Fabrication of peppermint essential oil nanoemulsions by spontaneous method: Effect of preparing conditions on droplet size

Peppermint essential oil is used in food, cosmetics, and pharmaceutical industries. The fabrication of peppermint nanoemulsion can increase its efficiency at lower concentrations. Spontaneous emulsification is a low-energy method based on the characteristics of the surfactant. Factors affecting the interfacial properties of organic and aqueous phases at the time of mixing can influence the formation and droplet size of emulsions. In this study, nonionic surfactants of Tween 20, Tween 40, and Tween 80 were used to prepare peppermint microemulsions. After selecting an appropriate surfactant, the effect of stirring speed, organic phase temperature, storage for 90 days, and different surfactant-to-oil ratios were evaluated to investigate the emulsifying ability of the surfactant under different conditions and concentrations. The results showed that the best microemulsion was formed using Tween 80 with a droplet size of around 50 nm. Varying the surfactant-to-oil ratio from 50 to 225%, we found that a nanoemulsion was formed at all concentrations. Moreover, we found that the speed of mixing of organic and aqueous phases and the temperature of the organic phase affects the droplet size. At higher surfactant-to-oil ratios, storing the microemulsion for 90 days decreased its droplet size and increased transparency.