CO2-Switchable High Internal Phase Pickering Emulsions Stabilized by Small-Molecular Surfactants and Hydrophilic Silica Nanoparticles

High internal phase Pickering emulsions (HIPPEs) have received increasing attention recently due to their unique viscosity and rheological characteristics. Nevertheless, their high viscosity might limit their applications in the transportation field. Developing HIPPEs with adjustable stability, controllable viscosity, and easy preparation remains a challenge. Here, we reported an O/W HIPPEs costabilized by a novel CO₂-switchable nonionic surfactant (NCOEO₃) and hydrophilic silica nanoparticles with low concentrations, facilitated by the adsorption of NCOEO₃ onto the nanoparticles through hydrogen bonding. Upon increasing the NCOEO₃ concentration to 1 mM, the oil volume fraction of the emulsions can be adjusted to 92%. The HIPPEs demonstrate remarkable CO₂ responsiveness due to the reversible transformation of the surfactant structure between nonionic (NCOEO₃) and cationic-nonionic (N⁺COEO₃) forms triggered by CO₂. This responsiveness enables efficient demulsification at room temperature as well as the recycling and recovery of the surfactant within the aqueous phase. More importantly, the inverted highly viscous HIPPEs could be reversibly converted into flowable low viscous HIPPEs through the CO₂ trigger. This research offers an effective method for preparing intelligent HIPPEs with adjustable properties, such as stability, viscosity, and an aqueous recyclable emulsifier, which can meet various practical application needs.