Emulsions stabilized by highly hydrophilic TiO₂ nanoparticles via van der Waals attraction

Hypothesis: Highly hydrophilic nanoparticles are generally considered not suitable for stabilizing Pickering emulsions, since they could not be effectively wetted by the oil phase at the water-oil interface. However, highly hydrophilic nanoparticles with good dispersity are possibly absorbed and packed onto the surface of the oil droplets in water via the van der Waals attraction between the nanoparticles and the oil droplets. Hence, a novel “van der Waals emulsion” should be possible to be stabilized by highly hydrophilic nanoparticles. Experiments: Oil-in-water emulsions solely stabilized by pristine TiO₂ nanoparticles (i.e., TiO₂ without any modification or additives) were prepared. The emulsification behavior under varying pH value, oil fraction, particle content and temperature of the emulsion were explored. Composite wax-based beads which encapsulated chemical sunscreen and was coated by TiO₂ nanoparticles, was also fabricated using the obtained emulsion as the templates. Findings: The emulsions displayed the highest stability near the isoelectric points of the TiO₂ nanoparticles, which was attributed to the van der Waals attraction between TiO₂ nanoparticles and oil droplets. Such mechanism was supported by a theoretical analysis based on calculation of the Hamaker constants and experimental evidences. Therefore, this work presents a simple, general and green method for preparing particle-stabilized emulsions.