200 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS H H . "-7---'% I I O• O•CloH1 Call17 (^) Figure 11. "Through space" hydrogen bonding interaction in octyl salicylate (A) and menthyl anthranilate (B). disubstituted benzoates (parabens) have a X max around 260 nm (in the UV-C re- gion), whereas the ortho-disubstituted benzoates (salicylates) have a X max of 308 nm (in the UV-B region). In the ortho-disubstituted compounds, internal hydrogen bonding to form a new six-membered ring is possible due to the proximity of the two polar groups (as shown by the dotted bonds in Figure 11), whereas in the para-disubstituted derivatives such internal cyclical hydrogen bonding is not possible. This internal hydrogen bonding facilitates the electron delocalization, and hence the electron cloud is extended futher over the entire molecule. The H-cloud extension results in the lowering of the energy requirements for electronic transitions in the ultraviolet spectrum. Thus a bathochromic (red) shift to higher wavelengths is usually observed. Evidence of this trend can be found in numerous examples in the literature (17), not only in sunscreen chemicals as can be seen in Table II. EFFECT OF pH pH may have a marked effect on the ultraviolet absorption spectra of acidic and basic compounds. In the case of acidic compounds, the use of alkaline conditions (pH over 9) will assist in the formation of anions that tend to cause increased delocalization of electrons (18). Again, this electron delocalization would decrease the energy require- ments for the electronic transition in the ultraviolet spectrum, and hence a bath- Table II The X Max of Ortho- and Para-Disubstituted Compounds Ortho-disubstituted compounds Wavelength Para-disubstituted Wavelength (nm) compounds (nm) Ortho nitro phenol Ortho nitro aniline Ortho hydroxy benzaldehyde Ortho hydroxy acetophenone Ortho hydroxy benzoic acid Ortho amino benzoic acid 351 Para nitro phenol 317 412 Para nitro aniline 381 325 Para hydroxy benzaldehyde 282 329 Para hydroxy acetophenone 276 301 Para hydroxy benzoic acid 251 327 Para amino benzoic acid 284

UV ABSORPTION BY SUNSCREENS 201 OH- Figure 12. Phenolate anions formed by the action of alkali on phenol. HzO ochromic shift is observed (longer wavelength or h. max). For example, phenol in an alkaline environment will experience this anticipated bathochromic shift due to the formation of the phenolate anion as depicted in Figure 12. This phenolate anion will participate in resonance delocalization of electrons as pre- viously depicted in Figure 5. For aromatic amines, acidic conditions (pH below 4) will assist in the formation of cations. Here, a hypsochromic shift towards lower wavelength would be predicted since the protonation of the unbounded lone pair of electrons with acid would prevent any resonance delocalization of the electrons originally possible. Thus aniline, for example, would form the anilinium cation at low pH (see Figure 13) and a considerable hyp- sochromic shift occurs. EFFECT OF SOLVENT Solvent shifts in sunscreen chemicals have been observed by several researchers in the field (3, 19-21). Riegelman and Penna (22) concluded that the use of different solvents in cosmetic formulations may profoundly influence the effectiveness of a sunscreen chemical. These shifts in the ultraviolet spectrum are primarily due to the relative degrees of solvation of the ground state and the excited state of the chemical by the solvent. Thus, to predict the effect the solvent has on a particular chemical, the interac- tion (mostly hydrogen bonding) between the solvent and the sunscreen chemical has to be reviewed closely. For polar sunscreens, e.g., PABA (see Table III), their solvation with polar solvents such as water and ethanol will be quite extensive. This large amount of solvation stabi- lizes the ground state, thereby inhibiting the electron delocalization, leading to the excited state illustrated in Figure 8, where Y = NH 2 and R- OH. The net result would be that hypsochromic shifts to lower wavelengths are observed. This extensive interac- tion (hydrogen bonding) between PABA and the solvent would inhibit this electron NH: NHz Figure 13. Anilinium cation formed by the action of acid on aniline.