CLASSIFICATION OF SURFACE-ACTIVE AGENTS 317 The amount of emulsifier used in these tests, 10 gm. for 95 gin. of oil, is much more than is usually re- quired to obtain emulsification. Usually in a given system, the de- gree of emulsification increases with increasing amounts of emulsifier to a point. Beyond this point, the ad- dition of emulsifier does not im- prove or harm the emulsion. The high proportion of emulsifier was selected as providing maxim ex- pected emulsification from the agents under test so that those hay-. ing a poor over-all efficiency would indicate trends. The oils and waxes usually included in an evaluation series are listed in Table 3. These values may be utilized more completely than for a gross estimate of the behavior of the emulsifier. In Fig. 1, we show sev- eral series of samples in which heavy mineral oil is emulsified with several combinations or blends of emulsi- fiers. In each series, the only differ- ence is the types of emulsifiers em- ployed. Two emulsifiers are used in each row, one having a low and one a high HLB. The samples on the extreme left are emulsified with 100% of low HLB agents the samples on .the extreme right are emulsified with 100% of high HLB agents. Blending permits coverage of the complete range of values be- tween the two emulsifiers. From top to bottom, the emulsifiers are blends of laurate, palmirate, stea- rate, and oleate ether-esters (the latter in addition being tri esters). You will note that the weight per- centages of the low and high HLB products required are not similar for best emulsification and that the ß 25'1,'•. __ :'%' .. • • .. , . Figure 2.--Behavior of various oils with one pair of emulsifiers (different ratios best for different oils)

318 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS gross efficiency of the systems are not identical. The important fact to us is that in each instance, the best emulsion is obtained at about an HLB of 10. This is one of the major points on which this system is based. A given oil has an opti- mum HLB for a given type of emul- sification. Different oils require different values as ,nay be seen in Fig. 2. In The optimum required HLB val- ues indicated in Fig. 2 were for O/W emulsions. It will be noted that at tintes a W/O emulsion is formed. This usually appears at the extreme left-hand side of the figure. When this occurs, the oil is suitable for use in W/O formula- tions in major amounts. Often good emulsions are obtained of both types and the-oil can have an opti- mum HLB for either type of emulsion. When we speak of a required HLB, reference is usually made to the O/W type. ,,o0 Fortunately, the range covered by the inverse • . or W/O type appears to ..• be quite narrow and in -'.,• the low HLB's so that •o,.o the difference is readily apparent. •."-'- Required HLB values :T • for blends of oils •nay be calculated in a manner similar to that tbr blends of entulsifiers so long as the requirements for the components are known. Basis for this is illustrated in Fig. 3 in which all mineral oil is emulsified in the top row, 50/50 mineral oil-stearic acid in the middle row, and all stearic acid in the bot- tont row. The optimum emulsifier blend progresses front about 10 for 100% mineral oil to about 16 for 100% stearic acid. A natural question is the adher- ence of the HLB system to ionic or soap emulsions. Monovalent soaps, generally used for emulsification, ex- Figure 3.--Effect of blending oils and waxes this figure, one of the pairs of non- ionic emulsifiers used in Fig. 1, Span 40-Tween 40, is pictured with eight different oils or waxes cot- tonseed oil, lanolin, petrolatum, mineral oil, paraffin, cetyl alcohol, .beeswax, and stearic acid. It will be apparent that to choose an emul- sifter for a formula we need to know the required HLB for the oil blend, as well as that of the proposed emul- sifiers.