Effects of Extended Surfactant Structure on the Interfacial Tension and Optimal Salinity of Dilute Solutions

Extended surfactants with the oxypropylene (PO) group and ethoxylated anionic surfactants with the oxyethylene (EO) group have a high salt tolerance capability. Most of the researches on extended surfactants and ethoxylated anionic surfactants focused on the microemulsion, solubilization, and interfacial tension (IFT) of concentrated surfactant solutions, whereas a few researches focused on the IFT of dilute surfactant solutions. Moreover, a previous work focused only on surfactants with PO numbers greater than 4 and copolymers of PO and EO. The effects of extended surfactants containing short PO chains and no EO groups have not been examined. We measured the IFT and optimal salinity between n-alkanes and dilute solutions of extended surfactants or ethoxylated sulfonates at 30 °C. The effects of the surfactant structure on the equilibrium interfacial tension (IFT_eq) and optimal salinity of the system were studied in detail. As for the effects on IFT, results indicate that the introduction of PO groups leads to their enhanced capability to reduce the IFT prior to cross-salinity and a reduction in the IFT between n-alkanes and surfactant solutions to ultralow values (smaller than 0.01 mN/m) near the optimal salinity. It was also found that extended surfactants with different alkyl chains also entail a cross-salinity; at values lower than the cross-salinity, the IFT reduction capacity of extended surfactants with a long alkyl chain (C16P3SO3) is better than that of extended surfactants with a short alkyl chain (C13P3SO3). As for the effect on the optimal salinity, it was found that the optimal salinity of extended surfactants is lower than that of ethoxylated sulfonates for the same oil phase. It was also found that the optimal salinity of extended surfactants first increased and later decreased with increasing PON. This finding is first proposed based on summarizing some researchers' studies and our experiments.