pH-Switchable\nSurfactant-Based Microemulsions: Reversible\nTransition between Microemulsification and Demulsification Triggered\nby Suitable Acids and Bases

pH-switchable surfactant-based microemulsions (SBMEs)\nare those\nthat can switch reversibly between a monophasic state and a fully\nphase-separated state under the alternation of acids and bases, which\nis rarely reported. By using an equimolar mixture of sodium dodecyl\nsulfate and N,N-dimethyldodecylamine (SDS-C<sub>12</sub>A) as a pH-switchable\nsurfactant, a pH-switchable SDS-C<sub>12</sub>A-based microemulsion\n(SDS-C<sub>12</sub>A-ME) has been fabricated for the first time. The\nmain principles of the reversible switching are the reversible destruction/formation\nof the emulsifier, SDS-C<sub>12</sub>A-<i>n</i>-butanol,\nfilm at the oil–water interface due to the alternating protonation/deprotonation\nof C<sub>12</sub>A caused by acids and bases. The byproducts, H<sub>2</sub>O and salt, had an adverse effect on the reversibility of\nSDS-C<sub>12</sub>A-ME, with salt having a greater adverse effect\nthan H<sub>2</sub>O. However, the reversibility of SDS-C<sub>12</sub>A-ME could be enhanced by suitable acids and bases. For example,\nfor the same oil-in-water (O/W) SDS-C<sub>12</sub>A-ME, the number\nof switching cycles with HCl-choline hydroxide (ChOH) as a stimulus\ncan be as large as 11, but only 3 with HCl-NaOH as a stimulus. By\nusing the methyl methacrylate photochemical polymerization as a model,\nsuch a pH-switchable SBME can function as a recyclable reaction medium,\nwhile the resultant poly(methyl methacrylate) has a considerably reproducible\nmolecular weight and narrow molecular weight distribution (polydispersity\nindex is around 1.2) over three cycles. It is anticipated that the\nresults presented in this work will serve as a reference for the design\nand fabrication of pH-switched SBMEs and also that such pH-switched\nSBMEs may have potential applications in practical technological areas\nsuch as industrial reaction media, drug delivery, microreactors, etc.