*2.2.1 Spontaneous emulsification*

An emulsion can be fabricated by diluting a biphasic system leading to diffusion of one phase to another. This is usually done by adding the organic phase into the aqueous phase and then a surfactant which is water miscible. The migration of the surfactant causes disorder at the interface of the two phases leading to an increase in the surface area along with the formation of oil droplets in the aqueous phase [16].

To obtain nanoemulsions, the same dilution process is performed on microemulsions. The properties of the nanoemulsion depend on the oil viscosity, surfactant hydrophilic–lipophilic balance and solvent miscibility with water. With the help of an appropriate dilution procedure and composition, both W/O and O/W microemulsions can be used to obtain nanoemulsion. While obtaining it from O/W microemulsion the composition of microemulsion and the procedure of dilution does not matter, whereas while starting with W/O microemulsion the dilution procedure and /or the composition of microemulsion matters. O/W and W/O nanoemulsions can be formed even without a surfactant, this is called the Ouzo effect also known as Solvent displacement method [24, 27, 28]. This phenomenon has mainly been used for fabricating polymeric nanoparticles or nanocapsules using nanoemulsion as a template in drug delivery [25].

#### *2.2.2 Phase inversion*

As addressed earlier, there are different types of Nanoemulsions, either oil in water (O/W) or water in oil (W/O). Phase inversion, as the name suggests, is a fabrication method that involves conversion of O/W to W/O emulsion or vice versa. It utilizes the energy released during this conversion for the formation of droplets. This physical process can be brought about by varying the temperature or phase

volume ratio, giving rise to phase inversion temperature (PIT) method and phase inversion composition (PIC) method [29].

In these emulsification methods it is very important to know the behavior of the surfactant as it plays a significant role in minimizing both droplet size and polydispersity of the nanoemulsion formed. Its properties also depend on the kinetics of the emulsification process, especially if they have high viscosity [27].
