344 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS On the other hand, if the emulsi- fier is a very effective stabilizer 'but does not rapidly yield a low inter- facial tension for a freshly formed interface, then the use of the homo- genizer or colloid mill may be highly desirable. Otherwise the droplets will not be as fine as they should be, and the emulsion will appear coarse and will tend to cream, if not to separate. In some cases the work can be done physico-chemically in- stead of mechanically. Combining oils plus free fatty acid with water plus alkali, as discussed, is an ex- ample. USE OF ANTAGONISTIC EMULSIFIERS It is frequently taught that emul~ sifters which form O/W emulsions, for instance soaps and the water- soluble anionic and non-ionic agents, are-antagonistic to those which form W/O emulsions, for instance lanolin, cetyl alcohol, and many oil- soluble surface-active agents. The logical .conclusion would be that it was always wrong to use such op- posed emulsifiers together. That this is not necessarily true is at- tested by. many useful formulas. Although an emulsifier sometimes breaks an emulsion stabilized by an antagonistic emulsifier, it is often found that a limited amount of a correctly chosen antagonistic emul- sifier aids in stabilizing an e•mulsion. This may be due to the formation of a dual interfacial film,' with one emulsifier-film more deeply in the water phase and the other more deeply in the oil phase. This would make the transition at the interface more gradual and would also in- crease the thickness of the emulsify- ing layer. It is therefore not uncommon for limited proportions of lanolin or fatty alcohols such as cetyl or stearyl to be used along with soap or sodium lauryl sulfate in stabilizing O/W emulsions. A combination of an oil-soluble and a water-soluble non-ionic agent is sometimes a bet- ter emulsifier than either one alone. One of the largest manufacturers of non-ionic agents recommends use of such combinations in formulas for skin creams, sun-tan lotions, hair dressings, and other cosmetic prep- arations. DIsPEl•SIOr/ OF So•.II•S The dispersion of solids will not be discussed in detail. Solid par- ticles, unlike liquid droplets, cannot ordinarily coalesce completely. They can and do group together, under certain conditions, to form loosely attached flocs. It is much easier to break a floc into its con- stituent particles than to break up the particles themselves. How- ever, to do either is useless if the particles will coalesce into riocs again the moment agitation has ceased. Dispersing agents owe their effectiveness to sorption at the in- terface between solid particles and liquid, thus preventing the par- ticles from flocculating. Here again it is necessary to integrate the me- chanical and the physico-chemical factors or the product will be less satisfactory than desired, and money

SURFACE-ACTIVE AGENTS will be wasted in useless processing or in useless raw materials. An interesting aspect of pigments in powders, lipsticks, and other pig- mented cosmetics, is that their coloring power increases as they are made finer--up to a certain point-- whether subdivision is caused by grinding or by deflocculation. This is true only within limits. Beyond the limit of fine particle size, the coloring power decreases. Particles which are substantially smaller than the wave length of light have no color. Related to this is the fact that the whiteness of an emulsion is con- trolled by particle size. If an emul- sion is quite coarse, it will be grayish and translucent. As the particle size is made smaller, it will become whiter and more opaque, since more surfaces at which light can be re- flected and diffracted are created. However, if the particle size becomes too small, the emulsion will lose whiteness and opacity. The ex- treme example is a colloidal sol which, like a solubilized liquid, is transparent to transmitted light. Usually the problem is that the particles are too coarse rather than too fine. So proper milling and de- fiocculation of such products as lip rouge is important, not only to ob- tain a uniform product, but to ob- tain maximum color. Variations in particle size cause differences in shade. Deflocculation may be important for several reasons. If it is desired to suspend a powder in a liquid, as in calamine lotion or some liquid IN COSMETIC INDUSTRY 345 make-up preparations, flocculation must be avoided. Otherwise a coarse, unattractive precipitation will occur quickly. The consistency of a paste depends largely on degree of fiocculation. Often a very firm paste will break down at once to a liquid or semiliquid when a trace of defiocculating agent . is added. These considerations become very important whenever it is desired to obtain a paste of standardized thickness. CONCLUSIONS The foregoing discussion has neces- sarily been incomplete. Important fields of surface chemistry and cos- metic practice have been omitted or mentioned briefly in passing. To those who are not familiar with the science of surface chemistry, the treatment has probably been sug- gestive rather than informative. Today, with so many new ma- terials and new techniques available, workers in the field of cosmetic science require at least a working knowledge of the principles of sur- face chemistry. Without that, one is reduced to wasteful trial and er- ror or dependence on manufac- turers' recommended formulas. Par- ticularly is this true for the use of the surface-active agents. In the versa- tility of the newer non-ionic agents, in the bactericidal action and unusual wetting properties of the cationic agents, in the incompletely explored special properties and synergistic effects of all classes of these agents, lie the unborn cosmetics of the fu- ture.