How reactive is water at the nanoscale and how to control it?

TL;DR

This study uses advanced molecular dynamics simulations to explore how water's reactivity at the nanoscale is influenced by confinement, material properties, and thermodynamics, providing insights for controlling water chemistry in nanodevices.

Contribution

It demonstrates that water reactivity at the nanoscale is governed by thermodynamic factors, reconciling previous conflicting observations and offering a framework for modulation.

Findings

Reactivity varies with water density, confinement, and surface chemistry.

Chemical potential and interfacial interactions explain dissociation trends.

Nanoscale environment can significantly alter water reactivity.

Abstract

Nanoconfined water plays a key role in nanofluidics, electrochemistry, and catalysis, yet its reactivity remains a matter of debate. Prior studies have reported both enhanced and suppressed water self-dissociation relative to the bulk, but without a consistent explanation. Here, using enhanced sampling molecular dynamics with machine-learned potentials trained at first-principles accuracy, we investigate dissociation behavior in water confined within 2D slit pores and nanodroplets, using graphene and hexagonal boron nitride as model materials. We find that reactivity is extremely sensitive to water density, confinement width, geometry, material flexibility, and surface chemistry. Despite this complexity, we show that chemical potential -- together with interfacial interactions -- governs dissociation trends and explains the variability observed in prior studies. This thermodynamic perspective reconciles previous contradictions and reveals how nanoscale environments can drastically shift water reactivity. Our findings provide molecular-level insight and offer a design lever for modulating water chemistry at the nanoscale.