Molecular Dynamics Simulation of the Cross-Scale Role of the Transition Layer in Droplet Coalescence under a DC Electric Field

Abstract

Electrocoalescence technology is widely employed for oil-water separation in crude oil dehydration due to its high efficiency and process stability. However, the presence of an enriched intermediate transition layer between oil and aqueous phases often leads to operational challenges, including increased operating currents and electric field breakdowns, significantly impeding separation efficiency. This study investigates the cross-scale regulatory role of the Span80-enriched transition layer on droplet-interface coalescence in water-in-oil emulsions under a DC electric field, utilizing an integrated approach combining molecular dynamics (MD) simulations and macro-scale experiments. Simulations reveal that electrostatic polarization induces axial droplet stretching, increasing intermolecular distances and reducing cohesive energy, thereby facilitating deformation. Thicker transition layers prolong droplet penetration and delay hydrogen bond reorganization, while expanding the surfactant contact area during embedment triggers a transient energy surge. Analysis of interaction energy, solvent-accessible surface area, radius of gyration, and hydrogen bonding confirms that thickened layers enhance the free energy barrier and elevate the critical electric field required for partial coalescence. This work provides molecular-level insight into how transition layers hinder electrocoalescence, offering a theoretical basis for designing demulsifiers and optimizing electric field strategies to overcome efficiency bottlenecks in electro-dehydration processes.