Applied Cosmetic Science and Technology Applied Cosmetic Science and Technology

Lipid-based nanocarriers, including nanoemulsions, are distinguishable and promising vehicles in the cosmetic and pharmaceutical fields. Nanoemulsions are advantageous because they enhance both the efficacy and aesthetic properties of formulations. To maximize these advantages, efforts are needed to increase the surface area of nanosized oil droplets and to control the physicochemical properties of oil, such as the solubility of drugs and skin feeling. However, nanoemulsions are thermodynamically unstable; therefore, difficulties are associated with obtaining stable nanoemulsions, particularly those containing high amounts of polar oil. The aim of the present study was to formulate stable nanoemulsions with high amounts of polar or nonpolar liquid oil. Three different types of oil, silicone oil (SCO), sunflower oil (SFL), and ester oil, were used. Nanoemulsions containing approximately 16% SCO, SFL, and octyl dodecyl lactate (ODL) were successfully formulated. These nanoemulsions had oil droplets of approximately 40 nm in diameter and were transparent or translucent. SCO and SFL were stable at 50°C and RT for 1 month. ODL was unstable at room temperature (RT) for 1 month but was stable for 1 month in a 50°C oven and solidified at RT several hours after its removal from the oven. Small-angle/wide-angle X-ray scattering, cryo-transmission electron microscopy, and differential scanning calorimetry measurements revealed that an α-gel formed at the interface of oil droplets, and it significantly impacted the lipid structure and the stability of nanoemulsions. Namely, ODL, an unstable nanoemulsion, formed an α-gel, while SCO, a stable nanoemulsion, appeared to form a nonperiodic bilayer or a non-lamellar phase with hexagonal chain packing. These results provide important information for the formulation of stable nanoemulsions with various oils.