A biphasic liquid preparation containing two immiscible liquids, one of which is dispersed as minute globules into the other, is termed as pharmaceutical emulsion. The liquid in the form of minute globules is the dispersed phase, while the liquid containing the dispersed globules is the continuous phase.

An emulsifying agent is added to disperse two immiscible liquids for a prolonged time period. This agent covers the globules to scatter them indefinitely in the continuous phase, resulting in a stable emulsion.

The diameter of globules in an emulsion lies in the range of 0.25−25μm. The coarse emulsions are known to contain large-sized globules, whereas fine emulsions are known to possess globules of mean diameter below 5μm. The micro-emulsions’ are known to possess globules of diameter as small as 10 nm, and are milky and transparent in appearance.

Classification

Depending on their dispersed phase, emulsions can be categorised into:

Water-in-Oil Emulsions: In these emulsions, the water (dispersed or internal phase) is dispersed as globules in the oil (continuous phase or dispersion medium). For example, butter and salad dressings.

The water is in the dispersed phase and oil is in the continuous phase in w/o emulsions. Usually, these emulsions are applied topically as lotions and creams. These emulsions also provide emollient action. Wool fat, resins, bees wax, synthetic compounds, and soaps formed from divalent bases (like Ca⁺⁺. Mg⁺⁺, and Zn⁺⁺) are a few emulsifying agents used for preparing w/o emulsions.

Oil-in-Water Emulsions: In these emulsions, the oil (dispersed or internal phase) is dispersed as globules in the water (continuous phase or dispersion medium).

Examples of o/w emulsions meant for both internal and external use are:

External: Benzyl benzoate emulsion (Ascabiol), gamma benzene hexachloride emulsion (Seaboma), etc.

Internal: Vitamin A in corn oil in water, liquid paraffin in water (Agarol), Griseofulvin in corn oil in water, etc.

Microemulsions: These emulsions are known to contain globules of size around 0.01μm. Globules of such sizes are incapable of refracting light and are invisible to the naked eye; thus, microemulsions are transparent. They are also unstable because the colloidal particles are in a state of Brownian motion; thus they collide and lead to coalescence and formation of large globules with reduced mobility. Microemulsions enhance the bioavailability of poorly water-soluble drugs.

Fine Emulsions: These emulsions are known to have a milky appearance and contain globules of 0.25−25μm.

Multiple Emulsions: These emulsions are generally referred to as emulsion-within-emulsion. In an oil-in-water-in-oil (o/w/o) emulsion, very small oil droplets are dispersed in water globules of a water-in-oil emulsion; while in a water-in-oil-in-water (w/o/w) emulsion, water droplets are dispersed in the oil phase of an oil-in-water emulsion.

Multiple emulsions are prepared in two stages. For example, a w/o/w emulsion is prepared by forming a water-in-oil system (primary emulsion): then this system is dispersed in a second aqueous phase. Multiple emulsions are used for delivering sustained release dosage forms since the drug in the innermost phase passes through two other phases and then gets released for absorption.