Interfacial Curvature, not Simply Size, Controls Spontaneous Hydrogen Peroxide Formation in Water Microdroplets

Micron-sized water droplets promote redox reactions that are absent in bulk water, yet the kinetics of these interfacial processes remain poorly understood. Here we use real-time fluorescence imaging to monitor spontaneous hydrogen peroxide (H₂O₂) generation in individual microdroplets. Both the apparent production rate and equilibrium concentration of H₂O₂ increase with decreasing droplet size, even after normalizing for surface area, revealing an intrinsic curvature-dependent enhancement. This effect arises from the geometry-driven amplification of interfacial electric fields, and this enhancement of electric field strength accelerates redox processes. Modulating interfacial electrostatics further alters reactivity: organic surfactants suppress H₂O₂ formation to varying degrees, whereas simple salts like NaCl and KCl have minimal impact, even at relatively high concentrations. These results highlight curvature and interfacial charge as key determinants of microdroplet reactivity, governed by mesoscale electrostatic fields that drive chemical transformations at and near the surface of the microdroplet.