Experimental cookery

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 con- tinuous 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 266 THE PHASE-VOLUME THEORY 267 coalescing or running together. This third substance is known as an emul- sifying 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 per- manence to the system. Classes of Emulsions Clayton states that two very distinct classes of emulsions exist. ( 1 ) The very dilute emulsions. These emulsions are simple emulsions, containing only oil and water. In this class only the oil in water emulsions are known. (2) The concentrated emulsions. The more concentrated emulsions may be either of two types according to whether the oil or the water is the dis- persed phase. In the ( 1 ) oil-in-water type of emulsion the drops of oil are the dispersed or divided phase. In the (2) water-in-oil type of emulsion the water is the dispersed phase. The factors determining the type of emul- sion formed will be considered later. The Theory of Emulsification Bancroft states that the necessary conditions for forming a stable emul- sion are that the drops of the dispersed phase shall be so small that they will stay suspended and that there shall be a sufficiently viscous film around each drop to keep the drops of the dispersed phase from coalescing. Many theories have been advanced to account for the way or means by which the emulsion is stabilized by the emulsifier. At the present time no theory has been postulated that seems to apply universally to all emul- sions. As Fischer suggests, probably a number of factors play a role, and the relative importance of each varies not only in different emulsions but in one and the same emulsion under different circumstances. Clayton in his latest book on emulsions gives a summary of the various theories for emulsions. Only a few will be mentioned here. The electrical double layer. The oil globules in a pure oil and pure water emulsion carry a negative charge. The water ionizes so that both hydrogen and hydroxyl ions are present. The negative charge on the oil may come from adsorption of the OH ions. These adsorbed hydroxyl ions form a layer around the oil globules. A second layer of oppositely charged ions forms a layer in the liquid outside the layer of negative ions. These two layers of oppositely charged ions are known as the Helmholtz double layer. They are not confined to emulsions but accompany all boundary phenomena. The electric charge is a factor in all emulsions, even those stabilized with emulsifying agents.