Why Interfacial Electric Fields May Appear Weaker: An Electron-Density-Based 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 intermolecular electrostatics at the flat air–water interface. The calculated local intermolecular quantity may be viewed as a microsolvation electric-field descriptor of the environment experienced by a molecular species approaching or residing near the interface. By classifying molecules using two complementary interface definitions and going beyond classical partial-charge methods, we obtain field maps directly from the electronic charge density. Similarly to recent experimental findings, the bulk and interfacial field distributions overlap strongly, while their mean values exhibit a modest interfacial reduction of ∼10%. Correlations with the density profiles obtained from different reference frames indicate that density depletion near the surface contributes to this attenuation. Beyond offering an electron-density-based methodology for evaluating local electric fields, these findings suggest that exceptionally strong local fields alone may not fully account for the enhanced reactivity reported in microdroplets.