314 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS mining the "dye solubilization" ef- ficiency of surface-active agents as previously described by McBain (3). A comparison of our estimated HLB values and some of the data pub- lished by Lambert and Busse is most interesting. In their derivation of an equation for a solubilizing iso- therm, n and K are constants which are characteristic for each agent. In Table 1 we compare HLB values and TABLE I Esti- log K mated Agent n (2) (2) HLB Igepal CA 0.80 o.•9 •2.8 Emulphor ELA 0.83 o.3 t t3.3 Emulphor ON o. 9• o. • • • 5.4 Sodium Oleate • .06 0.24 18 their data for n and K for several agents. While there is no apparent relationship between HLB and K, there is remarkable agreement be- tween HLB and n. This is being studied further in an effort to es- tablish the existence of the agree- ment. However, until this or some other system of estimation is de- vised, estimation by cross reference of a large number of emulsification tests with established materials ap- pears to be the most satisfactory, though laborious, method. Our original estimation of empiri- cal HLB values for many Atlas sur- face-active agents was based •n re- suits observed in a large number of emulsification studies conducted over several years. These studies were predominately of O/W emul- sions. We found that the emulsi- tiers most often used as O/W emul- sifiers had assigned HLB values within the range of about nine to twelve. The values of our surface- active agents used for other pur- poses were then correlated, with the results shown in Table 2. After TABLE 2 HLB Range Use 4-6 W/O emulsi fiefs 7-9 Wetting agents 8-I 8 O/W emulsifiers t3-• 5 Detergents 5-• 8 Solubilizing we had conceived the idea of HLB as applied to our own materials and had assigned values to many of them we began investigating expan- sion of the idea's usefulness. The Atlas Span and Tween emul- sifiers had found their way into industry as detergents, wetting agents, etc., by every conceivable method and there were a multitude in the development stage aimed at these various uses. When our lab- oratory began correlating HLB of industrially used compounds and development stage materials versus their use, there occurred a self- alignment that was remarkable to see. One could appreciate the poten- tial advantages of any method whereby we could connect HLB data on our own materials to HLB data for all surface-active agents. Widely applied, this could simplify immensely the choice of emulsifiers for a given industrial task. Consideration of these possibili-

CLASSIFICATION OF SURFACE-ACTIVE AGENTS 315 ties led to the realization that here we may have a common denomina- tor that would enable us to relate and pin together all surface-active agents regardless of type which heretofore at best were considered by groups. ESXlMATION OF HLB V^LUES EMULSIFIERS AND OILS In our proposed system, HLB values for new emulsifiers are esti- mated from a series of tests in which actual emulsification behavior is compared. Comparison, by means of blends or mixtures, is made with agents of known HLB values, such as the series of Atlas emulsifiers. Emulsifiers, as used in industry, are almost always blends. The blend is usually most efficient if it combines lipophilic and hydrophilic emulsi- fiers. We usually recommend that Span* and Tween* emulsifiers be used blended. Glyceryl mono- stearate, self-emulsifying grade, is a blend. Even the monovalent soaps, generally used for O/W emulsification, are blends of soap and hydrolyzed fatty acid. Variation of the proportions of the blended emulsifiers has been taught as preferred practice to ob- tain best results. When two emulsi- fiers of known HLB are thus blended for use with a given oil there is an optimum ratio that gives best emulsification and the HLB at this ratio is said to be the required HLB for the oil (to give that type' of emulsion, whether O/W, W/O solu- bilization, etc.) This is expressed by the equation: optimum• /42'•tHLB•t +/4•BHLB• \ ratio ! /4/•t +/.•B = HLB oil wherein: /'•l = the amount (weight) of the first emulsifier (At) used, and /4• = the amount (weight) of the second emulsifier (B) used at the "op- timum ratio," giving the best emulsion HLB•t, HLB• = the assigned HLB values for emulsifiers At and /• HLB oil = the "required HLB" of the oil for the type of emulsion being studied Since good emulsification may occur over a wide range of emulsifier ratios, or since emulsification in the entire series may be only mediocre, the optimum ratio may be obscured. By averaging results with several emulsifiers and several oils the esti- mation is made more precise. In conducting a number of these tests it will be apparent that the infor- mation will have relationship to many factors. To establish the necessary cross relationships, series of emulsions were prepared over a standard for- mula. The emulsion in these tests were prepared in a uniform manner in so far as possible. Ten grams of emulsifier, or the appropriate amount of fatty acid for a soap, was weighed into an 8-oz., tall form, wide-mouthed jar. Ninety-five grams of oil, or molten wax, was added and the jar and contents agitated until the ingredients were well mixed. Ninety-five cubic cen- timeters of water was then added at one time (cold or hot depending upon the melting point of the oil or wax) and the jar capped and shaken. If required, alkali or amine is added