High-pressure microfluidization
is used to prepare a series of
oil-in-water Pickering nanoemulsions using sterically-stabilized diblock
copolymer nanoparticles as the Pickering emulsifier. The droplet phase
comprised either <i>n</i>-octane, <i>n</i>-decane, <i>n</i>-dodecane,
or <i>n</i>-tetradecane. This series of oils enabled the
effect of aqueous solubility on Ostwald ripening to be studied, which
is the primary instability mechanism for such nanoemulsions. Analytical
centrifugation (LUMiSizer instrument) was used to evaluate the long-term
stability of these Pickering nanoemulsions over time scales of weeks/months.
This technique enables convenient quantification of the fraction of
growing oil droplets and confirmed that using <i>n</i>-octane
(aqueous solubility = 0.66 mg dm<sup>–3</sup> at 20 °C)
leads to instability even over relatively short time periods. However,
using <i>n</i>-tetradecane (aqueous solubility = 0.386 μg
dm<sup>–3</sup> at 20 °C) leads to significantly improved
long-term stability with respect to Ostwald ripening, with all droplets
remaining below 1 μm diameter after 6 weeks storage at 20 °C.
In the case of <i>n</i>-dodecane, the long-term stability
of these new copolymer-stabilized Pickering nanoemulsions is significantly
better than the silica-stabilized Pickering nanoemulsions reported
in the literature by Persson et al. (<i>Colloids Surf., A,</i> <b>2014,</b> <i>459,</i> 48–57). This is
attributed to a much greater interfacial yield stress for the former
system, as recently described in the literature (see P. J. Betramo
et al. <i>Proc. Natl. Acad. Sci. U.S.A.,</i> <b>2017,</b> <i>114,</i> 10373–10378)
