Surfactant-free switchable emulsions using CO2-responsive particles.

Surfactant-free emulsions are prepared using bi-wetting particles which occupy the oil-water interface to effectively reduce the oil-water interfacial area. The equilibrium position of the particle at the interface is determined by its wettability. CO2-reponsive chemical functional groups are grafted onto the surface of silica particles. Particles with only CO2-switchable functional groups are capable of stabilizing oil-in-water emulsions. Particles prepared with both CO2-responsive and hydrophobic chemical functional groups on its surface are capable of stabilizing water-in-oil emulsions. Emulsion stability is disturbed when the wettability of the stabilizing particle is altered by introducing CO2 gas to the biphasic mixture, leading to phase separation of emulsions prepared using the functionalized particles. The emulsion stability can be re-established by the removal of CO2 through air sparging. The presence of CO2 imposes positive surface charge to the responsive particles, increasing wettability and, consequently, the ability of the particles to destabilize emulsions.