328 JOURNAL OF THE SOCIETY TABLE 1--C LASSlFICATION OF SURFACE- ACTIVE AGoeNTS ACCORDING TO HYDROPHILIC Gl•ouPs I. Ionogenic A. Anionic 1. --COOH 2. --O--SOaH 3. --SOaH 4. Phosphoric and miscellaneous B. Cationic 1. Non-quaternary nitrogen bases 2. Quaternary nitrogen bases 3. Non-nitrogenous C. Ampholytic II. Non-Ionic A. Polyhydroxy compounds B. Polyether compounds rectly or indirectly from the natural fatty acids. (2) Branched chain aliphatic hydrocarbon groups de- rived frotan kerosene or polymerized lower alkenes. (3) Branched chain aliphatic hydrocarbon groups de- rived from purely synthetic sources, e.g., 2-ethylhexanol. (4) Alkyl aro- matic hydrocarbon groups, in which the alkyl group is usually branched, and ranges from 3 to 20 carbon atoms. (5) Terpene groups, de- rived from rosin and similar sources. A third basis for classification is the nature of the group which serves to link together the hydrophobic and hydrophilic portions of the mol- ecule. The main types of linkage are: (1) Direct linkage as in the fatty alkyl sulfates or alkyl aro- matic sulfonates, (2) e'ster groups, (3) acid amide groups, (4) ether groups. It is apparent, even from this highly abridged classification system, that several hundred different types of surface-active compounds and al- most innumerable individual prod- ucts may be prepared. There are commercially available today about OF COSMETIC CHEMISTS 30 different types represented by some hundreds of individual prod- ucts. Relatively few are available in pure form but most manufactur- ers will furnish data on the nature and extent of the diluents or other substances in the mixture. All of these types and individual products differ quantitatively from one an- other in their surface active proper- ties, and a very wide choice of sur- face behavior or characteristics is accordingly available to the tech- nologist. From the physical chemist's point of view the surface-active agents 'are soluble substances whose pri- mary characteristic is their power to lower the surface tension of their solutions to an extraordinary extent. This effect is intimately related to a strong adsorption of solute mole- cules or ions at the phase bounda- ries of the solution. An equally im- portant characteristic of surface- active agents is the strong associa- tion of their molecules or ions in solution. The associated particles or micelles are large enough to con- fer the typical properties of a colloid to the solution. The association reaction is reversible and rapidly assumes an equilibrium state. Sur- face-active solutions are accordingly molecularly dispersed in extremely dilute solutions and only assume the typical associated miceilar form when a certain critical concentra- tion range is reached. This critical region for most common surface- active agents is in the neighborhood of 0.01 to 0.5 per cent. The technologist regards surface-

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