We report the first
example of a non-aqueous Pickering nanoemulsion,
which comprises glycerol droplets dispersed in mineral oil. The droplet
phase is stabilized by hydrophobic sterically stabilized poly(lauryl
methacrylate)-poly(benzyl methacrylate) nanoparticles which are prepared
directly in mineral oil using polymerization-induced self-assembly.
First, a glycerol-in-mineral oil Pickering macroemulsion with a mean
droplet diameter of 2.1 ± 0.9 μm is prepared via high-shear
homogenization using excess nanoparticles as an emulsifier. Then,
this precursor macroemulsion is subjected to high-pressure microfluidization
(a single pass at an applied pressure of 20,000 psi) to produce glycerol
droplets of approximately 200–250 nm diameter. Transmission
electron microscopy studies indicate preservation of the distinctive
superstructure produced by nanoparticle adsorption at the glycerol/mineral
oil interface, thus confirming the Pickering nature of the nanoemulsion.
Glycerol is sparingly soluble in mineral oil, thus such nanoemulsions
are rather susceptible to destabilization via Ostwald ripening. Indeed,
substantial droplet growth occurs within 24 h at 20 °C, as judged
by dynamic light scattering. However, this problem can be suppressed
by dissolving a non-volatile solute (sodium iodide) in glycerol prior
to formation of the nanoemulsion. This reduces diffusional loss of
glycerol molecules from the droplets, with analytical centrifugation
studies indicating much better long-term stability for such Pickering
nanoemulsions (up to 21 weeks). Finally, the addition of just 5% water
to the glycerol phase prior to emulsification enables the refractive
index of the droplet phase to be matched to that of the continuous
phase, leading to relatively transparent nanoemulsions.