216 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Absorbance units Absorbancr units 100% ETHYL ALCOHOL W•velength (nm•) I I ! ! i • i ! • I ! I ! • I -- 8.88 Wavelength (nm) Figure 3. UV absorption spectra of octyl salicylate showing no apparent shift in the X max in the UVB region (290 to 320 nm) from a non-polar solvent (hexane) to a polar solvent (ethanol). sunscreen formulation is placed upon the skin, then other less volatile ingredients in high concentration will affect the UV characteristics of the sunscreen chemical. If the sunscreen is polar (i.e., PABA, dioxybenzone, sulisobenzone, and oxybenzone), then interactions with polar solvents will cause the UV absorbance spectra to shift towards shorter wavelengths (Table II and Figure 1). Thus PABA becomes less effective as the polarity of the solvent increases and k max shifts from the UVB region to the UVC region. In the case of the three benzophenones tested, a hypsochromic shift occurs within the UVA region.

SOLVENTS AND SUNSCREEN ABSORBANCE 217 100% ETHYL ALCOIIOL Absorbance units AL•o•noe units 0 425 A / /' / Wavelength (nm), I -8.415 - .0.005 HEXANE Wavelength (nm) Figure 4. UV absorption spectra of octyl p-methoxy cinnamate showing a bathochromic shift in the max in the UVB region (290 to 320 nm) from a non-polar solvent (hexane) to a polar solvent (ethanol). On the other hand, if the sunscreen is less polar (i.e., octyl dimethyl PABA, octyl cyanodiphenylacrylate, octyl p-methoxycinnamate, butyl methoxydibenzoylmethane, and ethyl dihydroxypropyl PABA), then interactions with polar solvents will shift the UV absorbance spectra to longer wavelengths (Table IV Figures 4 and 5). In the case of octyl dimethyl PABA, it still remains within the UVB region regardless of the polarity of the solvent in which it dissolves. However, as the solvent becomes either more or less polar, k max is displaced from 308 nm, the wavelength of optimum protection for a UVB absorber. In the case of octyl p-methoxycinnamate, formulating with non-polar solvents should be avoided since such solvents will shift the k max away from 308 nm.