Acid-resistant ion-pair amphiphile for pH-switchable emulsion liquid membranes

Emulsion liquid membranes (ELMs) have found extensive applications in separation technologies and wastewater treatment. The stabilization of ELMs generally necessitates high surfactant loadings; however, achieving efficient and rapid demulsification remains a major challenge. In this work, we report a pH-responsive ELM system employing a pH-responsive ion-pair amphiphile, hexadecyldimethylammonium dodecylbenzenesulfonate (C16DMA-DBSA), which achieves facile and rapid demulsification post-extraction. Compared with other ion-pair surfactants, C16DMA-DBSA displayed superior emulsifying performance in acidic media. Notably, this ion-pair amphiphile enabled the stabilization of diverse emulsion types (O/W, O/W/O, and W/O) through pH manipulation. Accordingly, a W/O/W ELM system with a strongly acidic internal aqueous phase was constructed for the efficient extraction of metal ions (e.g., Cd (II), Pb (II), Cu (II), Mn (II)) from wastewater, affording an outstanding Cd (II) extraction efficiency of 98.29 ± 0.18% within only 4 min. Furthermore, efficient phase separation under mild conditions was achieved via the pH-triggered demulsification of the ELM system after extraction, eliminating the need for conventional demulsification methods, such as heating, centrifugation, or organic demulsifiers. The recovered oil phase was successfully reused for five consecutive extraction-demulsification-re-emulsification cycles, while maintaining a Cd (II) extraction efficiency of 90.62 ± 1.75%, demonstrating the excellent recyclability and operational stability of the ELM system. This study not only provides a low-energy demulsification strategy for ELM but also offers a novel application direction for the molecular design of switchable ion-pair surfactants.