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

Abstract

Oil–brine interfaces play an important role in oil recovery and oil–brine separation, in which the effects of salinity on interfacial tension (IFT) have been much of debate in the past in experiments and modeling studies owing to complex oil compositions. In this work, we use molecular dynamics (MD) simulations to study the oil–brine interfacial properties by designing seven systems containing different oil compositions (decane with/without polar compounds) and the salinity in brine of up to ∼14 wt %. We carefully investigate the salinity and polar component effects by analyzing IFTs, density profiles, orientation parameters, hydrogen bond densities, and charge density profiles. 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 the 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 in the system containing phenol, which shows a decrease. Phenol and decanoic acid incline 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 an insignificant effect on the orientation of polar molecules and hydrogen bond density in the interfacial region. The charges at the interfaces on the brine and oil sides are negative and positive, respectively, and the polar compounds disturb the arrangement of water molecules in the interfacial regions, while the addition of salt ions result in the higher peak values of charges in terms of water and system. Our study should provide new insights into the oil–brine interfacial issues and clarify some unsettled disputes.