Mapping Collective Electron-Density Rearrangements at Oil-Water Interfaces Using Optimal Transport

The electronic and spectroscopic properties of aqueous hydrophobic interfaces continue to be a topic of active research. One such open question is rooted in the origin of the apparent negative charge of oil droplets. Here, we deploy an information-theoretic optimal transport approach dubbed VIPER-CT (Voronoi-Informed Partitioning of Electron Redistribution for Charge Transfer), to construct the electronic T-matrix derived from the real-space electron density determined from first-principles simulations of an extended decane-water interface, sampled using neural-network-based Deep Potential Molecular Dynamics. This matrix operationally separates directional interphase charge transfer (CT) from intraphase density rearrangements. While decane-water molecular clusters of various sizes show small net CT from decane to water, the extended interface displays a systematic opposite electronic asymmetry, yielding a net CT from water to the hydrocarbon phase producing an average surface charge density of ∼0.003 e- nm-2 on the oil phase. This imbalance is accompanied by much larger intraphase self-polarization, particularly within the hydrocarbon phase, demonstrating that collective many-body polarization dominates the interfacial electronic response.