Insights into the mechanisms affecting water/oil interfacial tension as a function of salt types and concentrations

The interfacial tension (IFT) between oil and water impacts many industrial operations. For example, in low salinity flooding, water with optimal salinity is injected into an oil reservoir, to change the oil/water interfacial tension and fluid/rock interfacial energy to mobilize trapped oil and enhance the displacement of oil. Theoretical and experimental studies have shown that different ions can enhance or inhibit the production. To develop a fundamental understanding of IFT in these systems, particularly in the context of salinity and the ion-type effect, molecular dynamics simulations were used to probe the change in IFT with the salt types and concentrations. In agreement with experimental measurements, our results show that the oil/water IFT increases with increasing salt concentration, with the increase dependent on the salt species. The results revealed that the presence of divalent anions at the interface increases the electrostatic interactions, which leads to a smaller increase in the IFT compared to other salts. Moreover, the analysis showed that the increase in IFT is due to ion depletion near the interface. The water orientations at the interface and the bulk explain why this depletion occurs.