Probing Water Droplet Coalescence in Acidic Water-in-Oil Emulsions under a Direct Current Electric Field

Naphthenic acid (NA), the predominant organic acid in acidic crude oil, contributes to severe emulsification, complicating the electrostatic demulsification process. To elucidate the mechanism of electro-coalescence in water-in-oil (W/O) emulsions with NA, we employed both molecular dynamics (MD) simulations and experimental methods to examine the dynamic behavior of water droplets when subjected to DC electric fields. The effects of electric field strength (E) and naphthenic acid concentration (c_n) on droplet coalescence were explored. Results indicate that the intermolecular interaction between NAs and water droplets diminishes with decreasing solvent accessible surface area (SASA). Graphical interaction analysis shows that NAs adopt a "head-to-head" configuration between droplets, producing a repulsive exchange energy of 26.03 kJ·mol^-1 that impedes droplet coalescence. At c_n = 0.0441 mol·L^-1, both contact and coalescence times were significantly extended. In this scenario, NAs interact in a "head-to-shoulder" configuration, producing a dispersive attraction energy of -11.32 kJ·mol^-1, which facilitates droplet coalescence. Furthermore, increasing c_n reduces the interfacial tension (IFT) between water and oil. The application of an electric field induces alignment of NA molecules along the field direction, generating steric hindrance that impedes droplet contact. As E increased, the reorientation and alignment of both water molecules and NA intensified, enhancing the formation of H-bonds between the droplets and NAs. This work offers molecular insights into the droplet electro-coalescence mechanisms in acidic W/O emulsions.