CO<sub>2</sub>‑Switchable High Internal Phase\nPickering Emulsions Stabilized by Small-Molecular Surfactants and\nHydrophilic Silica Nanoparticles

High internal phase Pickering emulsions (HIPPEs) have\nreceived\nincreasing attention recently due to their unique viscosity and rheological\ncharacteristics. Nevertheless, their high viscosity might limit their\napplications in the transportation field. Developing HIPPEs with adjustable\nstability, controllable viscosity, and easy preparation remains a\nchallenge. Here, we reported an O/W HIPPEs costabilized by a novel\nCO<sub>2</sub>-switchable nonionic surfactant (NCOEO<sub>3</sub>)\nand hydrophilic silica nanoparticles with low concentrations, facilitated\nby the adsorption of NCOEO<sub>3</sub> onto the nanoparticles through\nhydrogen bonding. Upon increasing the NCOEO<sub>3</sub> concentration\nto 1 mM, the oil volume fraction of the emulsions can be adjusted\nto 92%. The HIPPEs demonstrate remarkable CO<sub>2</sub> responsiveness\ndue to the reversible transformation of the surfactant structure between\nnonionic (NCOEO<sub>3</sub>) and cationic-nonionic (N<sup>+</sup>COEO<sub>3</sub>) forms triggered by CO<sub>2</sub>. This responsiveness enables\nefficient demulsification at room temperature as well as the recycling\nand recovery of the surfactant within the aqueous phase. More importantly,\nthe inverted highly viscous HIPPEs could be reversibly converted into\nflowable low viscous HIPPEs through the CO<sub>2</sub> trigger. This\nresearch offers an effective method for preparing intelligent HIPPEs\nwith adjustable properties, such as stability, viscosity, and an aqueous\nrecyclable emulsifier, which can meet various practical application\nneeds.