COSMETIC FUNCTIONS OF SYNTHETIC DETERGENTS 329 active agents as substances whose dilute solutions may promote wet- ting, spreading, penetration, emul- sification, detergency, foaming or similar important utilitarian effects. It should be emphasized that these ef}ects are neither clearly distinct from one another nor mutually ex- clusive. They are complex phe- nomena, difficult to define quantita- tively, whose molecular mechanisms have been explained only in a quali- tative manner. The distinction between detergents, wetting agents, emulsifying agents, etc., is a decep- tive oversimplification in terminol- ogy. It stems from some of the pioneering application work on sur- face-active agents in the textile in- dustry, and has lost much of its significance as the number of indi- vidual applications in other indus- tries has multiplied. It has become accepted usage to employ the less cumbersome word "detergents" for the whole range of surface-active agents as well as for those products which exhibit outstanding cleaning power in a given system. The dis- tinction in meaning is usually quite apparent, from the context. The practical utility of any deter- gent may be limited or promoted by a number of properties not related to surface activity. Among these are solubility, chemical reactivity, physical form, color, and odor. Of the two detergents which were well known before the present era, soap is limited by its instability toward hydrogen ions and heavy metal ions the sulfated oils are limited by their physical form and lack of chemical homogeneity. With liter- ally hundreds of new detergents now available it is usually possible to find a product with just the right combination of physical, chemical, and surface-active properties to ful- fil1 any desired function, including many functions' which heretofore could not be fulfilled at all. The cosmetic industry has been quick to take advantage of this situ- ation. Among outstanding exam- ples are the newer cream formula- tions and the latest shampoos. Creams based on acid-resistant ionic detergents and non-ionic deter- gents are much more versatile than the older soap-based creams. Many of the modern depilatory creams, antiperspirant creams, and medi- cated creams could not possibly be formulated on a soap base. The synthetic detergent shampoos, which function well regardless of water condition, have surpassed the traditional liquid soaps in popu- larity. Numerous specialized items such as bubble baths, certain liquid cream lotions, and skin cleansers for soap-sensitive individuals depend on synthetic detergents of one type or another. Many modern indus- trial skin and hand cleansers de- pend on synthetic detergents rather than on soaps, and the success of synthetic detergents in dentifrices is well established. These spectacu- lar successes have been achieved largely by cut and try methods, with little emphasis on the theoreti- cal basis underlying the utilization of detergents. The field of emul- sions and emulsification is a possible

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