Redox-switchable microemulsions with efficient phase separation and surfactant recycling.

Hypothesis: Switchable microemulsions (MEs) are those capable of adaptively responding to the action of internal or external stimuli. For redox-switchable MEs to obtain high-efficiency phase separation and surfactant recycling, it may be one of the keys to adequately turn off the interfacial activity of surfactants and reduce the solubility of the closed surfactants in the oil phase.

Experiments: Monophasic MEs consisting 11-butylselanyl-undecyl sulfate sodium (CSeCSONa), n-butanol, n-octane, and water were fabricated using the pseudo-ternary phase diagram method. Their structural features and droplets size were characterized by conductivity, dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM), respectively. The redox response of MEs was studied using a combination of visual observations and DLS, cryo-TEM, nuclear magnetic resonance (NMR) and thin-layer tomography. The efficient recycling of CSeCSONa from a well-emulsified eluent is conceptually demonstrated.

Findings: The reversible transition between CSeCSONa and CSeOCSONa is achieved under the alternating action of HO and NH, by which CSeCSONa-based monophasic MEs are able to efficiently demulsify and regenerate, respectively, regardless of their type. After HO-induced demulsification of the MEs, CSeOCSONa can be efficiently recycled with the water phase. We hope that such a redox-switching method may benefit some technological applications. For example, it offers exciting possibilities for simultaneous recycling CSeCSONa and removal of oil from a well-emulsified eluent. Around 97.1 ± 0.3 % of CSeCSONa could be recycled over five cycles with no apparent loss. After a simple and conventional treatment with anion-exchange resin and active carbon, the total organic carbon and chemical oxygen demand of the waste water were 17.4 ± 2.8 and 26.2 ± 1.4 mg/L, respectively.