The origin of the negative charge at hydrophobic–water interfaces has long been debated, with the prevailing explanation attributing it to the preferential adsorption of hydroxide ions (OH^–). However, a new perspective attributes this charge to charge-transfer (CT) interactions between water and oil molecules. In this study, we investigate these charge transfer mechanisms at oil–water interfaces using polarizable AMOEBA force field calculations. We apply energy decomposition analysis (EDA) based on absolutely localized molecular orbitals (ALMOs) and charge-displacement analysis, leveraging the CT decomposition framework developed by Martin Head-Gordon. This method provides deeper insights into charge transfer between water and oil at the molecular orbital level, offering a more refined understanding than traditional spatial analysis.
We evaluate whether this ion-free CT mechanism can adequately explain the negative charge observed at hydrophobic–water interfaces. Moreover, we analyze the placement of the slip plane based on the water/oil interfacial charge transfer and assess its consistency with established interfacial dynamics.
The CT between interfacial water and hexane, as determined through EDA, serves as a proxy for surface charge. Using this data, we perform nonlinear Poisson-Boltzmann equation (PBE) calculations to compute the zeta potential, which is then compared to experimental zeta potentials measured under neutral conditions without ions.
Lastly, we investigate whether this charge transfer mechanism alone can sufficiently explain the stability of hydrophobic droplets in the absence of ions, and assess the adequacy of CT as the primary source of surface charge for maintaining droplet stability.

Ruoqi Zhao

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