On the growth mechanisms of nanoemulsions

Abstract

The shelf stability of nanoemulsions made by ultrasound, phase inversion composition, and the Ouzo effect was studied using a range of hydrocarbons, as the model oils, and surfactants. The cube of the average drop radius of the nanoemulsions displayed a linear increase with time. Both Ostwald ripening and coalescence can exhibit such behaviour. A new approach, based on the time evolution of drop size distribution, is proposed for unravelling the aging mechanism of nanoemulsions. Sequences of fall and rise in the average drop size of nanoemulsions were clearly observed. The decrease in the drop size could unambiguously be attributed to Ostwald ripening, but the increase could be due to either Ostwald ripening or coalescence/flocculation. Coalescence was identified as the dominant growth mechanism at low surfactant concentrations evidenced by drop size distribution broadening with time associated with the rise in the average drop size. Ostwald ripening was the dominant mechanism at higher surfactant concentrations where the drop size distributions broadened with time during the falls and narrowed with time during the rises of the average drop size. The nanoemulsions produced via the Ouzo process, displayed a coalescence-dependent transient stage and an Ostwald ripening dominated asymptotic regime in the absence of surfactant. The nanoemulsion produced via phase inversion was found to be the most stable one, however, still showed vulnerability to Ostwald ripening and flocculation in the long term.