Long-Term Stability of n-Alkane-in-WaterPickering Nanoemulsions: Effect of Aqueous Solubility of Droplet Phaseon Ostwald Ripening

摘要

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 n-octane, n-decane, n-dodecane, or n-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 n-octane (aqueous solubility = 0.66 mg dm^–3 at 20 °C) leads to instability even over relatively short time periods. However, using n-tetradecane (aqueous solubility = 0.386 μg dm^–3 at 20 °C) leads to significantly improved long-term stability with respect to Ostwald ripening, with all dropletsremaining below 1 μm diameter after 6 weeks storage at 20 °C.In the case of n-dodecane, the long-term stabilityof these new copolymer-stabilized Pickering nanoemulsions is significantlybetter than the silica-stabilized Pickering nanoemulsions reportedin the literature by Persson et al. (Colloids Surf., A,>2014,459, 48–57). This isattributed to a much greater interfacial yield stress for the formersystem, as recently described in the literature (see P. J. Betramoet al. Proc. Natl. Acad. Sci. U.S.A.,>2017,114, 10373–10378).