To develop a mild, effective, and clean strategy for recovery and recycling of anionic surfactants in CO2/N2‐switchable emulsions, a CO2/N2‐switchable anionic surfactant, which is a combination of dodecyl seleninic acid (DSA) and N,N,N′,N′‐tetramethyl‐1,2‐ethylenediamine (TMEDA), here referred to as DSA–TMEDA, was used to stabilize an oil‐in‐water (O/W) emulsion. Upon stimulation with CO2, DSA–TMEDA was switched off to form insoluble DSA and the water‐soluble TMEDA bicarbonate. Upon N2 bubbling and heating, the OFF state of DSA–TMEDA was restored to the surfactant of DSA–TMEDA. In this manner, O/W emulsions stabilized by DSA–TMEDA can be switched reversibly between demulsification (phase separation) and re‐emulsification (recovered emulsion) by triggering with CO2/N2 over ten times. After breakage of the emulsion, nearly all of the OFF state surfactant could be separated conveniently away from the oil phase, thus facilitating recovery and recycling of the surfactant afterward in emulsifying oil. No obvious adverse changes in the dispersed oil particles size and the relative stability of the regenerated emulsions were observed over five cycles, and the surfactant loss can be neglected during the recycling.
To better control the interfacial activities of selenium‐containing surfactants, a redox‐switchable anionic surfactant containing 2 selenium atoms, namely sodium 3‐((3‐(benzylselanyl)propyl)selanyl)propyl sulfate (SBSe2S), has been designed and synthesized. Upon oxidation by H2O2, the 2 divalent selenide groups in the hydrophobic tail of SBSe2S are converted into the corresponding selenoxides. The initial hydrophobic tail of SBSe2S gets separated into 3 segments by 2 hydrophilic selenoxide groups, destroying the bola‐type structure. The interfacial activities of SBSe2S in aqueous solution could thereby be switched off to a greater degree than in the case of its counterparts containing only a single selenium atom. After reduction with Na2SO3, the 2 selenoxide groups in SBSe2S‐Ox are restored to the initial selenide. Consequently, the interfacial activities of SBSe2S could be reversibly switched by alternate addition of H2O2 and Na2SO3, without obvious deterioration over 5 cycles.
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