Abstract:Pickering emulsions have been used in a variety of industries including petroleum, food, biomedicine, pharmaceuticals, and cosmetics due to their significant advantages such as stability, compatibility, toxicity-free and low cost over traditional surfactant stabilized emulsions. Recently, Pickering emulsions with stimuli-responsive properties have attracted increasing attention because of excellent and unique properties in controlled stabilization and destabilization. Stimuli-responsive Pickering emulsions can be prepared by stimuli-responsive particles possessing tunable surface properties and surface wettability, which in turn alter their position at the two-phase interfaces. In this work, poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) was used to modify the silica particles to make the responsive emulsifier (SiO₂-PDMAEMA) as well as responsive Pickering emulsions. Because of the tertiary amino groups at the end of the polymer chain and the hydrophobic alkyl backbone, PDMAEMA has pH and thermal responsive properties. Under low pH condition and/or at low temperature, the polymer becomes more hydrophilic due to the protonation of the amino groups and the hydrogen bonding between the amino groups and water molecules. Due to such thermo/pH dual-responsive property of PDMAEMA, the hydrophilic-hydrophobic surface property of SiO₂-PDMAEMA emulsifier can be easily modified. As a result, SiO₂-PDMAEMA particles can stabilize the hexane-water biphasic system and form stable Pickering emulsions at room temperature. The obtained Pickering emulsion is sensitive to pH and de-emulsification occurred at lower pH and emulsification is recovered by increasing pH. Zeta potential of SiO₂-PDMAEMA particles in aqueous dispersions show an isoelectric point (pI) about 9, indicating the deprotonation of the amino group of PDMAEMA when pH>9. Although SiO₂-PDMAEMA particles exhibit an obvious thermo-responsive behavior in aqueous suspensions with a lower critical solution temperature (LCST, 51℃), the Pickering emulsion system don't show significant thermo-responsive behavior. High temperature will benefit to the stability of the Pickering emulsion due to the increased hydrophobic aggregation of the emulsifier particles. The new emulsification system with stimuli-responsive properties will show a great perspective in many applications e.g. drug delivery, multiphase catalysts and surface science, etc.