Microscopic mechanism for electrocoalescence of water droplets in water-in-oil emulsions containing surfactant: A molecular dynamics study

Abstract

Surfactant has distinctive effects on the coalescence of water droplets in water-in-oil (W/O) emulsions under electric field. In an attempt to reveal the molecular mechanisms underlying the electrostatic coalescence of droplets containing surfactant, molecular dynamics (MD) simulations were performed to study the coalescence behavior of water droplets with different surfactant contents. Our findings indicated that Span-80 is mainly held with n-hexane and water by basically stable dispersion attraction, and a tiny amount of hydrogen bonding between hydrophilic group and water was formed. The solvent accessible surface area (SASA) and the number of hydrogen bonds (H-bonds) between droplets could be available to determine the time nodes regarding droplet coalescence. Low concentration of surfactant has a promoting effect on droplet coalescence, while the reverse occurred at high concentrations. When the number density of surfactants is 0.0174 nm−3, the coalescence period of the two droplets was the shortest, approximately 1865 ps. The electrostatic attraction between water molecules dominates the intermolecular interaction during droplet coalescence. Positive and negative charges tended to agglomerate at both ends of the surfactant layer, which further enhanced the dipole-dipole coalescence of droplets. The stability of H-bonds in droplets deteriorated due to the extrusion of the surfactant, hindering the coalescence of water droplets. A moderate amount of surfactant could reduce the oil-water interfacial tension (IFT) and increase the critical electric field strength (Ec) for droplet coalescence. The research provides a reference for explaining the coalescence behavior of water droplets containing surfactants at an electric field in molecular perspective.