Effects of surfactants on droplet deformation and breakup in water-in-oil emulsions under DC electric field: A molecular dynamics study

Abstract

The presence of surfactants in water-in-oil (W/O) emulsions can increase the susceptibility of water droplets to breakup under an electric field, resulting in intensified emulsification. To elucidate the mechanism underlying the electrodynamic behavior of surfactant-containing water droplets in oil, this study has employed molecular dynamics (MD) simulations to investigate the impact of surfactants on droplet deformation and breakup under a DC electric field. The results indicate that despite the formation of hydrogen bonds between surfactants and water molecules, a higher number of van der Waals interactions were formed between them. Upon reaching maximum deformation, the droplet experiences a reduction in deformation due to increased steric hindrance of the oil phase and decreased surfactant adsorption. Eventually, the droplet reaches a steady state with a lower degree of deformation. The enhanced dispersion attraction between n-hexane and Span-80 facilitates the migration of surfactants to the oil phase, weakening the interaction between surfactants and the droplets. The migration of the surfactant drags the surrounding water molecules, increasing droplet deformation. During droplet deformation, hydrogen bonds with a lifetime of 0.88 ps can be formed between the surfactants. The combined influence of hydrated ions, surfactant-related interactions, media effects, and migration effects leads to the breakup of surfactant-containing droplets. During droplet breakup, there is an uneven distribution of surfactants on the droplet surface. Increasing the concentration of surfactants appropriately can prolong the time required for droplet breakup. The simulation findings presented in this paper offer new insights and inspiration for enhancing the electrostatic demulsification of emulsions containing surfactants.