Molecular Dynamics Study on the Effects of Metal Cations on Microscale Interfacial Properties of Oil–Water-Surfactant System

Water flooding is a secondary oil recovery process where oil from porous rocks is displaced by high-pressure water. Often, water with high salinity was used in this process. In this work, the effects of metal cations on interfacial properties of oil–water-surfactant flooding systems were investigated using molecular dynamics simulation. The results show that the addition of metal cations can cause a slight decrease of the interfacial thickness and the greatly enhanced interaction between the polar head group of surfactant and water molecules, which effectively restrain the water molecules nearby the polar head group. The diffusion coefficients of water molecules in oil–water-surfactant system decrease in the presence of the metal cations. The results of the interface information energy show that the addition of metal cations makes the oil–water-surfactant system more stable. The order parameter analysis showed that the surfactant molecules become more ordered in the normal direction of the interface. Our results demonstrated that the enhanced interaction between water molecules and surfactant head groups due to the existence of metal cations is the driving factor for the change of interfacial properties, especially the interfacial orientation of surfactants. In brief, the most important contribution of our work is the first demonstration of that the enhanced interaction between water molecules and surfactant head groups due to the existence of certain metal cations is the driving factor for the change of interfacial properties especially the interfacial orientation of surfactants. Our work should be valuable for the understanding of interfacial phenomenon of oil–water-surfactant systems and their industrial applications such as for oil displacement.