This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Some substances like alcohol and water are miscible in all proportions, i.e., they mix intimately however small or large the proportions used, whereas other substances like oil and water are not miscible.
When a liquid is dispersed in a second liquid with which it is non-miscible the product is called an emulsion. The boundary surface between these two non-miscible liquids is referred to as the liquid/liquid interface, or the dineric interface. Most emulsions are colloidal systems in which the dimension of the dispersed phase is greater than 0.5/x, the upper limit of the colloidal realm. However, the stabilizing film in the concentrated emulsions may have colloidal dimensions.
In cookery, the liquid of the emulsion may be water, milk, a weak acetic acid solution, or some similar liquid. The fat or oil may be any fat or oil used as a food. Mineral oils may also be used for emulsions. If emulsions are mentioned in connection with food preparation, mayonnaise is usually the one suggested first. However, all thickened gravies, sauces, and cream soups are emulsions. Fillings for pies like chocolate cream, and French and cooked salad dressings, may be added to the list. In many of the batter products the fat or oil may be partially or wholly emulsified.
Forming emulsions. To form emulsions, work must be done. The function of the work is not only to separate the dispersed phase into smaller particles, but to increase the surface area, which gives a better opportunity for the two phases to come in contact with each other and increases the area for adsorption of the emulsifying agent. Stirring, beating, shaking, grinding, or some other method may be used to disperse one of the sub-stances.
Dispersed and continuous phases. The substance broken into small portions is called the inside, the discontinuous, or the dispersed phase; the one surrounding the dispersed phase is designated as the outside, the continuous phase, or the dispersing medium. If oil and vinegar are shaken together, French dressing is made, but after standing a few minutes the oil and vinegar separate. This type of emulsion that lasts for a few minutes only is sometimes called a temporary emulsion.
Emulsifiers. A permanent emulsion of pure water and pure oil can be formed only when the proportion of oil is very small, less than 2 per cent, and usually not more than 1 part of oil in 10,000 parts of water. For a permanent emulsion with a high percentage of fat or oil, a third sub-stance must be present to prevent the drops of oil or of water from coalescing or running together. This third substance is known as an emulsifying agent, an emulsifier, or a stabilizer. Its function is to form a film around the oil or water drops and thus keep them dispersed, giving permanence to the system.