CLASSIFICATION OF SURFACE-ACTIVE AGENTS BY "HLB"t By Wx•,•,x• C. G•,•FFx•r ,/it/as Powder Company,//Filmington, Del. SINCE rH•. •NXUODtrCX•ON of the "HLB" system of classifying and selecting emulsifiers (6) numer- ous requests have been received concerning its derivation. The term "HLB" comes from the words hydrophile-lipophile balance. Emul- sifiers consist of a molecule that combines both hydrophilic and li- pophilic groups (or polar and non- polar groups) and it is the balance of the size and strength of these two opposing groups that we call HLB. Surface-active agents have been classified in many ways: including chemical types and according to ionization. Classification by HLB permits some prediction of behavior and reduces the amount of work in- volved in the selection of an emulsi- fier, wetting agent, or other type of agent. Emulsifiers constitute one of the widest used subdivisions of surface- active agents and we will use this group as an illustration of the manip- ulation of the HLB system, with reference to other applications later. )Presented at the October i '1, 1949, Meeting, Chicago Chapter, Chicago, I11. A complete system for selecting an emulsifier would provide the best emulsifier to give the desired form of product for the desired raw ma- terials. Thus, knowing the materi- als to be emulsified, the most ef- ficient emulsifier could be chosen for the desired type of emulsion. AcTioN ov EMtrzsiviZl•S An emulsifier has two actions that are distinctly different. The accepted action is that of promoting the formation of an emulsion mak- ing the emulsion easier to. prepare producing a finer particle size and aiding the stability of the emulsion. The second action, which occurs along with the preparation of the emulsion, consists in controlling the type of emulsion that is to be formed, O/W or W/O. This second action appears to be a function of HLB. In evolving a system for the selec- tion of emulsifiers, we will first con- sider briefly the theory of emu}sifica- tion. For practical purposes, an emulsion consists of two immiscible liquids, one being dispersed as a multitude of small particles in the 311

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