312 JOURNAL OF THE SOCIETY OF COSMETIC'CHEMISTS other. The former is called the dis- persed phase and'the latter is called the continuous phase. Industry demands further that an emulsion must exhibit a certain stability under a variety of conditions. Emulsifiers and stabilizers are added to attain the desired stability. Between the two phases of an emulsion, a large area or interface exists which surrounds each parti- cle. The adsorption of emulsifiers at the emulsion interface has been established. The nature of this interface, whether mono- or multi- molecular in thickness, is still in question as pointed out by Schwartz and Perry (5) in their new book. Since the particles of an emulsion are considerably larger than molecu- lar in size, we may consider the in- terface that surrounds each particle as being similar to the interface which separates the phases when present in bulk. If we were to shake mineral oil with water we ob- tain an emulsion that will break quickly into its two phases. Ex- amination of the interface would re- veal that the interfacial tension is high, approximately 45 dynes/cm. With the addition of an emulsifier of suitable type this would drop to al- most zero. This reduction in inter- facial tension primarily assists in forming the emulsion though it also promotes stability. As mentioned above, the assist- ance in emulsion formation and subsequent stabilization appears to be only part of the action of an emulsifier. In addition, the emulsi- fier usually establishes the type of emulsion formed. In our study and use of emulsifiers, it appears that the action of emulsifiers may be related to their structure roughly as follows: First, "what the emulsifier or sur- face-active agents will do," that is, make an O/W or W/O emulsion, act as a detergent, or solubilize an oil, or have some other action, seems to depend on what we call the HLB of the emulsifier. This value is an expression of the relative simultane- ous attraction of an emulsifier for water and for oil (or for the two phases of a system to be emulsified). Emulsifiers consist of a molecule that combines both hydrophilic and lipophilic groups and it is the bal- ance of the size and strength of these two opposing groups that we call HLB. For purposes of convenience, the effective balance of these groups is assigned a numerical value. Second, how efficiently the emulsi- fier will work s•ems to be related to over-all chemical structure, that is, whether the emulsifier is a soap, a partial ester, a complete ester, whether the lipophilic group is satu- rated, and so forth. This latter ac- tion appears to be quite specific and no "rules" have been estab- lished. DEVELOPMENT OF HLB SYSTEM In our present system, an emulsi- fier that is lipophilic in character is assigned a low HLB number and an emulsifier that is hydrophilic in character is assigned a high number. The midpoint is approximately ten

CLASSIFICATION OF SURFACE-ACTIVE AGENTS 313 and the assigned values have ranged from one to forty. When two or more emulsifiers are combined or blended, the HLB values are addi- tive in behavior. Thus, if we blend three parts of emulsifier "A" having an HLB of 8 and one part of an emulsifier "B" having an HLB of 16, the resulting HLB of the blend will be the sum of three-quarters of 8 and one-quarter of 16, i.e. (6 4- 4) or 10. We should note that chemical type alone does not establish hydro- phile-lipophile balance. Thus, soaps may.range from strongly hydrophilic for sodium laurate to strongly lipo- philic for aluminum oleate esters, ether-esters, and ethers may range from low to high HLB's, sulfates and sulfonates may range from medium to high. HLB is no t the same as solubility, though there is an over-all relation- ship. Thus, materials having low values tend to be oil soluble and materials having high values tend to be. water soluble. However, two emulsifiers may have the same HLB and exhibit different solubility char- acteristics. In the preparation of an emul- sion, the reduction of interfacial tension makes it easier to disperse one of the phases in the other. The nature of the interface established by the adsorption of the emulsifier at the interface in some •manner influences the two immiscible liquids to such an extent that one breaks up into droplets while the other re- tains its continuity. The interface apparently "bends" more easily in one direction than the other. This would seem to decide the type of emulsion that is formed--whether O/W or W/O. Presumably, the be- havior of an emulsion could be ex- pressed by observing the proper characteristics of this interface. It occurred to us that the "bend- ing" tendency of the interface might be observed by determining the in- terfacial tension in different direc- tions (i.e., up and down), from one phase to the other and vice versa. A literature search revealed that Roberts (4) had already made a brief study of this effect in relation to natural petroleum emulsions. In his work, differences in interfacial tension were observed whether the duNouy ring was pulled up or pushed down through the interface. However, the existence of a differ- ence in interfacial tension is ques- tioned since interfacial tension is in itself the difference in free energies of the two phases. It is possible that the observed differences are due to the introduction of a third phase, the platinum ring. Whether or not this is true, with efficient emulsifier systems, observations are most dif- ficult because, by their very nature, these systems have an interfacial tension of almost zero (since they form emulsions spontaneously) and therefore the precision of measure- ment is low. Hence, this phase of the study was not considered further. Other experimental means of esti- mating HLB have been considered. Lambert and Busse (2) recently published a rapid method of deter-