Light-Switchable Pickering Emulsions Emulsified by Spiropyran-Based Ionic Liquid Surfactants and UiO-66-NH2.

Abstract

The main goals of catalytic reactions are to minimize catalyst use, reduce activation energy, and shorten reaction time. Pickering emulsions offer an effective platform for achieving these goals. A key challenge is developing simple, efficient methods that enable fast reactions, easy product separation, catalyst recovery, and emulsifier reuse. Herein, a light-switchable oil-in-water (O/W) Pickering emulsion composed of a spiropyran-based ionic liquid surfactant, UiO-66-NH₂, toluene, and water is presented. The system can reversibly transition between demulsification and emulsification under alternating visible and ultraviolet light. This feature enables the O/W Pickering emulsion to function as a precisely controlled interfacial catalysis reactor for synthesizing hydrophilic products via Click reaction. The catalytic platform is highly efficient, requiring minimal catalyst loading. It reduces activation energy from 33.6 to 8.5 kJ mol^-1, a fourfold decrease, and improves conversion efficiency by more than 50%. Under visible light, the spiropyran structure in water shifts from a hydrophilic open-ring MC form to a hydrophobic closed-ring SP configuration. This structural change enhances the adsorption capacity of the spiropyran-based ionic liquid surfactant on UiO-66-NH₂ and increases its partition coefficient between toluene and water. At the same time, the surfactant's surface activity decreases, which explains the light-responsive phase behavior of Pickering emulsions.