The droplet size of an emulsion, in the order of 10 to a few hundred nanometers, is indicative for its characterization as nanoemulsion. However, it should not be confused with microemulsions, which have smaller droplet sizes (i.e., 5—100 nm) and are thermodynamically stable but are created by low-energy emulsification systems (Gupta, Eral, Hatton, & Doyle, 2016; Mehrnia, Jafari, Makhmal-Zadeh, & Maghsoudlou, 2016). Oil-in-water (O/W) nanoemulsions serve as carriers of lipophilic bioactive agents, such as fish oil, essential fatty acids, plant sterols, carotenoids (like (3-carotene), a-tocopherol, dietary fats, or a combination of these compounds via different methods. Generally, the lipophilic compounds are protected in these emulsions since the oil phase entails nutritional oils. On the other hand, water-inoil (W/O) nanoemulsions can be used to encapsulate hydrophilic compounds including most polyphenols, water soluble vitamins, minerals, etc. (Donsi, Annunziata, Sessa, & Ferrari, 2011; Donsi, Sessa, Mediouni, Mgaidi, & Ferrari, 2011). The benefits of nanoemulsions concerning their high stability and bioavailability are extensively reported accordingly in the literature.
Recently, double nanoemulsions, a more complex type of liquid—liquid dispersion, have been receiving increased interest (Assadpour, Maghsoudlou, Jafari, Ghorbani, & Aalami, 2016a, 2016b; Faridi Esfanjani, Jafari, & Assadpour, 2017; Mehrnia, Jafari, Makhmal-Zadeh, & Maghsoudlou, 2017). Double nanoemulsions belong to a bigger group of multiple emulsions that themselves belong to structural emulsions. Double nanoemulsions due to their special structure could be used for nanoencapsulation of hydrophilic compounds in W/O/W emulsions (Assadpour, E., Maghsoudlou, Y., Jafari, S.-M., Ghorbani, M., & Aalami, M. 2016a, Assadpour, E., Maghsoudlou, Y., Jafari, S.-M., Ghorbani, M., & Aalami, M., 2016b; Esfanjani, Jafari, Assadpoor, & Mohammadi, 2015; Hemar, Cheng, Oliver, Sanguansri, & Augustin, 2010) or lipophilic compounds in O/W/O emulsions or both of them in a single double emulsion (Aditya, Aditya et al., 2015).