TRANSPARENT EMULSIONS 283 TABLE IV.--RATIO Or COMPONENTS IN TRANSPARENT EMULSIONS Tween 60 Span 80 6.12 Water Oil Transparent O/lff Emulsions A I 0.32 0.21 1.2 1.0 B 1 0.44 0.16 1.7 2.0 C 1 0.5l 0.21 1.2 2.5 D 1 0.53 0.24 1.3 2.9 E 1 0.53 0.24 1.8 2.9 F 1 0.65 0.32 1.2 1.0 G 1 0.65 0.32 1.9 1.0 H 1 0.65 0.32 1.2 2.6 I 1 0.88 0.32 3.2 1.0 Transparent lff /O Emulsions J 1 1.5 0.16 1.7 4.7 K 1 0.90 0.21 1.2 3.5 L 1 0.90 0.21 1.2 5.3 It is of interest to compare HLB values for microemulsion and macro- emulsion formation. The range of Span-Tween ratios in Table IV for O/W microemulsions correspond to an HLB range of 10.0 to 12.3. A re- cent Atlas bulletin (11) lists a required HLB for mineral oil O/W emulsions as 11 + 1, and the two values are consequently in good agreement. In the case of the W/O microemulsions of Table IV calculated HLB values are in the range of 8.5 to 9.9. In contrast, the same bulletin gives a re- quired HLB of 3 to 8 for W/O emulsions. Formation of microemulsions with anionic emulsifiers is analogous to that with nonionic emulsifiers. Consequently, only one example is shown in Table V. A 32% aqueous triethanolamine stearate solution was pre- pared. Two parts of this soap solution was heated with one part of iso- propyl palmirate. At the pH of this soap solution (pH 8.1), the micelles are highly condensed and very little of the isopropyl palmitate is taken into the micelies. Even with heating and vigorous stirring, most of the oil remains as a separate phase. Upon the addition of about 3 to 6% of 6.12, the soap micelles swell to take up all of the isopropyl palmirate and form an opaque O/W emulsion. The addition of a small amount of oleyl alcohol then condenses the interphase sufficiently to form a transparent emulsion. TABLE V.•/W EMULSIONS CONTAINING SOAP Parts By Weight-- 5d 5B 5C Triethanolamine stearate Water Isopropyl palmirate 2-Ethylhexanediol-l,3 Oleyl alcohol Appearance 9.5 9.5 20.5 20.5 15.0 15.0 3.0 2 separate Soap solution phases swells to take up oil, opaque 9.5 2O.5 15.0 6.0 0.5 Transparent above 25 øC

284 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS III. Discussion The theory of microemulsion formation can be useful in many ways for the preparation of cosmetic creams and lotions. 1. The theory provides a rationale for the preparation of transparent lotions and gel. 2. Since these transparent products are thermodynamically stable, they have a decided advantage over conventional emulsions. The addition of an opacifying agent to a transparent emulsion will give the appearance of a conventional lotion or cream. 3. Macro emulsions can be readily prepared without the use of homog- enizers and other high shear equipment by first dispersing the internal phase as a microemulsion. Upon subsequent dilution with the solute of the continuous phase, and cooling, the size of the dispersed droplets in- creases to a value consistent with the final concentration of emulsifying agents. 4. Because of the small size of the dispersed droplets in a transparent emulsion, biologically active ingredients (in the dispersed phase) may be absorbed more readily. Thus, these systems may be the preferred ve- hicles for topical application. However, a word of caution is necessary because conventional cosmetic ingredients may prove to be not quite so innocuous when applied in this manner. IV. SUMMARY The experimental study has shown that the conditions required for the preparation of transparent emulsions can be explained on the basis of the physical state of the interphase. If the interphase is expanded or too highly condensed, a transparent emulsion will not form. These results are consistent with Schulman's microemulsion theory. Only one deviation from the theory was observed in that transparent emulsions can be formed even though the hydrocarbon chain length of the nonpolar oil is greater than that of the emulsifying agents. This deviation does not detract from the theory but is simply an extension of experimental findings that broadens the applications of the theory. (Received March 25, 1963) REFERENCES (l) J. H. Schulman and J. B. Montagne,/inn. N.Y./icad. Sci., 92 (Art. 2), 366 (1961). (2) J. E. Bowcott and J. H. Schulman, Z. Elektrochem., 59, 283 (1955). (3) J. H. Schulman, W. Stoeckenius, and L. M. Prince, y. Phys. Chem., 63, 1677 (1959). (4) A. S.C. Lawrence, Ier Congr. mondial detergence et prods. tensio-actifs, (Paris) 1, 31 (1954). A.J. Hyde et al., Discussions Faraday Soc. No. 18, 239 (1954). (5) P. A. Winsor, Trans. Faraday Soc., 44, 376 (1948). (6) P. A. Winsor, Ibid., 46, 762 (1950). (7) J. Bromilow and P. A. Winsor, f. Phys. Chern., 57, 889 (1953). (8) P. A. Winsor, f. ColloidSci., 10, 88 (1955).