33O JOURNAL OF THE SOCIETY exception to this rule. The theory of emulsions has been extensively studied and discussed in the litera- ture, and is part of the working equipment of the cosmetic chemist. The fundamental factors involved in wetting, penetration, foaming, and detergency, however, are less well recognized, and it is logical to suppose that their application would facilitate even greater efficiency and effectiveness in the use of synthetic detergents. This point might best be illustrated by considering a few specific examples, the first of which concerns skin cleansifig prepara- tions. Cleansing of the skin is essentially a problem in detergency, and much of the knowledge gained in studying detergency on fabrics, metals, ce- ramics, etc., is directly applicable to this problem. A detersire system consists of three essential elements: 1. A solid object to be cleaned, called the substrate. In this case it is the human skin. 2. Water-insoluble "soil" or "dirt" attached to the substrate. The soil may consist of several phases but a typical soil is a mixture of liquid oily material with finely divided solid material. 3. A "bath" which consists es- sentially of a dilute aqueous solu- tion of the detergent. The total bath rather than the relatively small amount of detergent dissolved in it must be considered as the cleaning agent. The dissolved de- tergent confers enhanced cleaning properties to the bath, but the bath may contain builders, protective OF COSMETIC CHEMISTS colloids, and other effective ingredi- ents in addition to the primary de- tergent. The cleaning operation itself may be represented by the equations: Soil-substrate complex n t- bath = soil at- substrate d- bath Soil + bath = soil-bath complex Substrate or- bath = substrate-bath complex The course of these reactions is de- termined only by those observable factors which determine the course of any similar chemical reaction, namely: 1. Nature of the soil. 2. Nature' of the substrate. 3. Composition of the bath. 4. Relative quantities of soil, substrate, and bath. 5. Physical conditions of opera- tion, including temperature, duration, and mechanical agi- tation. The molecular mechanisms in- volved in disrupting the soil-sub- strate complex vary with the indi- vidual system concerned. Among those which are known to occur, separately or concurrently, in vari- ous systems are: 1. Differential wetting of the substrate by the bath and the soil, i.e., a decreasing of the contact angle at the sub- strate-soil-bath boundary line. This effect is connected with adsorption of surface-active ions or molecules on soil and substrate. 2. Electrostatic or coulombic ef- fects, conditioned by the ad- sorption of ions of high spe- cific charge. 3. Protective colloid effects, con- ditioned by the adsorption of

COSMETIC FUNCTIONS OF SYNTHETIC DETERGENTS 331 . surface-active micelies or other colloids in the bath. Solubilization of the soil. Emulsification of liquid soils. Defiocculation and suspension of solid soils. Detailed consideration of these mechanisms is, however, of far less practical importance than the di- rectly observable factors listed above. The skin, considered as the sub- strate in a detersire system, is unu- sually complex. Neglecting the pores (for the time being) the outer- most surface of the skin consists of a reasonably smooth layer of flat, dead keratinized cells. Except in specialized areas this layer, the stratum corneum, is so thin that the sensory nerves in the living lay- ers below can easily perceive minor disturbances on the surface. The dead superficial cells are continu- ously being removed by external attrition and by the progressive maturing and outward migration of new cells from the lower layers. In the last stages of their attach- ment to the body it might be dif- ficult to decide whether they are part of the substrate or part of the soil. With regard to their permea- bility, soil-binding power and reac- tions toward the bath they are probably quite similar to a.hair or a wool fiber. This relatively simple picture is complicated, however, by the presence of the several million skin glands and their open pores. The lining of these pores consists of living unkeratinized epithelial cells and certainly has markedly different properties from the stratum cor- neum. The sudoriparous glands secrete a clear fluid and thus tend to keep themselves clean and free of foreign soil. The sebaceous glands secrete the products of their own necrosis including solid, oily, and aqueous cell debris. The solid debris tends to clog the pore of the gland and in this light it can be re- garded as undesirable soil. When successfully excreted, however, the sebum spreads over the stratum cot- ß neum and forms an oily protective layer, which is said to "lubricate" the skin. This lubrication is desir- able since it renders the surface less liable to abrasive injury. The sebum probably has no direct ef- fect in making the epidermal lay- ers more pliable, after the manner of oil in tanned leather. It doubt- less has an indirect effect, however, since the thin oily layer inhibits the evaporation of moisture and consequent desiccation of the skin. It is apparent that a precise defi- nition of desirable cleanliness with regard to the skin is highly elusive, and is essentially a problem for the dermatologist. Certainly all solid and most liquid soil of foreign origin should be removed. It might also be desirable to remove excesses of oily or solid sebum from the stra- tum corneum. It is questionable whether the sebum should be re- moved completely, although it would probably be desirable to re- move excess solid cell debris from the canals of overactive sebaceous glands before they become clogged and give rise to an ache condition.