CHANGES DURING EVAPORATION OF EMULSION 27 (c) (d) Figure 8 (continued). (c) Bottom layer, without crossed polarizers. (d) Bottom layer, with crossed polarizers. Magnification: (a-c) 400x (d) lOOx.

28 JOURNAL OF COSMETIC SCIENCE bination used to stabilize the emulsion in combination with other polar water-soluble substances that may be part of the emulsion formulation. This conclusion is supported by the pattern of particles in Figure 4c. It is certainly tempting to associate these particles with the anisotropic material, an amphiphilic association structure, in Figure 8. As to the kind of such an amphiphilic association structure, the first inclination may be to identify it as a lamellar liquid crystal, an L ex phase against the extensive fundamental treatment of such structures in emulsions (20-23) after their initial introduction (24). However, the optical pattern in Figure 8d does not support such an interpretation. Instead, the most probable identi­ fication is a gel phase, an 113 phase (25), an interpretation supported by the obvious stability of the emulsion. Combining these conclusions with the numbers from the evaporation (Figures 3 and 7) and realizing that an association structure will contain water at an approximate level of 25-50%, the size of the bottom layer in Figure 7 is accounted for in a satisfactory manner. It is essential to realize the consequences of these results for the action of the emulsion on the skin. For this phenomenon, the structures in the original emulsion or those at 40% evaporation are not of primary importance because they disappear within less than 15 minutes after application. Instead, the basis for this activity is due to the structures found in Figure 8c and d. Unfortunately, information about this phenomenon is entirely lacking in the literature. Hence, it appears reasonable and justified to conclude that the efforts to relate the action of emulsions on the skin to their structure should preferably focus on structures such as those in Figure 8, paying less attention to the structure of the original emulsion. Since a large part of these structures obviously consists of surfactant association structures, it appears that more interest should be focused onto the influence of such structures on the skin. CONCLUSIONS The structural changes during evaporation in a commercial skin care emulsion were determined using optical microscopy and centrifugation. The results show the emulsion to have good stability against coalescence during the initial evaporation stage and illustrate most convincingly that the final structure on the skin after evaporation is only indirectly related to the original elements of the emulsion. REFERENCES (1) J. Sjoblom, Emulsions: A Fundamental and Practrical Approach (Kluwer, Amsterdam, 1991). (2) J. Sjoblom, Encyclopedia Handbook of Emulsions Technology (Marcel Dekker, New York, 2001). (3) P. B. Binks, Modern Aspects of Emulsion Science (Royal Society of Chemistry, Cambridge, 1998). (4) S. E. Friberg, G. K. Larsson, and J. Sjoblom, Food Emulsions (Marcel Dekker, New York, 2004). (5) D. T. Downing, "Epidermal Composition," in Dry Skin and Moisturizers, 1st ed., M. Loden and H. E. Maibach, Eds. (CRC Press, New York, 1999), pp. 13-26. (6) K. R. Feingold, The regulation and role of epidermal lipid synthesis, Adv. Lipid Res., 24, 57-70 (1991). (7) G. F. Odland, "Structure of the Skin," in Physiology, Biochemistry and Molecular Biology of the Skin, 1st ed., I. A. Goldsmith, Ed. (Oxford University Press, New York, 1991), pp. 3-15.