Emulsions are liquids containing in a suspended state microscopic droplets of other, insoluble liquids. A typical example of natural emulsions is milk. Fats in the blood and lymph are also in an emulsified state. Emulsification of fats in the intestine occurs with the participation of bile salts and is an essential stage of their assimilation by the body. Emulsions, as a rule, are unstable - they spontaneously coalesce droplets with each other (coalescence), leading to delamination of the emulsion-forming liquids. Technical emulsions are usually obtained by mechanically dispersing one liquid in excess of the other in the presence of emulsifiers, substances that prevent coalescence and, consequently, increase the stability of the emulsion. Emulsifiers can be alkaline and alkaline earth metal soaps, proteins, rubbers, etc. The process of obtaining an emulsion is called emulsification, emulsion destruction by demulsification.
Emulsions are of great importance in the preparation of many medicinal and cosmetic products, butter, cream, margarine, mayonnaise, etc.
See also Dispersed systems.The
Emulsions are disperse systems consisting of two mutually insoluble liquids, one of which is distributed into the other in the form of droplets, sometimes distinguishable only in a microscope. Both liquid phases of emulsions usually consist of molecules that are sharply different in character: if one liquid consists of polar molecules, the other is composed of non-polar molecules. Less polar phase in emulsions is usually called "oil." A special position is occupied by emulsions of mercury, to which this classification is not applicable.
Typically, emulsions are prepared by dispersing one liquid into another. Of the two pure liquids, stable emulsions are not obtained, since the droplets merge (coalesce) and the emulsions quickly break down. The formation of a stable emulsion is possible only in the presence of so-called emulsifiers, which, adsorbed on the boundary of both liquids, reduce the surface tension. The presence of adsorption layers on emulsion droplets often prevents their fusion. Sometimes, in order to impart a stability emulsion, it is necessary that the emulsifier develop a protective coating that is mechanically strong, preventing the coalescence of the droplets. In some cases such protective shells are formed by particles of powders, more often they are films of polymers or semicolloids.
To obtain stable concentrated emulsions, the emulsifier must be introduced in relatively large amounts, while for the formation of dilute emulsions, it is often sufficient to have only traces of surface active substances.
Emulsion droplets usually have a spherical shape; in very concentrated emulsions, the droplets are strongly deformed. According to their structural and mechanical properties, concentrated emulsions are similar to gels (see). Emulsions, as a rule, are unstable and destroyed during storage, mainly due to the coalescence of droplets, as well as the combination of very small droplets in flakes [the so-called flocculation, or coagulation (see)] in which individual droplets still exist.The
To accelerate the destruction of emulsions used a variety of techniques: centrifugation, the action of the electric field, the addition of electrolytes and non-electrolytes, etc. Of nonelectrolytes, a particularly strong destructive effect on emulsions stabilized by gel-like films of proteins and semicolloid is provided by those that have high surface activity, but do not form gellike and strong films on the surface. Stable emulsions usually form fluids that are close in density.
When some substances are added, oil-in-water emulsions can be converted into water-in-oil emulsions and vice versa. This phenomenon was called "phase inversion of the emulsion." It can be caused, for example, by the introduction of an emulsifier in the emulsion stabilizing the opposite type of emulsion. Emulsifiers of different types with simultaneous application act antagonistically. Phase reversal with the introduction of an emulsifier of the opposite type occurs gradually: first, an initial emulsion breaks down and an unstable system appears in which both initial and inverse emulsion particles are simultaneously present, along with complex formations containing small droplets of one phase included in larger drops of the other (so-called multiple emulsions), and only after vigorous stirring and introduction of a sufficient amount of emulsifier complete phase reversal occurs.
Emulsions are of great practical importance. Lipids are found in the blood of humans and animals in the form of a highly disperse emulsion (chylomicron emulsion), and the assimilation and exchange of fats are closely related to their ability to form such an emulsion. An example of natural emulsions is milk. Cream and butter obtained from milk and similar artificially prepared systems (mayonnaise and margarine) are also emulsions. In the form of emulsions, many vitamins and medicinal substances are introduced into the body. Concentrated emulsions are many ointments, medical candles and other pharmaceutical preparations. See also Dosage Forms.