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

摘要

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 (IFTeq) 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) nearthe optimal salinity. It was also found that extended surfactantswith different alkyl chains also entail a cross-salinity; at valueslower than the cross-salinity, the IFT reduction capacity of extendedsurfactants with a long alkyl chain (C16P3SO3) is better than thatof extended surfactants with a short alkyl chain (C13P3SO3). As forthe effect on the optimal salinity, it was found that the optimalsalinity of extended surfactants is lower than that of ethoxylatedsulfonates for the same oil phase. It was also found that the optimalsalinity of extended surfactants first increased and later decreasedwith increasing PON. This finding is first proposed based on summarizingsome researchers’ studies and our experiments.