366 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Becher and Birkmeier (5), for example, suggested a gas chromatographic technique to determine the HLB of surfactants. Gorman and Hall found a correlation of the HLB values and the dielectric constants of the surfactants (6). Ross and coworkers reported a relationship between HLB and spreading coefficients (7, 8). From kinetic considerations, Davies found a relationship between the HLB of a surfactant and its solubilities in water and oil and suggested a method of calculating HLB values from the structural group numbers ot• the surfactants (9, 10). In a more recent work, Shinoda and Saito showed a possible relationship between HLB and phase inversion tem- perature of the surfactant (11). Although the HLB system has wide applications, perhaps the most valuable aspect of the HLB concept to a cosmetic chemist is its usefulness in selecting an effective surfactant system for emulsification. According to the concept, for a given oil and chemical type of surfactant, an emul- sion having an optimum stability can be prepared when the HLB of the surfactant or the surfactant mixture agrees with the "required HLB" value ot• the oil to be emulsified. The concept thus provides a means to reduce the number ot• trials required to arrive at the most effective sur- factant combination for the emulsification of an oil to form either a W/O or an O/W emulsion. To take advantage of this method, one would have to know, or be able to determine, the HLB value of each surfactant component and the "required HLB" value ot• the oil or oil blend. A relatively large amount of HLB data for surfactants is now available but only a small amount of data is known on the required HLB values of oils and waxes (12, 13). This is partly due to the difficulty in accurately determining the required HLB values. Although possible correlations between the required HLB and spreading coefficients, or between the required HLB and dielectric constants, have been suggested (6, 8), the present procedure for determining the required HLB values of the oils generally involves a trial emulsification method using a pair of surfactants with known HLB values. Indeed, compared to the information available on the HLB of sur- factants, data on the required HLB for oils have not been thoroughly ex- amined. Like the HLB values of surfactants, the required HLB of an oil mixture is generally regarded as a linear function ot• the required HLB values of the components. However, the work ot• Ohba suggested that this may not be exactly true in some systems (14, 15). Recently, Griffin

SURFACTANT LOCATION $67 and c()workers (16) conducted an investigation on a series of natural and symhetic oils but no direct correlation was found between the required HI,B values of the oils and the physical properties of the oils. It is generally known that the required HLB of an oil is dependent not only on the oil itself but also on the method by which the emulsion is prepared. This is a serious disadvantage in using the HLB system since, for a given value of required HLB, the exact method of emulsifica- tion must be specified. And, in using this value, the specified prepara- tire technique must be followed exactly to insure the desired results. Unless the extent of variations in required HLB caused by preparative variables is very small, such variations can, conceivably, cause serious manufacturing problems. In making emulsions, it is often difficult or impossible to keep the manufacturing conditions identical to the laboratory conditions used in developing the formulation. By necessity, or practicality, the mixing condition, cooling rate, the rate of phase combination, etc., in a full scale manufacturing operation are usually quite different from the laboratory scale operations. If, for example, a newly developed emulsion is stable only within a narrow range of HLB values, the variations introduced dur- ing the marefracturing operations can cause a shifting of the required HI,B and hence produce a mismatching of the HLB of the surfactant sys- tem and the required HLB of the oil system. This may lead to a produc- tion of emulsion with undesirable characteristics or poor stability. In spite of the apparent importance, very little is known about the extent effects of preparative variables on the required HLB of oils. In preparing emulsions, a combination rather than a single surfac- rant is generally used. The surfactant system may be initially placed in the oil phase, in the aqueous phase, or divided into both phases prior to emulsification. In earlier investigations, it was established that the initial location of the surfactants plays a significant role in determining the viscosity and particle size distribution, as well as the type of emulsion formed (17, 18). Since surfactants are generally somewhat soluble in both oil and water, a part of the surfactant placed initially in one of the phases will eventually migrate to the interface and the other phase to establish an equilibrium. However, the rate of such migration might be quite slow in some systems (19). The purpose ot• this study was to in- vestigate, specifically, if the initial surfactant location prior to emulsifi- cation had measurable effects on the required HLB of oils determined by emulsification. It was hoped that such information would be useful in