CO₂-switchable Pickering emulsions of chitosan with recyclable emulsifier in aqueous phase

Demulsification is necessary for the emulsions requiring temporary stabilization. However, the amphiphilicity of emulsifiers can create challenges for demulsification, and their residue in oil phase can also impair the oil quality. Herein, we describe a CO₂-switchable Pickering emulsions stabilized by chitosan aggregates (CAs), in which the emulsifiers could be recycled and reused in water. Various oils, including crude oil, could be emulsified by CAs (0.05 wt%) at pH greater than pK_aH value (acid dissociation constant of the conjugated acid, e.g., pH 6.5). CO₂-triggered reversible transformation between CAs and hydrophilic polymer chains (CCs) facilitated not only rapid demulsification (≤10 min) but also complete recycling of the emulsifier in water, with no residue in the oil. The interfacial tension of the separated oil after 6 cycles was comparable to that of fresh oil (50.2 ± 1.0 mN/m). Elemental analysis of the separated oil further confirmed the absence of emulsifier contamination in oil. More importantly, the Pickering emulsion could be reformed by fresh oil and the recycled aqueous phase after removing CO₂. The CO₂-responsive behavior of CAs was further analyzed with molecular dynamics simulation. This strategy effectively addresses some of the problems associated with other bio-based emulsifiers, such as difficulties in demulsification and potential contaminations in the oil phase, which has significant applications in pipeline transportation and biphasic catalysis.