PREPRINTS OF THE 1998 ANNUAL SCIENTIFIC SEMINAR 211 Rank IMluent % OHglnal Absorbance after Molecular Weight of 35 MED ((• 353nm} diluent additive I 99% cyclohexane./1% IPA 91.2% 68 2 Neat isopropanol 90.9% -- 3 99% cyclohexaneYl % octanol 88.7% 130 4 99% cyclohexaneYl % butyloctyl salicylat• 84.5% 306 5 99% cyclohexane./1% octyldodecanol 78.9% 298 6 99% cyclohexaned 1% tetrahydrofuran 69.2% 72 7 Neat cyclohexane 61.8% -- 8 99% cyclohexaneY1% diethyl adipate 58.8% 174 Table I Conclusions The tendency of avobenzone to lose absorbance in sunlight is explained in part by' UV-induced enol to keto tautomerization. Surprisingly, the tautomerization reverses when irradiation ceases. and something resembling the original enol-keto equilibrium results. Stability during [IV irradiation of the enol-keto equilibrium is increased in what appears to be a concentration-dependent manner by adding hydroxylic (protic) materials to the solvent. Butyloctyl salicylate increases stability of the enol-keto equilibrium disproportionately when compared to the nearly equimolar octyldodecanol. Polar materials which are not hydroxylic do not appreciably improve stability of the equilibrium. Our hope is that these findings will lead to improvements in avobenzone formulations and, therefore, to more protective, dependable, and efficacious sunscreens. References 1. Food and Drag Administration, Announcement of Enforcement Policy, Fed. Register, Vol 62, No. 83, April 30, 1997 2. C. Bonda, Formulating Stable, High SPF, Broad Spectrmn Sunscreens with Avobenzone, from poster presented at SCC Annual Scientific Meeting, NYC, (1997) 3. W. Schwack and T. Rudolph, Photochemistry of Dibenzoyl Methane UVA Filters Part I, J. Photochem. & Photobiol., 28, 229-234, (1995) 4. L. Andrae et al, A UVA Filter (4-tert-butyl-4'-methoxydibenzoylmethane): Photoprotection Reflects Photophysical Properties, J. Photochem. & Photobiol., 37, 147-150, (1997) Acknowledgements The authors gratefully acknowledge the valuable assistance and contibufions of Yin Hessefort, Magda Roth, Urvil Shah, and Robert McMillin of the C.P. Hall Company, Dr. John Harwood of the University of Illinois-Chicago, and David Steinberg of Steinberg & Associates.

212 JOURNAL OF COSMETIC SCIENCE