unaffected by nanoscale confinement, maintaining bulk-like behavior even in very small aggregates or narrow pores.

Contribution

It provides a fundamental understanding that water autoionization energetics are insensitive to nanoscale confinement, challenging previous assumptions about interfacial effects.

Findings

Water dissociation free-energy profiles are similar in bulk and nanoscale environments.

Nanoscale confinement does not significantly alter the energy barrier for water autoionization.

Bulk water dissociation behavior persists down to aggregates of a dozen molecules or pores below 2 nm.

Abstract

Nanoconfinement effects on water dissociation and reactivity remain controversial, despite their importance to understand the aqueous chemistry at interfaces, pores, or aerosols. The pKw in confined environments has been assessed from experiments and simulations in a few specific cases, leading to dissimilar conclusions. Here, with the use of carefully designed \textit{ab-initio} simulations, we demonstrate that the energetics of bulk water dissociation is conserved intact to unexpectedly small length-scales, down to aggregates of only a dozen molecules or pores of widths below 2 nm. The reason is that most of the free-energy involved in water autoionization comes from breaking the O-H covalent bond, which has a comparable barrier in the bulk liquid, in a small droplet of nanometer size, or in a nanopore in the absence of strong interfacial interactions. Thus, dissociation free-energy profiles in nanoscopic aggregates or in 2D slabs of 1 nm width reproduce the behavior corresponding to the bulk liquid, regardless of whether the corresponding nanophase is delimited by a solid or a gas interface. The present work provides a definite and fundamental description of the mechanism and thermodynamics of water dissociation at different scales with broader implications on reactivity and self-ionization at the air-liquid interface.