CO₂-Triggered Pickering Emulsion Based on Silica Nanoparticles and Tertiary Amine with Long Hydrophobic Tails

We describe a strategy of fabricating CO₂-triggered oil-in-water Pickering emulsion based on silica nanoparticles functionalized in situ by a trace amount of conventional CO₂-switchable surfactant, N-(3-(dimethylamino)propyl)alkyl amide (C_nPMA). By alternately bubbling CO₂ and N₂ at a moderate conditions (30 °C, 80 mL min^-1), silica nanoparticles reversibly switch between amphipathic and hydrophilic as a result of the adsorption of ammonium (CO₂) and the desorption of tertiary amine (N₂). The emulsion can then be smart switched "on (stable)" and "off (unstable)", along with homogenization, without needing cooling and heating. The switching of the current tertiary-based system is simple, moderate, and environmentally friendly, without contamination and the restriction of rigorous conditions. The surfactant concentration window of the Pickering emulsion is closely related to the length of hydrophobic tail, and the upper limit is no more than 0.20 cmc of that of the corresponding ammonium surfactant. Such a strategy is also suitable for commercial alkyl tertiary amines, without needing complicated organic synthesis.