LAUSR

HYPOTHESIS Stimuli-responsive surfactants (also known as switchable surfactants) can undergo reversible conversions between active and inactive forms under particular stimuli, affecting surface and interfacial activity, aggregation structure, emulsification and solubilisation. Selenium-containing surfactants are of reversibly redox-responsive. Hence, microemulsions (MEs) stabilized by selenium-containing surfactants should reversibly respond to redox reactions. EXPERIMENTS The formation of MEs, consisting of sodium dodecylselanylpropyl sulfate (reduced form, SDSePS-Re) or its oxidized form (SDSePS-Ox), n-butanol, n-heptane, and water, was verified based on a pseudo-ternary phase diagram. Changes in molecular structure between SDSePS-Re and SDSePS-Ox were verified by nuclear magnetic resonance spectrometry and electrospray ionization mass spectrometry. The transition between SDSePS-Re- and SDSePS-Ox -based MEs was systematically characterized through electrical conductivity measurements, cryo-transmission electron microscopy and dynamic light-scattering. FINDINGS Both SDSePS-Re and SDSePS-Ox could stabilize the mixture of n-butanol-n-heptane-water to form MEs. A reversible transition between an SDSePS-Re-based ME and the corresponding SDSePS-Ox-based ME was achieved, which was realized by the oxidation of SDSePS-Re with H2O2 and then reduction with N2H4. Compared with SDSePS-Re, SDSePS-Ox has a lower surface activity, resulting in a difference in solubilization capacity of the oil between SDSePS-Re- and SDSePS-Ox -based MEs. After oxidation with H2O2, phase separation could be observed in some SDSePS-Re-based MEs; however, the SDSePS-Re-based MEs could be recovered after reduction of SDSePS-Ox-based MEs with N2H4.