Interfacial Properties of Extended-Surfactant-Based Microemulsions and Related Macroemulsions

摘要

Extended surfactants containing an intermediate-polarity spacer, such as polypropylene oxide, in between the hydrophilic head and the hydrocarbon tail are known to result in superior solubilization and low interfacial tension, though they exhibit slow kinetics. The present work seeks to evaluate both equilibrium and kinetic aspects of extended-surfactant-based micro- and macroemulsions. The interfacial morphology of the extended surfactant membrane, i.e., characteristic length (ξ) and interfacial rigidity (E_r) at optimum middle-phase microemulsion conditions, was characterized using the net-average curvature model. The results showed that extended surfactants resulted in a relatively rigid interfacial membrane compared with conventional surfactants having similar hydrocarbon chain length. In addition, both ξ and E_r parameters increased with the length of the polypropylene oxide spacer. Increasing E_r values correlated to the slow coalescence rates of extended surfactant emulsified systems. Two alternative approaches (the addition of combined linkers and co-surfactant) are shown to overcome the slow kinetics of coalescence while maintaining desirable high solubilization and low interfacial tension.