Colloidal Stability of Emulsifier-free Oil-in-Water Emulsions: Effect of Oil Properties

This study aims to determine the factors affecting the colloidal stabilization of emulsifier-free (EF) oil-in-water (O/W) emulsions prepared by mixing oil and water with a high-powered bath-type ultrasonicator (HPBath-US; 28 kHz, 300 W) in the absence of emulsifiers such as surfactants. The interrelation between the colloidal stability, oil properties (oil density, interfacial tension between oil and water, solubility parameter of oil, and oil viscosity), and emulsion properties (diameter and zeta-potential of oil droplets) of such EF-O/W emulsions were examined for this purpose. The colloidal stability of EF-vegetable oil-in-water emulsions (EF-VEG/W) was significantly higher than that of EF-hydrocarbon oil-in-water emulsions (EF-HDC/W) and EF-fatty acid-in-water emulsions (EF-FA/W). This can be attributed to the larger density of vegetable oils (VEG) (approximately 0.9 g cm^-3), the formation of smaller-sized oil droplets (diameter of approximately 0.2 µm) in the EF-VEG/W emulsions, and the lower solubility parameter of VEG (δ around 1). Furthermore, the formation of smaller-sized oil droplets in the EF-O/W emulsions correlated with the physical properties of the oil.