Collective Electronic Polarization Drives Charge Asymmetry at Oil-Water Interfaces

TL;DR

This study uses advanced molecular dynamics and information theory to reveal that collective polarization effects cause charge asymmetry at oil-water interfaces, explaining why oil droplets acquire a negative charge.

Contribution

It demonstrates that many-body polarization dominates interfacial electronic response and provides a microscopic origin for charge transfer asymmetry at oil-water interfaces.

Findings

Net charge transfer from water to oil phase (~0.006 e/nm^2)

Dominance of intra-phase self-polarization over interfacial charge transfer

Asymmetry in molecular motifs explains charge transfer direction

Abstract

Why kinetically stable oil droplets in water spontaneously acquire a negative charge remains one of the most vigorously debated questions in interfacial science. Here, we combine neural-network based deep potential molecular dynamics with a data-driven and information theory approach to probe the real-space electron density at an extended decane-water interface. While decane-water clusters show nearly symmetric forward and backward charge transfer (CT) and thus negligible net CT, the extended interface displays a systematic electronic asymmetry, yielding a net CT from water to the hydrocarbon phase producing an average surface charge density of $\sim0.006~e^{-}\,\mathrm{nm}^{-2}$ on the oil phase. This imbalance is accompanied by much larger intra-phase self-polarization, particularly within the hydrocarbon phase, demonstrating that collective many-body polarization dominates the interfacial electronic response. Structural analysis reveals an asymmetry between forward C--H$\cdots$O and backward O--H$\cdots$C motifs, providing a microscopic origin for a net CT from one phase to the other. Curiously, both the water O--H and decane C--H covalent bonds incur subtle contractions which originate from a response to the charge-separation layers at the interface. These features are fully consistent with the weak improper hydrogen-bonds forming at the oil-water interface that results in blue-shifts of the C-H modes.