Nanoemulsions of essential oils to improve solubility, stability and permeability: a review

There is a growing society demand for healthier, safer and environmental-friendly products. For instance, essential oils are emerging as a natural alternatives for replacing synthetic pharmaceuticals. Essential oils have indeed proven in vitro pharmacological activities to treat various diseases. Nonetheless, the application of essential oils is limited by their low solubility, low bioavailability, low permeability, uncontrolled volatility and low long-term stability. These issues can be solved by encapsulation of essential oils in nanoemulsions. Nanoemulsions display unique properties such as nanometric size, increased surface area and stability, which increase the efficiency of pharmaceutical dosage. Here we review nanoemulsion formulations, types of surfactants and oils, and their applications for essential oils encapsulation. We present methods to produce nanoemulsions such as high-pressure homogenization, microfluidizers, ultrasonic homogenization, phase inversion composition and phase inversion temperature. We also discuss nanoemulsion instability.