Film-Forming Microemulsions with Essential Oils: Elucidating Relationships Between Formulation Parameters, Thermodynamic Stability, and Quality Attributes

The incorporation of essential oils into the oil phase of oil-in-water microemulsions is an emerging strategy for the development of stable water-based topical formulations. The introduction of a suitable polymer to formulate film-forming microemulsions may improve topical administration; however, the effect of formulation variables on film quality attributes has not been studied. In this study, thermodynamically stable microemulsion concentrates consisting of surfactant (Kolliphor® RH40), alone or in combination with cosurfactant Transcutol® at surfactant-to-cosurfactant mass ratio 7:3, cosolvent (propylene glycol), and synthetic oils (medium-chain triglycerides or isopropyl myristate) with tea tree, cinnamon, or thyme essential oil were formulated and diluted with hypromellose solution in a water/isopropanol mixture (1:1 w/w) to produce film-forming microemulsions. The type and concentration of synthetic and essential oils and cosurfactant influenced the dynamics of structural transformations upon dilution as well as the rheological behavior, viscosity, and pH of film-forming microemulsions. Films obtained by casting film-forming microemulsions were opalescent, smooth, flexible, and swellable in artificial sweat and water. The weight and yield of films increase with the synthetic oils present and without cosurfactant added. Optimizing the ratio of essential oil/synthetic oil, the type of synthetic oil, and the inclusion/exclusion of cosurfactant allows for achieving the targeted film attributes for cosmetic and pharmaceutical applications, including wound treatment.