**5. Fundamentals of Nanoemulsions**

Emulsions are generally made of two immiscible liquids, commonly oil and water, forming a relatively stable mixture. Generally, emulsions are systems that contain a dispersed and continuous phase and can be classified according to the three-dimensional organization of the oil and water phases. Oil-droplets dispersed within an aqueous phase is named *oil-in-water* (*O/W*) emulsion, whereas water-droplets dispersed in the oil phase is classified as *water-in-oil* (*W/O*) emulsion, and they are the most common emulsions [14,56,57]. Figure 5 shows the schematically structures of *O/W* (Figure 5A) and *W/O* (Figure 5B) emulsions, emphasizing the micelle structures dispersed in the continuous phase.

**Figure 5.** Schematic representation of (**A**) *oil-in-water* (*O/W*) and (**B**) *water-in-oil* (*W/O*) emulsions, representing micelle structure dispersed in continuous phase for each system.

Emulsions are classified into three main classes according to thermodynamic stability, stable mechanisms, and physical properties: macroemulsion or conventional emulsion, nanoemulsion, and microemulsion. Conventional and nanoemulsions are thermodynamically unstable, while the microemulsion is stable. The droplet mean radius for conventional emulsions are bigger, which distinguishes them from nanoemulsions with a radius of less than 100 nm [11,57,58]. The droplet size in nanoemulsions is a key-point that influences

their capability to improve the bioavailability of added hydrophobic substances, such as carotenoids [58], and increase antimicrobial essential oil properties [59] or oil compounds [60]. The nanoemulsion classes will be further discussed in this article, with the focus of nanoemulsions as edible nanocoatings.
