Effect of inorganic salt concentration and types on electrophoretic migration of oil droplets in oil-in-water emulsion: A molecular dynamics study

• • Van der Waals interaction is the main weak interaction between oil and water phases. • • Dispersion attraction between oil phases is stronger than electrostatic attraction. • • Negative electrophoretic migration of oil droplets occurs under a DC electric field. • • Increasing ion concentration will reduce oil removal efficiency. • • The inhibitory effect of three inorganic salts on oil droplet is MgCl 2 >CaCl 2 >NaCl. Oil-in-water (O/W) wastewater generally contains inorganic salt ions, which cause energy consumption in the process of electric demulsification. To reveal the influence mechanism of inorganic salt ions on the micro movement of oil droplets in O/W emulsion, the electrophoretic migration of oil droplets with different ion concentration was studied based on molecular dynamics simulation. The results showed that Na + adsorbed the negative charge on the surface of the oil droplets while Cl - repelled it, leading to a decrease in the electrophoretic mobility and deformation of the oil droplets with the increase of ion concentration. The inhibitory effect of three inorganic salts on oil droplet electrophoretic migration was MgCl 2 > CaCl 2 > NaCl. The weak interaction between oil, water and ions was mainly van der Waals interaction. When the NaCl concentration was 0.4 mol/L, the oil-water interfacial tension was at its maximum value of 85.14 mN/m. and the deformation degree of oil droplets reached the minimum values of 0.45. The formation of hydrated ions reduces the freedom of water molecules. The above findings uncovered the effect of inorganic salt ions on the electrophoretic migration of oil droplets in O/W emulsions under a DC electric field, which can provide theoretical guidance for the purification of oily wastewater from the molecular level.