Why Interfacial Electric Fields Appear Weaker: A Quantum-Accurate Study of the Flat Air-Water Interface

The catalytic properties of water microdroplets are often attributed to strong interfacial electric fields, yet the nature of these fields remains debated. Here we combine quantum data-driven many-body molecular dynamics, ab initio resampling, and spatially resolved electric field evaluations to rigorously assess electrostatics at the flat air-water interface. By classifying molecules using multiple interface definitions and going beyond classical partial-charge methods, we obtain field maps directly from the electronic charge density. Consistent with recent sum-frequency generation (SFG) measurements [J. Am. Chem. Soc. 2025, 147, 46163–16173], our results show that local interfacial fields are weaker in absolute intensity than bulk fields, with an average reduction of ∼ 10%. Given the proportionality of the SFG intensity to the square of the field magnitude, we demonstrate that the observed attenuation arises primarily from density depletion near the surface, with minor contributions from changes in partial charges and dipoles. Beyond offering a methodology that treats local electric fields and molecular densities on an equal footing, these findings challenge the view that enhanced microdroplet reactivity originates from exceptionally strong interfacial electric fields.