Water Self‐Dissociation is Insensitive to Nanoscale Environments

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 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.

Beyond Local Structures in Critical Supercooled Water through Unsupervised Learning

“…These findings should motivate more work in trying to understand the relationships between local atomic descriptors and local molecular chemically inspired parameters and how they change our understanding of fluctuations across the phase diagram of water. We believe our work provides a general framework for understanding water’s structural and dynamic properties in other scenarios where long-range correlations may be important, such as at interfaces − as well as under confinement. , …”