(308b) Effects of Salinity and N-, S-, and O-Bearing Polar Components on Light Oil-Brine Interfacial Properties from Molecular Perspectives

Oil-brine interfacial tension (IFT) is a fundamentally important property in a number of engineering applications, including but not limited to, oil recovery process and oil-brine separation. Reducing oil-brine IFT to an ultra-low value (10^-2~10^-3 mN/m) can drastically reduce the residual oil saturation. In addition, oil-brine IFT also significantly affect the working efficiency of demulsifiers, during oil-brine separation process. However, the effects of salinity on oil-brine IFT have been much of debate in the past from both experimental and modeling perspectives. Besides, the effects of commonly present polar components (N-, S-, and O-bearing compounds) in the oil phase on oil-brine IFT have been rarely studied. Therefore, in this work, we use molecular dynamics (MD) simulations to study the oil-brine interfacial properties by designing seven systems containing different oil components (decane with/without polar component), and brine salinity up to ~14 wt%. We carefully investigate the salinity and polar component effects by analyzing IFT, density profiles, orientation parameters, hydrogen bonds, and charge distributions. The results indicate that O-bearing compounds (phenol and decanoic acid) can significantly reduce the oil-brine IFT, and exhibit the highest Gibbs surface excess relative to water, while others, including N-bearing compounds (pyridine and quinoline) and S-bearing compounds (thiophene and benzothiophene), only slightly decrease the oil-brine IFTs and show a relatively small Gibbs surface excess. Increasing salinity can slightly increase the oil-brine IFT, except the system containing phenol which presents a decrease. Phenol and decanoic acid have a preference to be perpendicular to the interface and generate numerous hydrogen bonds with water in the interfacial region, while others prefer to be parallel to the interface with much fewer hydrogen bonds with water. On the other hand, salinity has a marginal effect on orientation of polar molecules and hydrogen bond number in the interfacial region. The charges at the interfaces on the brine and oil sides are negative and positive, respectively, and the polar components disturb the arrangement of water molecules in the interfacial regions, while adding salt ions results in the higher peak values of charges in terms of water and system. Our study should provide some new insight into the oil-brine interfacial properties and clarify some unsettled disputes.