SPREADING, HLB, AND EMULSION STABILITY 331 HLB -- •'• DISTILLED WATER ON I% • EMULSIFIER IN CASTOR OIL --..... IO 8 6 4 2 o -30 -34 -38 -42 -46 -50 -54 -58 -62 -6E Sm Figure 4.--The correlation between spreading coefficient S2 for water and castor oil solutions of varying HLB. tier HLB, and, by means of what are after all quite simple measurements, determine the HLB at which the appropriate negative spreading coeffi- cient appears. On the other hand, let us realize that the spreading coefficient describes a physically realizable process. The spreading of one liquid on another can be observed by anyone with a shallow dish, two liquids, and some sort of dropping pipette. For example, Fig. 5 is a diagrammatic representation of what one ob- serves with a number of 31/2-inch Petri dishes filled with solutions of emulsifiers of varying HLB after one drops onto each liquid surface ap- proximately 0.5 mi. of toluene. Clearly, at about an HLB of 9 we have gone from a spread to a no-spread situation. Thus, HLB = 9 is very closely the required HLB for the formation of a stable toluene-in-water emulsion. It is inter.esting to note that in this particular case, at least, if the nonspreading droplet is left in contact with the substrate for a short time, a cloud of spontaneously-formed emulsion may be seen surrounding it. A similar type of observation could be made on a series of dishes filled with solutions of the same emulsifier by dropping a graded set of liquids of varying required HLB. This would perinit a definition of the HLB of the surface active agent. Similarly, if it is desired to establish the conditions for stability for a

332 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS HLB-- 16 HLB=15 COMPLETE SPREADING HLB=14 PARTIAL SPREADING HLB=13 HLB= 12 HLB-- II PARTIAL SPREADING HLB= I0 HLB= 9 HLB= 8 NO SPREADING Figure 5.--Diagrammatic representation of spreading of toluene on the surface of aqueous solutions of varying HLB. water-in-oil emulsion, the spreading of water on solutions of the surface active compound in the oil phase could be observed. Clearly, then, the extension of the theoretical significance of the spreading coefficient leads to a quick and simple method of determining both the HLB of new emulsifiers and the required HLB of new oil phases. REFERENCES (1) Clayton, W., "Theory of Emulsions," 4th edition, Philadelphia, The Blakiston Co. (1943), p. 127. (2) Griffin, W. C., ]. Soc. Cosmetic Chemists, 1, 311 (1949). (3) Griffin, W. C., Ibid., 5, 249 (1954). (4) Becher, P., "Emulsions: Theory and Practice," New York, Reinhold Publishing Co. (1957), pp. 188-205. (5) Davies, J. T., Proc. 2rid. Intl. Cong. Surface ./lctivity, I, 426 (1957). (6) Ross, S., Chen, E. S., Becher, P., and Ranauto, H. J.• ]. Phys. Chem., 63, 1681 (1959). (7) Becher, P., op. cit., p. 86.