**2. Microemulsion systems: a brief background about the origin and characteristic features**

Hoar and Schulman gave the concept of microemulsion in the 1940s. They prepared the first microemulsion by dispersing oil in an aqueous solution of surfactants and co-surfactants, which finally provides a transparent solution [3].

The microemulsion, as defined by Danielsson and Lindman, in 1981 [4]. The definition was as given below:

"*a microemulsion is a system of water, oil, and an amphiphile which is a single optically isotropic and thermodynamically stable liquid solution."*

In microemulsion systems, surfactants and co-surfactant play an essential role, which stabilized the system by reducing the interfacial surface tension between two immiscible liquids and compensates the dispersion entropy and make the system thermodynamically stable.

#### **2.1 Types of microemulsion systems**

After the 5 years of microemulsion concept, Winsor studied the phase behavior of water, oil, and surfactant and classified microemulsion system in different phases, also known as Winsor phases as shown in **Figure 1**:

**Figure 1.** *Phases of the microemulsion system.*

*Microemulsion Formulation of Botanical Oils as an Efficient Tool to Provide Sustainable… DOI: http://dx.doi.org/10.5772/intechopen.91788*

