Molecular insights into the effect of anionic-nonionic and cationic surfactant mixtures on interfacial properties of oil-water interface

The influence of surfactants is of great importance to the interfacial properties, which is related by the characteristics of molecular arrays at the interface of multicomponent and multiphase systems. Comparing with single type of surfactant, the mixtures of surfactants usually provide superior performances in improving interfacial properties of the oil-water interface in the application of enhanced oil recovery. In this study, molecular dynamics simulations were employed to explore the mechanisms and interfacial behaviors at the microscopic level of pure and binary mixture of anionic-nonionic surfactant alcohol ether sulfates (AES) and cationic surfactant dodecyltrimethylammonium chloride (DTAC) at oil-water interface. The results show that the sulfate groups of AES molecules could attract DTAC molecules in the mixed surfactants, thereby reducing the repulsion between the molecules. DTAC molecules present excellent molecular interfacial behaviors, which could improve the arrangement of AES molecules through the interactions between head groups. In addition, it is found that the number of ethylene oxide (EO) group of AES molecules has a great influence on the behaviors of interface. An increase of EO group number could cause AES molecules to exhibit varying degrees of bending. By controlling the proper number of EO group, the aggregation of surfactants could be avoided and satisfactory interfacial properties will be achieved. Moreover, it is observed that the positive charge of DTAC molecule shows a strong repulsion to salt ions, for example Ca2+ ions, thus enhancing the overall salt resistance of mixed surfactants. The molecular-level insights gained in this work could provide useful guidance for designing the surfactant formulations for tertiary/enhanced oil recovery in low-permeability unconventional reservoirs.