pH Switchable Microemulsions: Minimize Salt Formation to Enhance the Reversibility of Switching

The accumulation of water and salt constitutes one of the main factors impeding the attainment of superior reversibility for the pH-induced microemulsions (MEs) switching, with salt exerting a more significant effect. To address this problem, pH-switched MEs capable of reversible demulsification and microemulsification for over 10 times have been successfully fabricated based on a strategy that minimizes salt formation. The pH-switchable triethylamine laurate (C₁₂TEA) serves as a surfactant for the construction of MEs (C₁₂TEA-MEs), providing a reversible and rapid response to pH. The crucial principle lies in that the byproduct salt is significantly reduced by approximately 92.9–99.0% through ion exchange. Consequently, the types of C₁₂TEA-MEs that can be reversibly switched under pH variation include water-in-oil (W/O), bicontinuous (B.C.), and oil-in-water (O/W). More significantly, an exceptionally smooth phase inversion from W/O to B.C. to O/W is accomplished during reversible switching. Such ME can function as a recyclable reaction medium for styrene polymerization, and the resulting polystyrene exhibits a highly reproducible molecular weight and a narrow distribution over three cycles. Meanwhile, the chemical oxygen demand of the wastewater from the polymerization is significantly reduced to less than 30 mgO₂ L^–1 after a simple and conventional treatment with active carbon and anion-exchange resin. It is expected that the results presented in this work will serve as a reference for the design and fabrication of pH-switched MEs, and also that such pH-switched MEs will have potential prospects in relevant technological fields.