282 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS didon of use will determine its ultraviolet absorption properties in relation to the solvent effects mentioned above. A concentration of a sun screen that looks good in one solvent may not be effective in the final preparation or a finished formulation may not show the expected response on skin testing. EXPERIMENTAL The following sun screens were studied: Ethyl p-amino benzoate (benzocaine), p-amino benzoic acid, phenyl salicylate (salol), 2-ethoxyethyl p-methoxy cinnamate (Giv-Tan F Sindar Corp.), glyceryl p-amino ben- zoate (Escalol 106 Van Dyk & Co.), 2,4-dibenzoyl resorcinol (DBR Dow Chemical Co.) and an alkyl phenyl benzotriazole derivative (Tinuvin P Geigy Industrial Chem.). The first three compounds were recrystallized twice from appropriate solvents and confirmed to be authentic samples by ½erification of their melting points and by other physical tests. The com- mercial samples were used as received. Stock solutions of all sun screens were made up to 0.5 per cent concentra- tion in ethanol and chloroform. The solutions were diluted to 10 milligram per cent solutions in the selected solvents. The following solvents were used: Notation Used Solvent in Figures I.iquid petrolatum (Nujol) Isopropyl palmirate (Propal--Robinson Wagner) Polyethylene glycol 400 (Union Carbide Chem. Co.) Ethanol 95% Water Polyoxyethylene monolaurate 5% in water (Tween 20--Atlas Powder Co.) Sodium lauryl sulfate 5% in water (Duponol C--DuPont) MO IP PE AL WA TW SL Since p-amino benzoic acid was the only free acid used in this study, it was converted to its sodium salt by dissolving it in 0.05 3//sodium borate solu- tion, pH 9.2. The ultraviolet absorption spectra were recorded on a Cary model 11 U.V. spectrophotometer using a 1.0 min. path length for both the solvent and solution. An identical solvent blank without the sun screen was used in each case. RESULTS AND DISCUSSION The selection of a 10 rag. per cent concentration and a 1.0 min. path length was made in order to obtain curves of a maximum accuracy. This

EFFECTS OF CHARACTERISTICS OF SUN SCREEN COMPOUNDS 283 is equivalent to studying a 100 mg. per cent solution in a 0.1 ram. path length. Therefore, multiplication of the optical density values by a factor of ten will convert them into the Kumler sun screen index. Figure 1 represents the data obtained for ethyl p-amino benzoate (ben- zocaine) in the solvents studied. The intersection of the spectral curves with the 308 mu wavelength is indicated by the vertical dotted line. It ß I0 NH (•OC.•'I 5 //// • AL • WA '• o5 ooL• 25O 50O 550 WAVE LENG. H (me) Figure 1. Ultraviolet absorption spectra of ethyl iv-amino benzoate in various solvents. Con- centration at 10 mg. % path length at 1.0 mm. See experimental section for identification of solvents. should be noted that the absorption maximum is below 300 m• so that the sunburn region is on the shoulder of the absorption curve. The solvent effects are therefore very pronounced and result in a striking shift in the relative sun screen efficiency in each solvent. Kumler (7) gave ethyl p- amino benzoate one of the highest sun screen indexes. However, it is apparent from the spectrum of the compound in alcohol relative to the other solvents that alcohol as a solvent leads one to an exaggeration of the compound's real usefulness as a sun screen. Since alcohol evaporates upon application of the sun screen preparation, the benzocaine would transfer to the residual nonvolatile components of the preparation. In isopropyl palmirate (IP) or mineral oil (MO), the compound loses much of its sun screen capacity. A detailed study of the spectral characteristics of ben- zocaine in solution of surface active agents was carried out by Riegelman (10) while investigating the effects of the surfactants on the stability of the benzocaine. The surfactants appear to shift the spectrum to longer wave- lengths, thereby potentially reversing the "quenching" effect of the non- polar solvents. However, inclusion of the surfactant in the formulation in relatively large amounts would create a residual film which would be easily washed off with perspiration or while bathing.