Stable emulsions can be produced by using emulsifying agents. These agents reduce the interfacial tension between the two phases and then separate the globules by creating a barrier at the interface.

Surfactants are mainly used as emulsifying agents. The emulsifying agent’s solubility determines the kind of emulsion to be prepared. If using hydrophilic emulsifying agents which are water-soluble, water is the continuous phase and an o/w emulsion results. In contrast, if lipophilic emulsifying agents are being used which are more oil soluble, oil is the continuous phase and a w/o emulsion is produced.

The phenomenon in which any added substance modifies the solubility of the emulsifying agent and change the emulsion type is known as phase inversion.

Usually an ideal emulsifying agent is colourless, odourless, tasteless, non-toxic, non-irritant and is capable of producing stable emulsions at low concentrations.

The efficient emulsifying agents can be grouped into three categories:

Naturally Occurring Emulsifying Agents: These agents are obtained from vegetables or animals. Since these agents are prone to microbial contamination and degradation, their quality differs from batch to batch.

Some of the examples of naturally occurring emulsifying agents are:

Polysaccharides: In the preparation of an oral emulsion, acacia is considered the best emulsifying agent due to its ability to form a thick film at the oil-water interface and provide a barrier against coalescence. But, acacia is too sticky to be used externally; thus, tragacanth is used as a viscosity enhancer which also prevents creaming. Starch, pectin, and carrageenan are some of the popularly used emulsifying agents in this category.

Semisynthetic Polysaccharides: Less viscous grades of emulsifying agents like methylcellulose and carboxymethylcellulose are used for preparing o/w emulsions.

Sterol-Containing Substances: Beeswax, wool fat, and wool alcohols are some of the examples of emulsifying agents of this category which form w/o emulsions.

Synthetic Surfactants: These emulsifying agents are divided as follows on the basis of their ionic properties:

Anionic Surfactants: The surface active anions present in these organic salts (anionic surfactants) provide them their surface-active properties. These may be non-compliant with some of the organic and inorganic cations, and with other large organic cations (like cetrimide). These are o/w emulsifying agents meant for topical applications, and are effective in ionised forms. Emulsions comprising anionic surfactants are more stable at alkaline pH.

Few popular examples of pharmaceutically used anionic surfactants are:

1. Alkali metal and ammonium soaps such as sodium stearate (o/w),
2. Soaps of divalent and trivalent metals such as calcium oleate (w/o),
3. Amine soaps such as triethanolamine oleate (o/w), and
4. Alkyl sulphates such as sodium lauryl sulphate (o/w).

Cationic Surfactants: Since these quaternary ammonium compound comprise of surface-active cations, they show sensitivity towards anionic surfactants and drug. These are employed for preparing o/w emulsions intended for topical use. These surfactants also are more effective in their ionised forms and their emulsions are more stable at acidic pH. Few cationic surfactants like cetrimide and benzalkonium chloride possess antimicrobial activity.

Non-Ionic Surfactants: These synthetic agents, constituting the largest group of surfactants, are required for producing both o/w or w/o emulsions for external as well as internal applications. These surfactants are compliant with anionic and cationic substances. These remain unaffected to pH change. The emulsions formed depend on the balance between hydrophilic and lipophilic groups provided by the HLB (Hydrophilic-Lipophilic Balance) number. Glycol esters, glycerol esters, macrogol ethers, sorbitan esters, and polysorbates are few of the popular examples of non-ionic surfactants.

HLB (Hydrophilic-Lipophilic Balance) System: HLB number ranging from 1-20 is assigned for emulsifying agents which reflects the comparative properties of both lipophilic and hydrophilic parts of the molecule. High HLB numbers from 8-18 designate a hydrophilic group which produces an o/w emulsion. In contrast, low HLB numbers from 36 designate a lipophilic group which produces a w/o emulsion.

Oils and waxy materials are designated with separate HLB numbers, which is essential for selecting only required emulsifying agent for preparing a specific emulsion. For example, liquid paraffin is allotted two HLB numbers, 4 for a w/o emulsion and 12 for an o/w emulsion. However, two or more surfactants can be combined to produce a desired surfactant with required HLB value.

Table presents the HLB values of some of the popularly used emulsifying agents: