Electrophoretic coalescence behavior of oil droplets in oil-in-water emulsions containing SDS under DC electric field: A molecular dynamics study

Abstract

The presence of oil droplets in oil-in-water (O/W) emulsions seriously affects the efficiency of wastewater purification, and the appropriate addition of surfactants can effectively improve the demulsification process. In this paper, the electrophoretic coalescence behavior of oil droplets containing anionic surfactant sodium dodecyl sulfate (SDS) under a direct current (DC) electric field was studied based on molecular dynamics (MD) simulations, and four typical stages of oil droplet migration coalescence were defined. The weak interactions between SDS and water and between SDS and n-hexane are mainly van der Waals interactions, while a small number of hydrogen bonds are formed between SDS and water and between water molecules internally. With the increase of electric field strength (E), the deformation of oil droplets after aggregation increases, and the contact angle with the electrode plate becomes smaller. The interfacial tension between oil and water decreases and then increases with increasing surfactant concentration, reaching a minimum value of 32.52 mN/m when the number of SDS molecules (n) around each oil droplet is 55. When E = 1.2 V/nm and n = 55, the coalescence time of oil droplets reaches the minimum value of 2.425 ns. The migration rate of oil droplet far from the anode is greater than that of oil droplet near the anode, which is due to the larger combined forces on the former. The results reveal the microscopic mechanism of oil droplet migration and coalescence under the coupling effect of DC electric field and anionic surfactant, which could provide theoretical guidance for the improvement of oily wastewater purification process.