An emulsion is a dispersed colloidal system consisting of two immiscible liquid phases generally stabilized with one or more suitable agents.
Typical pharmaceutical emulsions are prepared from immiscible aqueous and organic (oil) liquids.
Complex multiple-phase systems may exist in an emulsion.
Whether the organic or the aqueous phase is the dispersed phase depends on the volumes of the two phases, the emulsier chosen, and the method of preparation.
When an oil phase is dispersed in an aqueous phase, the emulsionis termed an oil-in-water (O/W) emulsion and water is referred to as the continuous phase.
When water is dispersed in oil, the emulsion is referred to as a water-in-oil (W/O) emulsion.
Emulsions have dispersed phases typically ranging from 0.1 to 100 μm.
Emulsions are opaque while microemulsions are usually transparent or translucent.
Microemulsions have dispersed phases less than0.1 μm.
Emulsions may exhibit three types of instability: flocculation, creaming, and coalescence.
Flocculation describes the process by which the dispersed phase comes out of suspension in the form of flakes.
Coalescence is another form of instability -- small droplets within the media continuously combine to form progressively larger droplets.
Emulsions can also undergo creaming, where one of the phases migrates to the top (or the bottom, depending on the relative densities of the two phases) of the emulsion.
To prevent occulation, creaming, and coalescence of the emulsions, manufacturers commonly add surfactants, pH-modifying agents, emulsifying agents to increase the stability of emulsions so that the emulsion does not change signicantly with time.
Emulsions are widely used as pharmaceutical dosage forms. Oral emulsions have been prepared to improve taste, solubility, stability, or bioavailability. Emulsions for topical administration are referred to as creams, lotions, and sometimes ointments. Parenteral emulsions have been used for anesthetics, parenteral nutrition, and to deliver poorly water-soluble drugs.
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