Molecular Dynamics Study of the Bulk and Interface Properties of Frother and Oil with Saltwater and Air

For water treatment purposes, the separation processes involving surfactants and crude oil at seawater-air interfaces are of importance for the chemical and energy industries. Little progress has been made in understanding the nanoscale phenomena of surfactants on oily saltwater-air interfaces. This work focuses on using molecular dynamics with a united-atom force field to simulate the interface of linear alkane oil, saltwater, and air with three surfactant frothers: methyl isobutyl carbinol (MIBC), terpineol, and ethyl glycol butyl ether. For each frother, although the calculated diffusivities and viscosities are lower than the expected experimental values, our results show that diffusivity trends between each frother agree with experiments but the method cannot be applied for viscosity. Binary combinations of liquid (frother or saltwater)-air and liquid-liquid interfaces are equilibrated to study the density profiles and interfacial tensions. The calculated surface tensions of the frother-air interfaces are like that of oil-air, but lower than that of saltwater-air. Only the MIBC-air and terpineol-air interfaces agreed with our experimental measurements. For the frother-saltwater interfaces, the calculated results showed that terpineol has interfacial tensions higher than those of MIBC-saltwater. The simulated results indicate that the frother-oil systems underwent mixing such that the density profiles depicted large interfacial thicknesses.