Preparation Of TiO₂ Stabilized Pickering Emulsion And Its Application In Sun Protection

Particle-stabilized Pickering emulsions are comparatively more advantageous than conventional surfactant-stabilized emulsions due to their lower dosage of emulsifier,lower toxicity/irritation,multifunctionality as well as higher physical/chemical stability,providing a versatile platform for preparing novel functional emulsions and composite materials.Particularly,Pickering emulsion polymerization/curing methods can be employed for the one-step fabrication of nanoparticle-loaded microspheres,which are commonly used as skin smoother and delievery systems in cosmetics and generally fabricated by two-step method.Nano-Ti O₂ is a physical sunscreen with excellent UV shielding property,but suffers from the problems including agglomeration,whitening effects and possible penetration into the skin follicles.These problems can be possibly solved by loading nano-TiO₂ onto microspheres by Pickering emulsion-templated method.However,it still remains a challege to obtain nano-TiO₂stablized Pickering emulsions with high stability and controlled droplet size.Therefore,the present thesis focuses on the controllable preparation and stablizing mechanisms of the nano-TiO₂ stablized Pickering emulsions.In the thesis,the oil-in-water?O/W?Pickering emulsions were successfully prepared by nano-TiO₂ with high stability and controlled droplet size,and they based on two different stablizing mechanisms.The formation of the emulsions under varying conditions was systemactically explored.Moreover,by using the Pickering emulsions as the templates,polymeric or wax microspheres encapsulated with chemical sunscreen and loaded with nano-TiO₂ were also fabricated and characterized.In part one,colloidal TiO₂ particles with near-neutral wettability were prepared by the sol-gel method and employed to effectively stabilize the oil?vinyl-terminated polydimethylsiloxane?-in-water emulsions.The effects of particle size,particle concentration,pH value,oil to water ratio on the emulsion formation were studied.The stability tests?temperature,pH value,storage time,salt?of the emulsions suggested the ultra-high stability of the emulsions,which should be attributed to the the near-neutral wettability of the particles.And as revealed by the infrared spectroscopy?FTIR?and thermogravimetric analysis?TGA/TDA?,the near-neutral wettability derived from the remained butyl-related groups on the surface of particles.Moreover,the droplet size and rheological behavior of the emulsion could be adjusted by tuning the size of TiO₂ nanoparticles.Then,silicone elastic microspheres?TiO₂@Silicone?with diameters of 1?20?m were prepared by the Pickering emulsion-templated method,exhibiting improved sun protection factor?SPF?and the visual effects of skin.Similarly,polymethylmethacrylate?PMMA?microspheres?1?5?m in diamters?encapsulated with avobenzone?AVB?and loaded with nano-TiO₂ were also fabricated?TiO₂@PMMA-AVB?.Compared to the mixture of PMMA,nano-TiO₂ and avobenzone,such composite structure showed higher SPF value,improved photostability and reduced transdermal effect of AVB.In part two,the thesis found that the commercial bare nano-TiO₂ could also effectively stablize Pickering emulsions by adjusting the pH to its isoelectric point,and the stablizing mechanism was preliminarily studied experimentally and theoretically.Considering the minimal electrostatic repulsion among the nanoparticles near their isoelectric point as well as the weak hydrophilic and hydrophobic effects at the oil-water interface,the emulsion stability was attributed to the van der Waals interaction between the particles and the van der Waals interaction between the oil droplet and particles,which led to the packed nanoparticles anchored at the oil-water interface.The effects of particle concentration,oil-water ratio,oil type,and p H value on the emulsion system were explored.Additionally,the stability of the emulsions was tested at different pH,storage time and temperature.It was found that the emulsion was unstable at very high or low pH and also exhibited freeze-thaw demulsification property.Finally,the wax beads?3?20?m in diamters?coated with nano-TiO₂ and embeded with AVB?TiO₂@Wax-AVB?were prepared by the Pickering emulsion for effectively reducing the photodegradation rate of AVB.