Double emulsions and colloidosomes-in-colloidosomes using silica-based Pickering emulsifiers.

Poly(ethylene imine) (PEI) has been adsorbed onto the surface of fumed silica particles at pH 10 in order to produce an effective "hybrid" Pickering emulsifier. Systematically increasing the PEI/silica mass ratio at a fixed silica concentration of 1.0% w/w modifies the silica particle surface and hence allows the formation of oil-in-water (o/w) Pickering emulsions prepared via homogenization of an aldehyde-rich multi-component fragrance oil (at 12,000 rpm for 2 min at 20 °C). Further increasing the PEI/silica mass ratio leads to phase inversion, producing water-in-oil (w/o) Pickering emulsions. Thus this approach allows formation of stable water-in-oil-in-water (w/o/w) double emulsions using two batches of hydrophilic and hydrophobic PEI/silica hybrid particles that differ only in their PEI/silica mass ratios prior to homogenization. Stable w/o/w double emulsions can be prepared with oil volume fractions ranging from 5 to 42%. Moreover, controlling the volume fraction of the w/o Pickering emulsion homogenized in the presence of an aqueous dispersion of the hydrophilic PEI/silica particles allows the mean diameter of the resulting oil droplets to be conveniently controlled between 20 and 160 μm. Fluorescence microscopy studies confirm that controlling the mean diameter of these oil droplets allows encapsulation of either single or multiple droplets within them. Although these double emulsions do not require cross-linking at either interface to withstand an alcohol challenge, epoxy-amine cross-linking between the physically-adsorbed PEI chains and either an oil-soluble or a water-soluble bisepoxy-based polymeric cross-linker can be achieved to produce novel colloidosomes-in-colloidosomes, which may offer payload retention benefits over conventional colloidosomes.