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Emulsions are liquid dispersion systems consisting of at least two immiscible liquids (or two liquids that are saturated with each other), one of which is dispersed as small globules (internal or dispersed phase) in another liquid phase (external or continuous phase). Usually stabilized by a third substance called an emulsifying agent. The process of forming an emulsion is called emulsification.
There are two basic types of emulsions
1. Oil-in-water (O/W) emulsions
2. Water-in-oil (W/O) emulsions
Oil in water emulsion
Oil-in-water (o/w) emulsions consist of oil droplets dispersed as globules in an aqueous continuous phase. An oil-in-water emulsion is usually formed when the aqueous phase is more than 45% of the total weight and water-loving emulsifiers such as sodium lauryl sulfate, triethanolamine stearate, sodium oleate, and glyceryl monostearate are used. Emulsifiers are present in the external, continuous phase and help stabilize the interface with the dispersed phase globules.
Fats or oils for oral administration, either as drugs on their own or as vehicles for oil-soluble drugs, are always formulated as o/w emulsions. Oil-in-water emulsions are non-greasy and easily removable from the skin surface. They are used externally to provide a cooling effect and internally to mask the bitter taste of the oil.
Water-in-oil emulsions
In a water-in-oil (w/o) emulsion, the aqueous phase is dispersed as globules in the oil continuous phase. A lipophilic emulsifier is used to prepare w/o emulsions. w/o emulsions are mainly used for external application and may contain one or more of the following emulsifiers: calcium palmitate, sorbitan ester (SPAN), cholesterol and wool fat. Thus, the use of a lipophilic emulsifier enables the formation of w/o emulsions with the oil phase as the external, continuous phase.
However, depending on the need, more complex systems (referred to as “double emulsions” or “multiple emulsions”) where oil-in-water or water-in-oil emulsions are dispersed in another liquid medium.
Considering the particle size, pharmaceutical emulsions can be
1. Macroemulsion (droplet size usually exceeds 10 mm)
2. Mini emulsions (droplet size typically 0.1-10 µm)
3. Microemulsions (droplet size typically 100-600 nm)
4. Nanoemulsions (droplet sizes typically below 100 nm)
Microemulsion
Microemulsions are visually homogeneous, transparent/isotropic systems with low viscosity. In their simplest form, microemulsions are small droplets (diameter 5–140 nm) of a liquid dispersed across each other due to the presence of relatively large amounts of surfactant(s) and cosolvent(s). Microemulsions have a very finely divided dispersed phase and often contain high concentrations of emulsifier(s) and a cosolvent (eg ethanol).
Microemulsions are thermodynamically stable for long periods of time. These can be dispersions of o/w or w/o. The type of microemulsion (w/o or o/w) formed is largely determined by the nature of the surfactant. Microemulsions can be used to enhance the bioavailability of poorly water-soluble drugs by incorporating them into an oily phase. Incorporation of etoposide and methotrexate diester derivatives into w/o microemulsions has been suggested as a potential carrier for cancer therapy.
