ΔG ¼ ΔArƴ þ TΔS

ð19:1Þ

where

ΔG is the Gibbs free energy or total free energy

ƴ is the interfacial tension

ΔAr is the change in the surface area of the interface

T is the temperature of the system

ΔS is the change in entropy

19.3

Types of Nanoemulsion

Nanoemulsions are mainly classified as oil in water (o/w), water in oil (w/o), and

bicontinuous phases. In principle, among the two immiscible liquids, one component

will be dispersed phase and the other will be regarded as continuous phase. By

convention, the phase with higher volume fraction becomes the continuous phase

and the other will become dispersed phase, but it is dependent on the type of the

emulsifier. Accordingly, the surfactant molecules will orient themselves around the

dispersed phase and impart kinetic stability. The nonpolar tail part of the surfactant

orients towards the hydrophobic phase, whereas the polar head towards the hydro-

philic phase. Hydrophilic-lipophilic balance (HLB) is a parameter which aids in the

selection of appropriate surfactant. HLB of a surfactant signifies a ratio of hydro-

philic segment to the lipophilic segment. This means the lower the HLB value

(around 3–8), the surfactant is more suited for the preparation of w/o type

nanoemulsion, and higher HLB (around 8–18) forms o/w nanoemulsion (Che

Marzuki et al. 2019). In the case of bicontinuous phase, both immiscible liquids

do not form globules in the dispersion; instead, they form irregular structures or

birefringence. This state is also known as liquid crystalline state.

19.4

Components of Nanoemulsion

19.4.1 Oil

Oil is one of the components of nanoemulsion and is employed to solubilize

hydrophobic drug molecules. Also, oil phase facilitates drug permeation through

the biological membrane of the gastrointestinal tract or skin and improves pharma-

cokinetic behavior. In association with surfactants, oil phase tends to emulsify and

form nano-micelles with hydrophobic drug in solubilized form. In general, for

pharmaceutical use, hydrolyzed vegetable oils, chemically modified triglycerides,

essential oils, and medium-chain fatty acids are preferred. The screening of oil is

done on the basis of drug solubility in oil phase, and also miscibility with surfactant

is also evaluated before the final selection of all excipients.

354

S. Sharma et al.