pH-Switchable Surfactant-Based Microemulsions: Reversible Transition between Microemulsification and Demulsification Triggered by Suitable Acids and Bases

pH-switchable surfactant-based microemulsions (SBMEs) are those that can switch reversibly between a monophasic state and a fully phase-separated state under the alternation of acids and bases, which is rarely reported. By using an equimolar mixture of sodium dodecyl sulfate and N,N-dimethyldodecylamine (SDS-C₁₂A) as a pH-switchable surfactant, a pH-switchable SDS-C₁₂A-based microemulsion (SDS-C₁₂A-ME) has been fabricated for the first time. The main principles of the reversible switching are the reversible destruction/formation of the emulsifier, SDS-C₁₂A-<i>n</i>-butanol, film at the oil-water interface due to the alternating protonation/deprotonation of C₁₂A caused by acids and bases. The byproducts, H₂O and salt, had an adverse effect on the reversibility of SDS-C₁₂A-ME, with salt having a greater adverse effect than H₂O. However, the reversibility of SDS-C₁₂A-ME could be enhanced by suitable acids and bases. For example, for the same oil-in-water (O/W) SDS-C₁₂A-ME, the number of switching cycles with HCl-choline hydroxide (ChOH) as a stimulus can be as large as 11, but only 3 with HCl-NaOH as a stimulus. By using the methyl methacrylate photochemical polymerization as a model, such a pH-switchable SBME can function as a recyclable reaction medium, while the resultant poly(methyl methacrylate) has a considerably reproducible molecular weight and narrow molecular weight distribution (polydispersity index is around 1.2) over three cycles. It is anticipated that the results presented in this work will serve as a reference for the design and fabrication of pH-switched SBMEs and also that such pH-switched SBMEs may have potential applications in practical technological areas such as industrial reaction media, drug delivery, microreactors, etc.