Impact of Oil Type on Nanoemulsion Formation and Ostwald Ripening Stability

TLDR

Stable transparent nanoemulsions must first be formed with all droplets below 80 nm and then be protected from Ostwald ripening. By optimizing surfactant architecture and exploiting the high viscosity and large molar volume of long‑chain triglycerides, the authors create remarkably small, kinetically stable nanoemulsions that resist Ostwald ripening. High‑viscosity long‑chain triglycerides alone yield larger droplets, but when mixed with low‑viscosity oils the resulting nanoemulsions are ≤40 nm, some of the smallest reported, and remain thermodynamically stable against Ostwald ripening for over three months when at least 50 % of the oil phase is insoluble triglyceride.

Abstract

The formation of stable transparent nanoemulsions poses two challenges: the ability to initially create an emulsion where the entire droplet size distribution is below 80 nm, and the subsequent stabilization of this emulsion against Ostwald ripening. The physical properties of the oil phase and the nature of the surfactant layer were found to have a considerable impact on nanoemulsion formation and stabilization. Nanoemulsions made with high viscosity oils, such as long chain triglycerides (LCT), were considerably larger ( D = 120 nm) than nanoemulsions prepared with low viscosity oils such as hexadecane ( D = 80 nm). The optimization of surfactant architecture, and differential viscosity eta D/eta C, has led to the formation of remarkably small nanoemulsions. With average sizes below 40 nm they are some of the smallest homogenized emulsions ever reported. What is more remarkable is that LCT nanoemulsions do not undergo Ostwald ripening and are physically stable for over 3 months. Ostwald ripening is prevented by the large molar volume of long chain triglyceride oils, which makes them insoluble in water thus providing a kinetic barrier to Ostwald ripening. Examination of the Ostwald ripening of mixed oil nanoemulsions found that the entropy gain associated with oil demixing provided a thermodynamic barrier to Ostwald ripening. Not only are the nanoemulsions created in this work some of the smallest reported, but they are also thermodynamically stable to Ostwald ripening when at least 50% of the oil phase is an insoluble triglyceride.