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.

218 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Absorbance units Absorbance units 100% ETHYL ALCOHOL ,/•'•\,\ ', •.--'"-"-",-,. .• ,./' I I -- "• " I •,, 800 0 • I W•vele•th (nm)l • I • I I I i I HEINE • I ' , /,.' I 1 , O Waveleith (nm) Figure 5. UV absorption spectra of octyl dimethyl PABA showing a bathochromic shift in the h max in the UVB region (290 to 320 nm) from a non-polar solvent (hexane) to a polar solvent (ethanol). Triethanolamine salicylate, octyl salicylate, homomenthyl salicylate, and menthyl anthranilate showed only minor variations in h max in both polar and non-polar sol- vents (Table III and Figure 3). The minimum shift of the h max is due to the ortho- substituted configuration of salicylates and anthranilates. The lone pair of electrons are involved in the six-membered cyclic arrangement and, as a result, are not available for interaction with polar solvents (Figure 6). Hence, ortho-substituted compounds tend to exhibit moderate to low • max shifts in polar solvents. Although oxybenzone and dioxybenzone both may form a six-membered cyclical arrangement, similar to the salicy- lates and anthranilates, they exhibit added polarity due to the additional hydroxy and methoxy groups. A more in-depth explanation for the shifts and changes observed in the • max that occur when sunscreen chemicals are dissolved in both polar and non-polar solvents has been provided in separate reports by Shaath (10, 11).