Influence of Electric Fields in Water Microdroplets and Organic Molecules: A Comparative Study of Surface and Internal Environments

Chemical reactions in water microdroplets can proceed orders of magnitude faster than in bulk water, yet the molecular origins of this behavior remain unclear. Increasing evidence points to strong and heterogeneous electric fields at droplet interfaces as a key factor, but how different functional groups couple to these fields─and which bonds are most susceptible to field-induced polarization─remains poorly understood. In this work, we use polarizable molecular dynamics (MD) simulations to quantify local electric fields in and on a 3 nm water microdroplet model and their influence on two representative solute molecules, protonated 5,5-dimethyl-1-pyrroline-N-oxide (DMPO-H) and 1-methylcyclopentanol (MCP), positioned at the interface and in the interior. We projected electric fields onto intramolecular bond vectors to evaluate bond-level sensitivity to microenvironmental heterogeneity. Simulations indicate that the droplet interface produces stronger, broader, and more asymmetric electric field distributions than the interior. Polar and solvent-exposed bonds─particularly O-H bonds in both molecules and C-C and C-O bonds in MCP─exhibit the largest fluctuations and most frequent extreme field events, whereas less polar or internally oriented bonds (e.g., O-N and N-C in DMPO-H) show narrower, symmetric distributions largely independent of location. These results demonstrate that electric field effects in microdroplets are strongly dependent on bond type, orientation, and local environment, providing a molecular-level framework for understanding how interfacial electric fields selectively polarize functional groups and may influence field-sensitive reactivity in confined aqueous systems.