Emulsions typically consist of a polar (aqueous) and a relatively nonpolar (an oil) liquid phase.

A. Based on nature of the internal & external phase:

Based on nature of the internal & external phase, emulsion can be classified into two types.

1. Oil-in-water emulsion

When the oil phase is dispersed as globules throughout an aqueous continuous phase, the system is referred to as an oil-in-water (o/w) emulsion. The emulsifier is present in the external, continuous phase and helps stabilize the interface with the dispersed phase globules.

Example: Hydrophilic emulsifier, such as sodium lauryl sulfate, triethanolamine stearate, sodium oleate, and glyceryl monostearate are used.

2. Water-in-oil emulsion

When the aqueous phase is dispersed, and the oil phase is the continuous phase, the emulsion is termed as water-in-oil (w/o) emulsion. A lipophilic emulsifier is used for preparing w/o emulsions. The w/o emulsions are used mainly for external applications.

Example of emulsifiers: calcium palmitate, sorbitan esters (Spans), cholesterol, and wool fats. Use of a lipophilic emulsifier enables the formation of w/o emulsions with the oil phase as the external, continuous phase.

B. Based on Globules Size

Depending on particle/globules size of the internal phase, pharmaceutical emulsions can be classified as

1. Micro emulsion

Micro-emulsion contains globules of size about 0.01 µm. Droplets of such dimensions cannot refract light and as a result, globules are invisible to the naked eye. So microemulsions are transparent in appearance.

2. Fine emulsion

Fine emulsion have a milky appearance and the globules size range from 0.25 to 25 µm (micrometre).

C. Multiple emulsions

Multiple emulsions are emulsions whose dispersed phase contains droplets of another emulsion. They can be considered as emulsions within emulsions.

Multiple emulsions can be classified as the following

1. Water-in-oil-in-water (w₁/o/w₂) emulsion
2. Oil-in-water-in-oil (o₁/w/o₂) emulsion.

Emulsifying a w/o emulsion using water-soluble surfactants (which stabilize an oily dispersed phase) can produce w/o/w emulsions with an external aqueous phase, which generally has a lower viscosity than the primary w/o emulsion. Oil-in-water-in-oil (o/w/o) type multiple emulsions on the other hand consist of very small droplets of oil dispersed in the water globules of a water-in-oil emulsion.

Application of Multiple emulsion- Sustained release of Drug– the drug that is incorporated in the innermost phase must cross to face boundaries before getting absorbed.

Also, Read…

Introduction to Emulsion

Identification of Emulsion

Formulation of Emulsion

Physical Instability of Emulsion