*2.2.2.1 Phase inversion temperature*

The Phase inversion temperature (PIT) method is used when the surfactants are sensitive to changes in temperature. The principle of this method is based on the changes in surfactant spontaneous curvature (molecular geometry) with temperature. For example, in poly(oxyethylene)-type non-ionic surfactant, increase in temperature causes dehydration of the poly(oxyethylene) chains whereas at low temperature these chains are hydrated and hence are hydrophilic in nature. At one temperature the surfactant exhibits both hydrophilic and lipophilic properties, this temperature is known as HLB temperature (Hydrophilic–Lipophilic Balance) [17, 27]. So, at this temperature the surfactant is equally soluble in the oil and aqueous phase [16, 17].

Using PIT very small sizes of droplets can be obtained. At the HLB temperature due to the low interfacial tension the surfactant forms a layer but as soon as the temperature is changed by quick cooling or heating, the surfactant molecules move from one phase into another resulting in the formation of small oil droplets. The movement of the surfactant molecules depends on its hydrophilicity or lipophilicity of its chain which in turn depends on temperature [16, 17] (**Figure 4**).

**Figure 4.**

*Shows the curvature of surfactant and the favorable emulsion formed by heating and cooling [16].*
