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

SURFACE-ACTIVE AGENTS may not be low enough for long- term stability of the emulsion. By using a combination of the two types of agent, it is possible to achieve quick effective emulsification and yet high stability. The low molecular-weight agent acts quickly to lower the interfacial tension moderately, and thus helps to dis- perse and temporarily stabilize the emulsion. On aging, the preferen- tially sotbed high molecular weight agent largely displaces the other from the interface, thus yielding a very low final tension. EvruLsIoN It is important to distinguish be- tween breaking and creaming of an emulsion.' Thus in an O/W emul- sion, the oil droplets may actually coalesce and float to the top as a free oil layer. This is breaking. However, the oil droplets will float toward the top even if they do not coalesce. After a time, a clear aqueous layer may be seen at the bottom and a concentrated emul- sion cream at the top. This is creaming. Since the emulsion has not broken, it can be redispersed uniformly by very slight agitation. As has been noted, creaming can promote breaking by forcing the droplets into contact with each other. However, if an effective emulsifier is used, the cream may re- main unbroken for a long time. If a sufficiently aged emulsion cream is drawn off and allowed to stand, it will show little or no tendency to further creaming. The emulsified IN COSMETIC INDUSTRY 343 droplets are already packed as closely as possible, approximating the theoretical calculated packing. Advantage has been taken of this effect to produce unusually stable emulsions by emulsifying 76 per cent of the inner phase with 24 per cent of the outer phase. The point is that close-packed uniform spheres occupy 76 per cent of their contain- ing volume. Of course the 76:24 ratio is not exactly correct, since the droplets are of different sizes and since they can be deformed from their spherical shape under the forces of buoyancy. Nevertheless, emulsions made in this way often show very little or no creaming. An emulsion in which the inner phase is more than about 76 per cent of the total volume is apt to be very stiff, since the droplets are distorted from their spherical shape and can inter- lock instead of rolling past each other. COMBINATIONS OF FoRcEs It is essential for best results that the physico-chemical and the me- chanical factors in emulsification be combined properly. For instance the use of homogenizers or other powerful mechanical disintegrators to form emulsions may actually be harmful if the emulsifying agent is inefficient or insufficient. Many droplets which are uncoated or in- completely coated with emulsifier are formed, and these coalesce rapidly. Less effective agitation would have formed a rather coarse but tolerably stable emulsion. This happens.