Molecular insight into interfacial tension modulated by mixed cationic and anionic surfactants

The oil/water interfacial tension plays a crucial role in tertiary oil recovery, and the low interfacial tension facilitates oil emulsification and stripping. Here, anionic surfactant sodium dodecyl sulfate and cationic surfactant hexadecyltrimethylammonium bromide were employed to study their ability to reduce the oil/water interfacial tension (IFT). Experimental results show that the mixed cationic and anionic surfactants could produce lower IFT than that of individual cationic or anionic surfactant, and the lowest IFT could be obtained at molar ratio of CTAB:SDS = 2:1. Employing molecular dynamics simulation method, the oil/water IFT was calculated and gave same changing trend with that of the experiment. The underlying mechanism of IFT modulated by mixed cationic and anionic surfactants was unveiled from the insight of interfacial structure, interfacial energy, interaction energy between different molecules and interface roughness, and so on. We found the hydration radius of counterion as well as the charge distribution of polar headgroup of surfactant together play crucial role determining the ion distribution inside water phase and surfactant/water interface, and further influencing the IFT. Our study provides an atomic-level understanding of the IFT modulated by cationic and anionic surfactants, and our results might have some references for enhanced oil recovery.