pH-Responsive Non-Pickering Emulsion Stabilized by Dynamic Covalent Bond Surfactants and Nano-SiO2 Particles

A novel stimulus-responsive non-Pickering emulsion stabilized by nano-SiO2 particles was prepared in our recent study. 4-formylbenzoic acid and hexylamine through a dynamic covalent bond form a surface-active substance, which was confirmed by Fourier transform infrared (FTIR) and H-1 NMR. Through optimization experiments, it was proved that a stable emulsion can be formed by low surfactant concentration (below cmc) and low nano-SiO2 particle concentration (0.5 wt %). In this emulsion, nano-SiO2 particles are not located at the interface of oil-water but dispersed in the continuous phase of the emulsion, which is different from the Pickering emulsion. The negatively charged nano-SiO2 particles and anionic surfactants repel each other, thereby synergistically stabilizing the emulsion so that the concentrations of surfactants and nanoparticles required to stabilize the emulsion are reduced. In addition, the system can also control the formation and fracture of dynamic covalent bonds by changing pH, thereby controlling the stability and demulsification of the emulsion. At the same time, this non-Pickering emulsion could be used as a microreactor for chemical synthesis and still had a high yield after three cycles. This study provides a new application direction for this environmentally friendly emulsion.