pH- and thermo-responsive Pickering emulsion stabilized by silica nanoparticles and conventional nonionic copolymer surfactants

[Display omitted] Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) could be adsorbed on the silica surface via the hydrogen bonding between PEO and silanol (SiOH) groups. This interaction would be inhibited once SiOH is dissociated to SiO- at an increased pH value. Besides, the adsorption should be affected by temperature considering the nature of hydrogen bond. Hence, we speculate that silica nanoparticles modified in situ by adsorbed PEO-PPO-PEO possess a pH- and thermo-sensitive surface activity, making them a stimuli-responsive Pickering emulsifier. Paraffin oil-in-water emulsions stabilized by silica nanoparticles and PEO-PPO-PEO were prepared. Stabilities, droplet morphologies and stimuli-responses were systematically studied using bottle test, optical microscopy and cryo-scanning electron microscopy (cryo-SEM). To clarify the emulsification mechanism, interfacial viscoelastic moduli and desorption energies were determined using the data obtained from drop shape analysis. Silica nanoparticles are hydrophobized and flocculated by adsorbed PEO-PPO-PEO at a relatively low pH and room temperature. Upon the pH or the temperature increased, particles regain their hydrophilicity and dispersity due to the desorption of surfactants. Emulsions stabilized by these surfactant-modified particles are pH- and thermo-responsive and can be repetitively switched between stabilization and destabilization. The switch temperature is controlled by the PEO length. The emulsification mechanism is verified in view of interfacial viscoelasticity and desorption energy. These findings demonstrate a novel and simple strategy of preparing pH- and thermo-responsive Pickering emulsions desirable to many industrial applications.