SOLVENTS AND SUNSCREEN ABSORBANCE 219 Table V Relative Lipophilicity of Sunscreen Chemicals Based Upon Their Calculated Partition Coefficients Between Octanol and Water CTFA name Other names Log P @25øc Glyceryl PABA Benzophenone-4 PABA Benzophenone-8 Cinoxate Benzophenone-3 Ethyl dihydroxypropyl PABA Amyl dimethyl PABA Butylmethoxy dibenzoylmethane Menthyl anthranilate Octyl salicylate Homosalate Octyl methoxy cinnamate Octocrylene Octyl dimethyl PABA 1,2,3-Propanetriol, 1-(4-aminobenzoate) - 0.02 Sulisobenzone - 1.51 p-Amino benzoic acid 0.74 Dioxybenzone 2.15 Ethoxyethyl methoxy cinnamate 2.55 Oxybenzone 2.63 Ethyl-4-bis(2-hydroxypropyl-aminobenzoate) 2.84 Amyl dimethyl PABA 4.53 Butylmethoxy dibenzoylmethane 4.86 Menthyl-O-aminobenzoate 5.05 2-Ethylhexyl salicylate 5.30 Homomenthyl salicylate 5.61 Ethylhexyl-p-methoxy cinnamate 5.65 Octyl cyanodiphenylacrylate 5.69 2-Ethylhexyl-p-dimethyl aminobenzoate 6.08 CHANGES IN MOLAR ABSORPTIVITY In addition to shifts in X max, both increases and decreases in the molar absorptivities (½) of sunscreen chemicals were also observed in the various test solvents. In general, and with the exception of PABA, the rest of the UVA and UVB sunscreens selected for study showed increased ½ values at X max in both polar (ethanol-water systems) and non-polar (hexane and mineral oil) solvents. In contrast, there were signif- icant decreases in ½ values at • max when the sunscreen chemicals were dissolved in semi-polar solvents such as hexylene glycol and C•2-C•5 alcohols benzoate. These data (taken from Data Tables II-IV) have been summarized in Tables VI and VII. Reduction in molar absorptivity varied between 12% and 55% for hexylene glycol and between 7% and 28% for C•2-C•5 alcohols benzoate. In addition, employing a spectro- photometric technique suggested by Vogelman et al. (12), overall reduction in the area H 6 ø I I 0 -- Menthyl 0 -- Octyl (a) (b) Figure 6. Steric configuration of ortho-substituted molecules, (a) menthyl anthranilate and (b) octyl salic- ylate.

220 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table VI Sunscreens and Solvent Systems That Showed Changes in Molar Absorptivity Sunscreen Approximate Solvent exhibiting Solvent exhibiting reduction in increased decreased molar molar absorptivity molar absorptivity absorptivity UVA Absorbers Dioxybenzone Sulisobenzone Menthyl anthranilate Butylmethoxydibenzoyl- methane Oxybenzone Hexane Ethanol 90%-water 10% Ethanol 70%-water 30% Ethanol 90%-water 10% Ethanol 90%-water 10% Hexylene glycol 40% Methyl carbitol 37% Isopropyl palmitate 25% Isopropyl myristate 25% Hexylene glycol 24% C12-C15 alcohols benzoate 24% Mineral oil 18% under the absorbance-vs-wavelength curve of 32% to 35% was observed for oxyben- zone, dioxybenzone, and octyl cyanodiphenylacrylate, and there was a 75% reduction for triethanolamine salicylate in going from polar to non-polar solvents. Klein has recently reported (4, 13) almost complete suppression of UV absorbance spectra of octyl dimethyl PABA, octyl p-methoxycinnamate, and benzophenones alone and in combination with each other in two branched-chain liquid fatty esters (i.e., isostearyl neopentanoate and isopropyl linoleate). Klein's finding for octyl dimethyl PABA in isostearyl neopentanoate has been confirmed in our laboratory. The principal characteristics of a UV absorption band are its position (k max) and its intensity or molar absorptivity (½). In the case of the latter, ½ values at k max of most of the sunscreen chemicals tested were greatest in polar and non-polar solvents and re- duced in hexylene glycol and C•2-C•5 alcohols benzoate. Drastic reductions in ½ value Table VII Sunscreens and Solvent Systems That Showed Changes in Molar Absorptivity Sunscreen Approximate Solvent exhibiting Solvent exhibiting reduction in increased decreased molar molar absorptivity molar absorptivity absorptivity UVB Absorbers PABA Ethyl dihydroxypropyl PABA Octyl dimethyl PABA Octyl p-methoxy cinnamate TEA salicylate Octyl salicylate Homomenthyl salicylate Octyl-2-cyanodiphenyl- acrylate Ethoxy ethanol Ethanol 33% Ethanol 90%-water 30% Hexylene glycol 23% Ethanol 70%-water 30% Ethanol 70%-water 30% Hexylene glycol 21% C12-C 15 alcohols benzoate 25 % Methyl carbitol Hexylene glycol Ethanol 70%-water 30% Hexylene glycol Hexane Hexylene glycol Isopropyl myristate Hexane C12-C15 alcohols Ethanol benzoate 29% 20% 55% 28%