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This work uses deep-learning molecular dynamics with *ab initio* re-sampling to characterize interfacial electric fields (IEFs) at air-water interfaces of varying curvature and charge. The study finds a consistent outward-oriented field of ~1.0-1.2 V/Å, directly correlated with the local hydrogen-bond network. Contrary to expectations, curvature and pH have negligible influence on the IEF at experimentally relevant droplet sizes, suggesting IEF variations are not the primary driver of enhanced reactivity in microdroplets.
Forget blaming droplet curvature for enhanced reactivity – this study shows interfacial electric fields barely change with droplet size or pH.
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.
