SURFACE-ACTIVE AGENTS IN THE COSMETIC INDUSTRY* By FOSTER DEE SNELL and IRVING R•.ICU Foster D. Snell, Inc., New York, N.Y. IN SOME RESPECZS AN ex- cursion into the realm of surface activity is like going with Alice into Never-Never Land, where up is down and down is up. More often it is an application by sound logic of familiar physico-chemical prin- ciples. SURFACE CHEMISTRY The formulation of cosmetics de- pends largely on the chemistry of surface forces and phenomena. In most such products, one phase has to be uniformly and permanently dispersed in another. Whether the particles dealt with are of colloidal size or are much larger the principles of colloid chemistry are those which apply. Classical colloid chemistry is the study of special properties of sub- stances which have been dispersed to the colloidal range. Such par- ticles are intermediate in size be- tween those in molecular solution and those just visible in an ordinary microscope. Surface chemistry is the study of interfacial phenomena, and as such it provides a theoret?cal * Presented at the May 20, 1949, Meeting, New York City. background for the behavior of col- loidal systems, and for the behavior of many other systems which are not ordinarily considere4 colloidal. However in general usage, the term colloid chemistry is often used as a synonym for surface chemistry. Suspensions of fine particles in liquids, as in liquid make-up or toothpaste, are frequently not of colloidal fineness. Nevertheless the specific interfacial area is great enough for surface effects to become important. Other types of systems with large interfacial areas are powders, emulsions, foams, and mists. Most cosmetic products either exist in one of these forms or are converted to one of them in use. IMPORTANCE OF SURFACE CHEM- ISTRY IN COSMETIC SCIENCE Considerations of surface chem- istry become important when large specific surface areas are involved. In applying cosmetic products to the skin or hair, usually a small amount of product must cover a large area. This introduces con- siderations of wetting and adhesive forces between the cosmetic and the 336

SURFACE-ACTIVE AGENTS IN COSMETIC INDUSTRY 337 skin, surface changes at the interface between cosmetic and skin and at the interface between cosmetic and air, penetration into skin crevices, and sorption by the skin. Many cosmetic products are .•emulsions of one type or another. Questions of emulsion formulation and stabilization are primarily ques- tions of surface chemistry. Other cosmetic products are mixtures of finely powdered materials or sus- pensions of finely powdered solids in aqueous or oily bases. In the latter class are toothpastes, lipsticks, mas- cara pencils, and liquid make-up. Securing properly formulated prod- ucts which will not tend to soften, harden, or separate, which will have and maintain proper color, covering power, texture, etc., depends on ap- plications of principles of surface chemistry the specific interfaces here are those between two liquids or between a liquid and a solid. Certain cosmetic ingredients de- pend for their effect on ability to form a foam or lather. Among these are lather shaving creams and bubble baths. Others are expected by the user to foam under conditions of use, and therefore adequate foaming must be secured. These include shampoos, soaps, and most dentifrices. In foam formation, the interface between water and air is the important one. Soaps, shampoos, and dentifrices are expected to clean skin, hair, and teeth, respectively. This introduces the broad field of surface chemistry known as detergency. That sub- ject is too extensive to be covered here, since so many phases of sur- face activity are involved in de- tergency. Accordingly we should like to limit this discussion to the particular phases of surface chem- istry dealing with emulsification and dispersion. DISPERSION AND EMULSIFICATION In making cosmetics, one may be dispersing a powdered solid in oils, as for lipsticks, or in water, as for toothpaste. One may be mixing and dispersing powders in each other, as in making face powders. One may disperse a liquid in a powdered solid, as when perfume oil is incorporated in face powder, or flavoring oil in tooth powder. Most commonly of all, one disperses' one liquid in another to form an emul- sion. Cold creams, vanishing creams, various special skin creams, cl.eansing creams, hand and face lotions, brushless shaving creams, permanent wave creams, and "milki- fled" shampoos are a few of the many types of cosmetic emulsions. Satisfactory appearance and sta- bility of the product depends upon the correct performance of the dis- persion step. That is why the theory is worth discussing in some detail. ROLE OF MECHANICAL FORCE Regardless of how stable a dis- perse system may be once it has been formed, mechanical work is usually necessary first, in order to obtain the dispersion. The importance ofproper dispersion in cosmetic manufac-