Preparation of Pickering emulsions and colloidosomes using either a glycerol-functionalised silica sol or core–shell polymer/silica nanocomposite particles

A commercial glycerol-modified 19 nm silica sol has been homogenised with sunflower oil to form stable Pickering emulsions and also covalently cross-linked colloidosomes. Colloidal core–shell polymer/silica nanocomposite particles produced using this glycerol-functionalised silica were also used to produce both Pickering emulsions and colloidosomes containing hybrid shells comprising both inorganic and organic components. The formation of stable oil-in-water Pickering emulsions required either low pH or the addition of electrolyte: this is rationalised in terms of the highly anionic surface character of the silica particles. Colloidosomes are readily obtained on addition of a polymeric diisocyanate, which reacts with the surface glycerol groups on the silica particles. This oil-soluble cross-linker is confined to the interior of the emulsion droplets, thus avoiding inter-colloidosome aggregation. The oil phase can be removed from the colloidosomes by washing with excess alcohol, resulting in microcapsules comprising either a 19 nm particulate silica shell or a 240 nm polymer/silica shell. These microcapsules can be imaged by optical microscopy in solution and by scanning electron microscopy in the dry state. The permeability of these colloidosomes with respect to small molecule release was also examined by incorporating an oil-soluble fluorescent dye during homogenisation that becomes water-soluble on raising the solution pH of the aqueous continuous phase. Finally, control experiments performed with a non-functionalised silica sol confirmed that Pickering emulsions cannot be converted into colloidosomes due to the absence of surface glycerol groups.