produced. On the other hand, low-energy processes, particularly the phase inversion

composition and solvent diffusion method, show spontaneous emulsification with

slight agitation. All methods of low-energy processes depend on inherent physico-

chemical properties of all components to form nanoemulsions such as solubility,

concentration, and impact of temperature. Unlike the high-pressure energy process,

the low-energy process does not utilize high energy input for reducing interfacial

tension which may generate additional heat in the system. This approach for the

fabrication of nanoemulsion recommends the use of medium-chain triglycerides for

oily phase, whereas these methods are not suitable for long-chain triglycerides.

Hence, this limits the use of several oils in which the herbal bioactive shows good

solubility. But this situation can be resolved in some of the cases by using a mixture

of medium-chain and long-chain triglycerides (Pathania et al. 2018).

19.5.2.1 Phase Inversion Temperature

In this method, a key role is played by the emulsifier which tends to change their

hydrophilic and lipophilic character with respect to temperature at fixed concentra-

tion. With an increase in temperature, the emulsifier changes their curvature at the

interface, and the process of phase inversion begins. For instance, o/w nanoemulsion

is subjected to higher temperature, and with an increase in temperature, the solubility

of the emulsifier tends to alter. Particularly, the solubility of nonionic surfactant

decreases in aqueous solution with an increase in temperature due to the dehydration

of the hydrophilic component of nonionic surfactant. Therefore, at a specific tem-

perature, the type of nanoemulsion reversed, i.e., from o/w to w/o nanoemulsion, due

to the change in the solubility of the emulsifier as a function of time. This tempera-

ture is known as phase inversion temperature. A continuous stirring is generally

required in this technique for the uniform distribution of temperature in the system

and ultimately the uniform influence of temperature on size reduction. Compara-

tively, higher concentration of the emulsifier (such as ceteareth-12, cetostearyl

alcohol, and tetra-ethylene glycol dodecyl ether) is required in this method as it is

mainly the inherent property of the emulsifier which is influenced by temperature

that plays a critical role in phase inversion, droplet size, type of emulsion, and

stability (Pathak 2017; Anton and Vandamme 2009; Förster et al. 1990).

19.5.2.2 Phase Inversion Composition

This is a type of low-energy method wherein the change in fraction of oil to water or

vice versa at fixed temperature leads to phase inversion. The mechanism involved a

change in emulsifier orientation with an increase in dispersed phase volume. Due to

the change in emulsifier orientation, the micelle transformed from one type to

another, i.e., o/w to w/o or vice versa. The process involves slow addition of one

component with slight stirring. In between the complete transition, a phase comes

where the content of both oil and water reaches almost in equal fractions.

Bicontinuous structures are formed in this phase and are also known as liquid

crystalline phase. After this phase further addition of oil or water forms the opposite

type of micelle and with an increase in dilution results a further droplet size reduction

(Sharma and Sinha 2018; Che Marzuki et al. 2019; Sharma 2018).

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