Introduction

An emulsion is a thermodynamically unstable two-phase system consisting of at least two immiscible liquids, one of which is dispersed in the form of small droplets throughout the other, and an emulsifying agent. The dispersed liquid is known as the internal or discontinuous phase, whereas the dispersion medium is known as the external or continuous phase. Where oils, petroleum hydrocarbons, and/or waxes are the dispersed phase, and water or an aqueous solution is the continuous phase, the system is called an oil-in-water (o/w) emulsion. An o/w emulsion is generally formed if the aqueous phase constitutes > 45% of the total weight, and a hydrophilic emulsifier is used. Conversely, where water or aqueous solutions are dispersed in an oleaginous medium, the system is known as a water-in-oil (w/o) emulsion. W/O emulsions are generally formed if the aqueous phase constitutes < 45% of the total weight and an lipophilic emulsifier is used.

Emulsions are used in many routes of administration. Oral administration can be used, but patients generally object to the oily feel of emulsions in the mouth. But some times, emulsions are the formulation of choice to mask the taste of a very bitter drug or when the oral solubility or bioavailability of a drug is to be dramatically increased.

More typically, emulsions are used for topical administration. Topical emulsions are creams which have emollient properties. They can be either o/w or w/o and are generally opaque, thick liquids or soft solids. Emulsions are also the bases used in lotions, as are suspensions. The term "lotion" is not an official term, but is most often used to describe fluid liquids intended for topical use. Lotions have a lubricating effect. They are intended to be used in areas where the skin rubs against itself such as between the fingers, thighs, and under the arms.

Emulsions are also used a ointment bases and intravenously administered as part of parenteral nutrition therapy. Their formulation and uses in these roles will be covered in the appropriate chapters.

The consistency of emulsions varies from easily pourable liquids to semisolid creams. Their consistency will depend upon:

1. the internal phase volume to external phase volume ratio
2. in which phase ingredients solidify
3. what ingredients are solidifying

Stearic acid creams (sometimes called vanishing creams) are o/w emulsions and have a semisolid consistency but are only 15% internal phase volume. Many emulsions have internal phases that account for 40% - 50% of the total volume of the formulation. Any semisolid character with w/o emulsions generally is attributable to a semisolid external phase.

W/O emulsions tend to be immiscible in water, not water washable, will not absorb water, are occlusive, and may be "greasy." This is primarily because oil is the external phase, and oil will repel any of the actions of water. The occlusiveness is because the oil will not allow water to evaporate from the surface of the skin. Conversely, o/w emulsions are miscible with water, are water washable, will absorb water, are nonocclusive, and are nongreasy. Here water is the external phase and will readily associate with any of the actions of water.

Emulsions are, by nature, physically unstable; that is, they tend to separate into two distinct phases or layers over time. Several levels of instability are described in the literature. Creaming occurs when dispersed oil droplets merge and rise to the top of an o/w emulsion or settle to the bottom in w/o emulsions. In both cases, the emulsion can be easily redispersed by shaking. Coalescence (breaking or cracking) is the complete and irreversible separation and fusion of the dispersed phase. Finally, a phenomenon known as phase inversion or a change from w/o to o/w (or vice versa) may occur. This is considered a type of instability by some.