SURFACE-ACTIVE AGENTS It has long been known that some emulsifiers tend to create and sta- bilize oil-in-water (O/W) emulsions, and others water-in-oil (W/O) emul- ' o s•ons. Many attempts have been ß :. made to classify emulsifiers accord- : ing to this tendency. Probably the most general rule is that emulsifiers will tend to make that liquid which .-•: preferentially wets them or prefer- i.: phase. Thus cetyl alcohol is strongly overbalanced in the non- polar direction, and it shows a defi- nite tendency to stabilize W/O emulsions. Glyceryl monostearate has a more effective polar group and can be made to stabilize either O/W or W/O emulsions. Because it is not far• from the point of optimum balance, for this specific purpose, its effect can be influenced by rela- tively small proportions of decidedly unbalanced emulsifiers. Sodium :.. stearate, which has • still more ef- fective polar group, shows a strong tendency to form O/W emulsions. Solid .emulsifiers may be classified similarly, depending on which phase preferentially wets them. Too strong preferential wetting by either phase, however, prevents concentration at the interface, and is similar to great unbalance in the structure of a molec- ularly-dispersed emulsifier, whereby the agent becomes ineffective. The classic emulsifier f6r W/O emulsion is lanolin. This material is rich in the higher alcohols known as steroIs. It is not too difficult to separate the sterol fraction and in- corporate it in a mineral oil or petrolatum base. This gives the IN COSMETIC INDUSTRY 34• "absorption bases" which are useful as emulsifiers without the odor and tackiness of lanolin. Lanolin and absorption bases are widely used as emulsifiers and skin emollients in cosmetic creams. It is easy to for- mulate creams analogous to cold creams, but with lanolin or an ab- sorption base as the emulsifier. The currently popular emulsion hair dressings generally contain lanolin, both for its emulsifying value and its effect on the hair. EMULSIFIER FORMED IN SITU A freshly created interface in a three-component system may re- quire considerable time to reach its equilibrium interfacial tension. Let us suppose that in a specific system the equilibrium tension is very low- less than one dyne--but that several minutes or hours are required to reach that value, and that a freshly formed in terrace has a high tension-- say 15 or 20 dynes per cm. During agitation, as soon as any portion of the oil begins to be divided, fresh interface with a high interfacial ten- sion is formed. Therefore the oil re- sists subdivision. However, if by vigorous action, the oil is subdivided and maintained in very small drop- lets for an appreciable time, those droplets will achieve the low equilib- rium tension, and the emulsion will be comparatively stable. Here is an example from the field of metal cleaners. We found that a solution of 5 per cent of an oleate soap in kerosene shows high initial tension--12-1{ dynes--against water. Over a period of several

342 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS minutes, this value falls to one dyne or less. On the other hand if the soap is formed at the interface by bringing together a solution of alkali in water and a solution of fatty acid in kerosene, the interfacial tension is at once found to be less than one dyne. From this, the emulsification behavior of these systems can be correctly predicted. When the al- kali in water is shaken very slightly with the fatty acid in kerosene, a highly stable, finely dispersed emul- sion forms at once. However, when the soap in kerosene is shaken with water, even though agitation is pro- longed and violent, most of the kerosene does not emulsify. As soon as shaking is stopped, it rises to the top as a free solvent layer. But if this mixture is passed through a mixer in which a great deal of work is put into the system, a colloid mill for example, a finely dispersed emul- sion is formed, and this emulsion is as stable as the one formed by com- bining alkali in water with fatty acid in kerosene. Advantage is taken of this in preparing many cosmetic emulsions. Usually free fatty acid is incorpo- rated with the oils and waxes, and free alkali is dissolved in the water. Then, at a suitable temperature, the two solutions are brought together. Wherever oil contacts the water, soap will form at the interface, reducing the interfacial tension in- stantly to an exceedingly low value. Thus a very fine emulsion can be formed without much agitation. If the soap were incorporated as such, much more vigorous agitation, possibly the use of a colloid mill or homogenizer, would usually be re- quired. The principle is used widely. In making vanishing creams, the mol- ten stearic acid is mixed with a water solution of the alkali. In making cold creams, a mineral oil solution of the beeswax, which contains free cerotic acid, is mixed with a solution of borax in water. Conventional hand lotions, which are essentially dilute vanishing creams• are made in a similar manner. Thus the stability of an emulsion may not be predictable by the ease with which the emulsion is formed. If we are primarily concerned with prolonged stability of the emulsion, and if very vigorous agitation can be used in the emulsification procedure, then low equilibrium interfacial tension is of primary interest. If, however, it is essential to emulsify an oil effectively without vigorous agitation, then initial interfacial tension is important. If that value is high, poor results will be obtained no matter how low the final value. On thermodynamic grounds, sur- face-active agents of high molecular weight show a stronger tendency to concentrate at oil-water interfaces than low molecular-weight agents of analogous structure and with the same polar-nonpolar balance. How- ever, such high molecular weight agents diffuse slowly and require considerable time to achieve equilib- rium interfacial tension at an oil- water interface. The low molecular weight agents reach equilibrium rapidly, but that equilibrium value