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An emulsion is a system consisting of two immiscible liquid phases, one of which is dispersed throughout the other in the form of fine droplets. A third component, the emulsifying agent, is necessary to stabilize the emulsion.

The phase that is present as fine droplets is called the disperse phase and the phase in which the droplets are suspended is the continuous phase. Most emulsions will have droplets with diameters of 0.1-100 µm and are inherently unstable systems; smaller globules exhibit colloidal behaviour and the stability of a hydrophobic colloidal dispersion.
Pharmaceutical emulsions usually consist of water and an oil. Two main types can exist: oil-in-water (o/w) and water-in-oil (w/o), depending upon whether the continuous phase is aqueous or oily. More complicated emulsion systems may exist: for example, an oil droplet enclosing a water droplet may be suspended in water to form a water-in-oil-in water emulsion (w/o/w). Such systems or their o/w/o counterparts are termed multiple emulsions and are of interest as delayed-action drug delivery systems.

Traditionally, emulsions have been used to render oily substances such as castor oil and liquid paraffin in a more palatable form. It is possible to formulate together oil-soluble and water-soluble medicaments in emulsions, and drugs may be more easily absorbed owing to the finely divided condition of emulsified substances.

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