CO2-responsive surfactants for switchable Pickering emulsions with a recyclable aqueous phase

Abstract

Residual surfactants in the oil phase after demulsification may cause several problems, such as self-emulsification of the oil and contamination of oil-soluble products. Due to the amphipathic properties of surfactants, recycling and reuse of surfactants from emulsions and related systems without leaving their residues in the oil phase remains challenging. Here, we report a novel CO₂-switchable surfactant (11-(N,N-dimethylamino) sodium undecanoate, NCOONa), which can stabilize CO₂-switchable Pickering emulsions together with a trace amount of alumina nanoparticles (0.1 wt%). Rapid and complete demulsification of Pickering emulsions can be achieved by bubbling CO₂, in which NCOONa is converted into a more hydrophilic inactive form (N⁺COONa) that almost completely enters the aqueous phase, avoiding residual surfactant in the oil phase. Moreover, the aqueous phase containing NCOONa and alumina particles can be recovered and reused after removal of CO₂ by bubbling N₂ at ambient temperature. This method represents a green and effective strategy for emulsion separation and oil product purification, and will have potential applications in biphasic catalysis, oil transportation, and emulsion extraction.