19.4.2 Surfactants and Cosurfactants

The surfactants or cosurfactants are also known as surface-active agents, and they

can be nonionic, cationic, anionic, and zwitterionic. The role of surfactant is to

determine the type of emulsion, size of dispersed droplet, and stability and some-

times also impart toxicity to the nanoemulsion. Among the toxicity issues, the excess

amount of surfactants may cause gastric irritancy if taken orally, skin hypersensitiv-

ity in topical use, and renal toxicity in parenteral use. Therefore, it is necessary to

critically monitor the type and concentration of surfactants while preparing

nanoemulsions for pharmaceutical use. Surfactant alone is not sufficient to reduce

interfacial tension and stabilize nanoemulsion. Therefore, an additional cosurfactant

is usually required for the preparation of nanoemulsion. Preferably, C3–C8 chain

alcohols (ethanol, glycerin, propylene glycol, polyethylene glycol 400, Transcutol

P) are employed as cosurfactant and are supposed to increase the fluidity as well as

synergistically

reduce

the

interfacial

tension

(Lawrence

1996).

Generally,

cosurfactants are needed in lower concentrations as compared to the surfactant.

For instance, in the preparation of o/w nanoemulsion with one surfactant, a small

area signifying the nanoemulsion region has been observed. However, in combina-

tion with suitable cosurfactant, an increase in the nanoemulsion region towards the

water-rich apex has been observed. Also, more oil can be transformed into nano-

emulsified droplets. Therefore, a proper optimization of surfactant mixture has to be

validated so that surfactant and cosurfactant mixture can provide an optimum

reduction in interfacial tension (Sharma 2018) (Table 19.1).

19.5

Technology Involved for Preparing Nanoemulsion

Nanoemulsions are a non-spontaneous and non-equilibrated system, which means

they require some extra energy to form. Nanoemulsion comprises numerous nano-

droplets which cause an increase in the surface area, and to increase the surface area,

additional energy input is required. Therefore, to fabricate nanoemulsion, the type of

constituents, processing methods (high-energy or low-energy method), and

processing conditions are the critical factors among the nanoemulsion formulation

considerations. The primary aim is to achieve the minimum interfacial tension with

maximum stabilizing capacity and small size. In some cases, the mixture formed

spontaneously by mixing components all together is coarse dispersion, and hence

such premixtures are then subjected to high-energy processes like high-pressure

homogenization, microfluidization, and ultrasonication. These methods have differ-

ent mechanisms to reduce the size from coarse to significantly small size range as

shown in Fig. 19.1. High-energy methods utilize mechanical devices to forcefully

break down the bigger droplets to ultrasmall size. On the other hand, low-energy

processes comparatively require low energy input, and size reduction is carried out

by phase inversion composition, phase inversion temperature, and solvent diffusion

method. Low-energy processes generally utilize the intrinsic properties of the

components.

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Nanoemulsions: A Potential Advanced Nanocarrier Platform for Herbal Drug. . .

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