Bancroft states that the necessary conditions for forming a stable emulsion 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 emulsions. 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.

The phase-volume theory. If spheres of the same diameter are packed as closely as possible, one sphere will touch 12 others and the volume the spheres occupy is about 74 per cent of the total volume. Thus if the spheres or drops of the dispersed phase remain rigid it is possible to disperse 74 parts of the dispersed phase in the continuous phase; but if the dispersed phase is increased to more than 74 parts of the total volume, a reversal of the emulsion will occur. However, the dispersed phase does not remain rigid in shape but the drops flatten out where they come in contact with each other, nor are all the dispersed particles the same size (see Figs. 27 to 30), so that it is possible for the dispersed phase to consist of from 1 to 99 per cent of the emulsion.

Hydration theory of emulsions. Fischer and Hooker state that hydrated colloids make the best emulsifiers. Fischer states the emulsifying agent, by which a permanent emulsion is obtained, invariably "proves to be a hydrophilic colloid when water and oil emulsions are concerned (a lyophilic colloid of some sort when other than aqueous mixtures are under consideration). Put another way, oil cannot permanently be beaten into water, but only into a colloid hydrate."

Fischer and Hooker have found albumin, casein, and gelatin to be good emulsifying agents. Casein when not hydrated, i.e., at its isoelectric point, is a poor emulsifying agent, but hydrated casein, i.e., acid or alkali casein is a good emulsifying agent.

Fischer states that all permanent emulsions can be explained on the basis of hydrated or lyated colloids. He says that when water changes to a colloid hydrate, its physical constants change; and these include, among others, surface tension, viscosity, and adsorption. The treatment of the colloid, such as freezing or heating, or the addition of substances which alter the water-holding capacity of the colloid may crack the emulsion or lessen its emulsifying ability.

Interfacial films. Clayton in discussing "Foods as Colloid Systems" describes interfacial films as follows: "As early as 1840, Ascherson observed 'that coagulation in form of a membrane occurs inevitably and instantaneously when albumin comes into contact with a liquid fat.' - Any solute which lowers the interfacial tension between oil and water will necessarily accumulate at that interface, and in the case of certain proteins, notably albumen, the act of adsorption leads to a change in the physical character of the emulsifying agent, this being 'precipitated' as a fibrous or membrane-fibrous solid, no longer soluble in its original solvent. The existence of such interfacial membranes was verified by Ramsden and other investigators."

In reading the various theories of emulsion one is impressed with the similarity of many factors.

Oriented wedge theory. This theory for the manner in which emulsions are stabilized has been developed from the work of Langmuir and of Harkins. It is based upon the concept that the molecules of the emulsifier orient themselves in the interface between the dispersed and continuous phases, forming a wedge, the curvature of which determines the size of the dispersed phase. Fuller accounts may be found in Clayton's book and in the articles of the authorities mentioned above.

Adsorbed film and interfacial tension theory. This theory has been developed or rather extended from earlier theories. At the present time it is probably the most universally accepted theory for the formation of emulsions. Bancroft stated the underlying principles, basing them upon Don-nan's early work of interfacial tension; but many others have extended the interpretations. Clayton states that with this theory "emphasis is laid upon the fact that emulsification is influenced by (1) the mass of the emulsifying agent present, (2) the ease with which this agent is adsorbed at the interfacial separating surface, and (3) the nature of the ions adsorbed by the resultant film."

The emulsifier may be adsorbed by the water or by the oil, but it is usually adsorbed more in one liquid than in the other and thus lowers the interfacial tension of one liquid to a greater extent than that of the other. If the tension of the water is lowered more than that of the oil, the water has less tendency to form drops, flows to form a film more readily, and becomes the continuous phase. Thus the type of emulsion formed depends upon the nature of the emulsifying agent. The above is often worded somewhat as follows: if the emulsifying agent is more soluble in water than in the oil the water becomes the continuous phase, or if the emulsifying agent is wetted more by the water than by the oil, the water becomes the continuous phase. When the tension on each side of the film or the emulsifying agent is the same no emulsion is formed. This may occur when opposing emulsifying agents are in the mixture and the effect of each counterbalances that of the other.

To form an emulsion the emulsifier must be adsorbed at the interfacial surface and form a sufficiently coherent film to stabilize the emulsion. The reversal of the emulsion depends upon the nature of the ions adsorbed by this film. Bhatnagar stresses more than previous workers the necessity for wetting of the adsorbed film.

The making of permanent emulsions is important for foods, cosmetics, pharmaceutical preparations, sprays, and other products. But breaking of emulsions is important in crude oil operations for recovery of the oil. Hence emulsions have been investigated from many angles and it is not surprising that no one theory applies to all types of emulsions.