Temperature-driven wettability switching of nanoparticles for regulating emulsion inversely thermoresponsive stability

Conventional emulsions generally fail to simultaneously maintain stability at high temperatures and undergo facile demulsification at low temperatures. Here, switchable-wettability nanoparticles (SW-NPs) were prepared by grafting a certain proportion of hydrophilic acrylamide (AM) and hydrophobic methyl methacrylate (MM) groups onto silica nanoparticles and were employed as particulate emulsifiers for oil-in-water Pickering emulsions. The effects of temperature and the AM/MM molar ratio on nanoparticle wettability, oil–water interfacial properties, and emulsion stability were systematically investigated. Among the tested formulations, the nanoparticles at an AM/MM feed molar ratio of 20/80 exhibited the most pronounced inverse thermoresponsive behavior. After 168 h, the water separation rate of the emulsion stabilized by 3.0 wt% AM20%/MM80% nanoparticles decreased from more than 80% at 25°C to only 18% at 60°C. Increasing the temperature weakens hydrogen bonding between the AM groups and water and enhances the contribution of the hydrophobic MM groups, thereby shifting the nanoparticles from a strongly hydrophilic state toward suitable amphiphilicity. The resulting wettability transition promotes nanoparticle migration from the aqueous phase to the oil–water interface, enhancing the elasticity and mechanical strength of the interfacial film. These findings establish a nanoparticle-based strategy for constructing Pickering emulsions that remain stable at elevated formation temperatures but readily destabilize under cooler surface conditions, offering potential for integrated emulsification and demulsification in enhanced oil recovery.