Nano-structured microemulsion reversibly responds to CO2 and N2, accompanied with an alternate transition from monophasic microemulsion to near-complete phase separation.

To develop a fast, effective, and reversible strategy for phase separation and re-emulsification of the surfactant-based emulsions, a strategy for using acid/base-mediated redox reactions was established to switch the emulsions formed from a redox-responsive anionic surfactant of potassium dodecyl seleninate (CSeOK). Upon acidification, CSeOK was reduced by KI to give didodecyl diselenide (CSe), a state of almost no surface or interfacial activity; upon basification, (CSe) was oxidized by I to give CSeOK again. The fractional conversion of CSeOK in the reversible switching processes was close to 100%. Consequently, an unusually large change in interfacial tension (ΔIFT) as high as ∼27.1 mN m was obtained at a wider concentration range starting from the critical micelle concentration of CSeOK; the highest IFT at the oil-water interface was obtained after an almost complete switch-off, giving an oil-aqueous solution interface very similar to that without any emulsifiers, which leads to the effective and fast phase separation of the CSeOK-based switchable emulsions.

How to simultaneously realize retrieving oil and recycling surfactant in the remediation of leaked oil-polluted soil by means of surfactant-enhanced soil washing is still a significant challenge. Here, we reported for the first time a novel CO2-switchable anionic surfactant, 11-dimethylamino-undecyl sulfate sodium salt (DUSNa), to retrieve leaked oil and recycle surfactant simultaneously in the process of oil-polluted sand remediation by DUSNa-enhanced soil washing. Because a CO2-switchable tertiary amine group had been incorporated into a traditional anionic surfactant of sodium alkyl sulfate to form DUSNa, DUSNa was readily converted into its inactive form of DUS upon CO2 treatment, leading to complete oil/water phase separation from a DUSNa-stabilized oil-in-water emulsion. The formed DUS was insoluble in both the oil and the water, but was suspended or precipitated in the lower aqueous phase, allowing subsequent retrieval of almost surfactant-free oil. Recycling of DUSNa was enabled by its recovery from DUS by treatment with N2 or NaOH. Upon CO2 treatment, both around 92.1% of the oil and around 90.8% of the DUSNa at least could be retrieved and recycled over three cycles of DUSNa-enhanced soil washing.

To develop a convenient and reversible strategy for phase separation and re‐emulsification of surfactant‐based emulsions, we established a method for pH switching of emulsions formed from a pH‐switchable anionic surfactant, potassium dodecyl seleninate (C12SeO2K). Upon acidification, C12SeO2K was protonated to give a precipitate of dodecyl seleninic acid (C12SeO2H); upon basification, C12SeO2H was neutralized restore C12SeO2K. The pH‐switchable window of the emulsion thus obtained was a pH range of 7 to 8. Reversible changes in both interfacial tension by ~10.2 mN m−1, as well as the mechanical, steric, and/or electrical barriers formed by C12SeO2K at the interface of the oil–aqueous solution account for the fully reversible phase separation and re‐emulsification of the C12SeO2K‐based emulsions. A stable emulsion (i.e. the time needed to separate 1 mL of H2O from 6 mL of emulsion at 25 °C is larger than 1 h) could be cycled at least 25 times when the pH was varied between 7 and 8.