COSMETIC FUNCTIONS OF SYNTHETIC DETERGENTS 333 into account the deposition of an adsorbed layer of the detergent on the skin. It has been proved that most detergents leave an adsorbed layer of their own molecules on tex- tile fibers after having removed all the original soil. Soap is known to leave a layer of fatty acids and acid soaps on textile fibers and also on the skin. These layers may have a variety of effects. If no layer is left and the pore linings are not injured as in the case of rubbing alcohol, the se- bum soon builds up and the skin feels normal. A layer of free fatty acid deposited from neutral soap, or of glyceride oil deposited from sul- fated oils, feels normal and pleasant to most individuals and apparently does not disturb normal functioning of the skin. The adsorbed layer of a "drying" detergent may possibly exert a tanning action on the kera- tinized outer cells, or it may ad- versely influence the living cells in the pore linings, thus diminishing the flow of sebum. The adsorbed layer may also, by its chemical nature, evoke the sensation of "dry- ness" in the nerve endings without objectively disturbing the skin func- tions. These are speculations, and have as yet no positive claim to va- lidity. They are not, however, con- tradicted by any known facts con- cerning the interaction of detergent and substrate in a detersire system. They are supported by the fact that the various detergents differ mark- edly in their drying effect on skin and this drying effect bears no re- lationship to their efficiency as oily soil removers, at least as measured on fabrics. The writer is unaware of any quantitative tests on the power of various detergents to re- move sebum from skin. Some very poor detergents such as the lower alkyl naphthalene sulfonates feel extremely drying on the skin, whereas some excellent detergents such as the alkyl aryl ether sulfates leave the skin feeling bland and pleasant. The above considerations hold true at comparable pH values, and although pH is known to have a marked effect on cutaneous reac- tions it is not the decisive factor in this case. With regard to shampoos the above picture is somewhat further complicated. The scalp and hair normally carty a higher soil load than the skin and the mechanical difficulties of cleansing are therefore magnified. The semidetached, ker- atinous, dead cells tend to accumu- late on the scalp in the form of dandruff, and these must be re- moved. Any adsorbed film left on the hair must be lustrous and non- tacky and should contribute to rather than detract from the soft- ness and manageability of the hair. Drying effect on the scalp is fully as important as it is in the case of skin cleansers. The possibility and effect of adsorbed films on both hair and scalp must accordingly be consid- ered. It should be noted that a de- tergent which leaves a lustrous, clean, soft hair shaft might never- theless leave a tight dry feeling on the scalp and thus be unsatisfactory to the user.

334 JOURNAL OF THE SOCIETY These examples indicate that the study of detergent effects in cos- metic preparations could be put on a more fundamental basis and that the current theories of detergency, although admittedly incomplete, could serve as a useful guide in such studies. A somewhat more com- plicated example is found in the emulsion creams. In using most creams the object is to spread a thin layer of the oily component on the skin and to use the aqueous com- ponent merely as a carrier. The problem of producing a satisfactory stable emulsion of good spreading properties has been thoroughly stud- ied. The behavior of such an emul- sion on contact with the skin, how- ever, is less well known. In many cases it is not known whether the emulsions break or remain stable on contact with skin and hair, although it is well known that contact with a foreign solid phase often causes emulsions to break. It has proved possible, in the case of metals and fabrics at least, to formulate emul- sions which will break as desired, with either the oil phase or the wa- ter phase clinging to the substrate. This could quite possibly be done in the case of cosmetic creams. The nature of the detergent or emulsify- ing agent is the major controlling factor in this phenomenon. Among the numerous products now avail- able it is probable that combinations can be found which will cause the emulsion to break in either manner or not to break at all. In shaving cream one of the essen- tial problems is the removal and OF COSMETIC CHEMISTS dispersion of the oily constituents which are added to increase the smoothness, lubricity, and emollient effect on the face. In this case a detergent of very high oil-dispers- ing power must be sought. This property must, of course, be cou- pled with the other well-known es- sential properties, but oil-dispersing power is relatively easy to measure and varies enormously among the different detergents. The synergy of various surface-active combina- tions with regard to emulsifying or oil-dispersing power has been em- phasized in several recent publica- tions, and some of these findings can doubtless be applied directly to the development of more rinsable shav- ing creams. Dentifrices present a problem which seems well suited to straightforward study of detergency. Here the nature of the substrate, tooth enamel, is well known and can even be simulated reasonably well by artificial mixtures. The na- ture of the soil, the so-called "den- tal plaque" is less well recognized, but .could probably be ascertained with no more than normal difficulty. The detergent must act in a diluted saliva medium, and it is known that the soil-substrate bond in this case is very strong. There is no doubt that a wide variation in tooth clean- ing power exists among the differ- ent individual detergents as well as among the different types. It is also possible that a systematic study of dental detergency would point the way toward an optimum tailor-made detergent, superior to