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

Abstract

The origin of enhanced reactivity in aqueous microdroplets remains unresolved, with interfacial electric fields (IEFs) frequently proposed as catalytic drivers. Here, we present a quantum-mechanical, spatially resolved characterization of the electric field at air-water interfaces by combining deep learning molecular dynamics with ab initio re-sampling. Across planar interfaces and nanodroplets spanning a range of curvatures and charge states, we consistently identify an outward-directed field of ∼1.0–1.2 V/Å along the intrinsic surface normal. Notably, the field magnitude scales linearly with the average number of hydrogen bonds per interfacial molecule, directly linking the IEF to the local hydrogen-bond network. Strikingly, curvature and pH exert only a moderate influence on the IEF, with differences becoming negligible at experimentally relevant droplet sizes and pH regimes. Accordingly, the reactivity enhancements observed in μm-sized droplets cannot be attributed to variations in the IEF, which changes by only ∼10–5 between 3 and 40 μm-sized droplets. Furthermore, the IEF is highly localized within the interfacial region, decaying over just a few Å. This pronounced spatial confinement ties the field to the local electronic structure, establishing the IEF as an intrinsic interfacial property rather than an independent mechanistic driver of “on-water” catalysis.