Salt-acid-surfactant synergistic effects on interfacial characteristics of water/oil systems: A molecular dynamics simulation study

Low salinity water injection with surfactants is one of the standard approaches in enhanced oil recovery (EOR). This method relies on the interaction of surfactants and ions at oil/water interfaces to reduce the interfacial tensions. However, the details of the mechanisms governing the synergistic effects of these components are still debatable. In this study, molecular dynamic (MD) simulations have been employed to investigate the challenging interactions of chemicals and saline water at a molecular scale. The model oil phase used in this study was defined as a mixture of toluene and decane. In addition, the synergistic materials are NaCl, sodium dodecyl ethoxy sulfate (AES), and decanoic acid. The results of density profiles, planar distribution maps, and PMF calculations imply that the acid molecules in the oil phase act as surfactants and form micelles. A few water molecules are also transferred to the oil phase to form water-acid micelles at high surfactant concentrations. Consequently, the number of acid components in the interfacial area is decreased; hence, the IFT reduction is alleviated. The results showed that micelle formation was decreased by adding ions to the aqueous phase. RDF profiles indicate that surfactant-acid interaction improves by adding ions to the water phase. The higher surfactant-acid interactions keep the polar hydrocarbon molecules near the interface and prevent them from moving to the hydrocarbon phase. This study also highlights the vital role of surfactant-acid interactions in the arrangements and orientations of surface-active materials. Orientation analysis indicates that salts in water and hydrophobic acids synergize to orient surfactants parallel to the interface. These facts signify that the surfactant-acid interaction is one of the main factors that critically affect the water-oil interfacial behavior.