Interfacial Electric Fields in Water Nanodroplets are Weakly Dependent on Curvature and pH

TL;DR

This study uses quantum-mechanical simulations to show that interfacial electric fields in water droplets are largely unaffected by curvature and pH, challenging their role in enhanced reactivity.

Contribution

It provides a detailed spatial characterization of interfacial electric fields, revealing their weak dependence on droplet size and pH, and their confinement to the water interface.

Findings

Interfacial electric fields are approximately 1.0–1.2 V/Å and scale with hydrogen bonding.

Curvature and pH have negligible effects on the electric field at relevant droplet sizes.

The electric field is localized at the interface and diminishes within a few Å, acting as a local property rather than a catalyst.

Abstract

The origin of enhanced reactivity in aqueous microdroplets remains debated, with interfacial electric fields (IEFs) often invoked as catalytic drivers. Here, we provide a quantum-mechanical, spatially resolved characterization of the electric field at air-water interfaces by combining deep-learning molecular dynamics with \emph{ab initio} re-sampling. Across planar interfaces and nanodroplets of varying curvature and charge state, we find an outward-oriented field of $\sim 1.0$--$1.2$ V/{\AA} along the intrinsic surface normal. Crucially, its magnitude scales linearly with the average number of hydrogen bonds per interfacial molecule, directly tying the field to the local hydrogen-bond network. Despite its large magnitude and contrary to common expectations, we find that curvature and pH exert only a minor influence on the IEF, becoming negligible at experimentally relevant droplet sizes and pH. Consequently, the reactivity differences observed in $\mu$m-sized droplets cannot be ascribed to variations in the IEF, which changes by a factor of only $\sim10^{-5}$ between $3$ and $40\mu$m-sized droplets. Moreover, the IEF is localized inside the interfacial region and rapidly vanishes within a few {\AA}. This strong spatial confinement renders the IEF strongly tied to the local electronic structure, identifying it as a local property of the air-water boundary rather than an independent physical driver of ``on-water'' catalysis.