**2.2 Characteristics features of ME**

The microemulsion system is a competent and stabilized carrier system for all types of active constituents. The characteristic features of microemulsion are:


most suitable carrier for the delivery of bioactive constituents of essential oils and has excellent potential for industrial applications. The microemulsion system is not only thermodynamically and kinetically stable but also possesses a small droplet size (preferably below 100 nm). It means the formulation can incorporate a large amount of bioactive essential oil in the disperse phase. Due to these characteristic features, essential oil-based ME favors a stabilized and intelligent approach for the delivery of their active ingredients into the targeted site and results in enhanced bio-efficacy. In this chapter, we will discuss the essential oil-based ME and their role

*Nano- and Microencapsulation - Techniques and Applications*

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

tants and co-surfactants, which finally provides a transparent solution [3].

*optically isotropic and thermodynamically stable liquid solution."*

Hoar and Schulman gave the concept of microemulsion in the 1940s. They prepared the first microemulsion by dispersing oil in an aqueous solution of surfac-

The microemulsion, as defined by Danielsson and Lindman, in 1981 [4]. The

"*a microemulsion is a system of water, oil, and an amphiphile which is a single*

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

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**:

in pest management.

**and characteristic features**

definition was as given below:

thermodynamically stable.

**Figure 1.**

**262**

*Phases of the microemulsion system.*

**2.1 Types of microemulsion systems**


#### **2.3 Theories of micro emulsion formation**

Three theories explain the microemulsion formation and stability.

i. **Interfacial or Mixed film theory**: According to this theory, the microemulsion is formed due to the formation of oil and water complex interface reduction by surfactant and co-surfactants. This theory depends on the reduction of interfacial tension and expressed as [Eq. (1)]:

$$\Upsilon\_{\mathbf{i}} = \Upsilon\_{\mathbf{o}/\mathbf{w}} - \Psi \tag{1}$$

Ψ = spreading pressure; ϒ<sup>i</sup> = interfacial tension; ϒo/w = interfacial tension between oil and water.

Interfacial of oil and water reduced to zero and increases the spreading pressure.


**Figure 2.** *Diagrammatic representation of the phase inversion method of microemulsion.*

phase to another contributes in enhancing the entropy which results into reduction of droplet size. The following thermodynamic equation expresses this theory as (Eq. (2):

$$
\Delta \mathbf{G}\_{\mathbf{f}} = \mathbf{\tilde{y}} \,\Delta \,\mathbf{A} - \mathbf{T} \,\Delta \,\mathbf{S} \tag{2}
$$

I. **The viscosity of the microemulsion**: The size of particle in a microemulsion is depended upon the viscosity of the mixture after adding all the surfaceactive agents and water and expressed by the following equation [Eq. (3)]:

*Microemulsion Formulation of Botanical Oils as an Efficient Tool to Provide Sustainable…*

where, η<sup>r</sup> = relative viscosity; η = viscosity of the dispersion; η<sup>o</sup> = solvent viscos-

In the microemulsion system, breaking up of droplets gives droplet volume fractions up to 0.2, the expected relative maximum viscosity is 1.5, which results in

II. **The ratio of water to surface-active agents:** The water level inside the spherical micelles gives the radius measurement by the following expression

where, Vm is the dispersed volume of water; s is the interfacial area by

active agents to protect the more substantial drop — consequently, particles undergo coagulation and flocculation. Therefore, the size of the droplet depends

upon the ration of water to surface-active agents (ώ).

As the water level is high, it lowers the stability due to less capability of surface-

III.**Nature of surface-active agents and concentration of aqueous reactants**: An increase in surface-active agents decreases the particle size. The surfaceactive agents stabilized the microemulsion by reducing the ration of water to surface-active agents (ώ). Therefore, surface-active agents control the droplet size as well as provide stability to the microemulsion system.

IV.**Temperature**: Temperature plays a significant role in droplet size reduction. As temperature decreases, the viscosity of the microemulsion increases, which results in particle agglomeration. Elevated temperature decreases the solubility

droplet interactions and destabilizes the microemulsion system [5].

*Diagrammatic representation of the titration method of microemulsion.*

*DOI: http://dx.doi.org/10.5772/intechopen.91788*

ity; ȹ = volume of droplets.

**Figure 3.**

surfactant molecules.

**265**

[Eq. (4)] and **Figure 4**.

*η*<sup>r</sup> ¼ η*=η*<sup>o</sup> ¼ 1 þ 5*=*2ȹ (3)

r ¼ 3 Vm*=*s (4)

ΔGf = free energy of formation; γ = surface tension of the oil–water interface; ΔA = change in the interfacial area after microemulsion; ΔS = change in entropy of the system after mixing; T = is the temperature.
