Effects of Surfactants\non Oil Droplet Demulsification\nin Oil-in-Water Emulsions under an Electric Field: A Molecular Dynamics\nStudy

The application of an electric field to demulsification\nof oil-in-water\n(O/W) emulsions has received extensive attention. However, microcosmic\ninformation about the effect of surfactant type on the demulsification\nof an O/W emulsion under an electric field is still rare. In this\nwork, the effects of cationic surfactant cetyltrimethylammonium bromide\n(CTAB) and anionic surfactant sodium dodecyl sulfate (SDS) on oil\ndroplet demulsification in an emulsion under a pulsed-DC electric\nfield were studied by molecular dynamics (MD) simulation. The MD simulation\nresults show that oil droplets underwent directional movement under\nthe action of an electric field. The larger the electric field strength,\nthe shorter the time required for the oil droplets to coalesce. However,\nthe movement direction of the oil droplets in the electric field was\ndifferent depending on the type of surfactant. Compared to oil droplets\ncontaining SDS, oil droplets containing CTAB molecules exhibited faster\ndeformation and easier migration coalescence under low electric field\nstrength. It was mainly attributed to the fact that the deformation\nof oil droplets will be accelerated when oil droplets contain asphaltene\nmolecules and surfactant molecules with different electronegativities\nunder an electric field. During the demulsification of oil droplets\ncontaining CTAB molecules, CTAB molecules generated electrostatic\nattraction with asphaltene molecules in adjacent oil droplets, strengthened\nthe interaction between oil droplets, and promoted the demulsification\nof oil droplets. In the process of oil droplet demulsification containing\nSDS molecules, the potential energy of the electrostatic interaction\nbetween oil droplets did not change, and the demulsification mainly\ndepended on the van der Waals force between oil droplets.