Substantivity of sunscreens: an appraisal of some quaternary ammonium sunscreens

Synopsis Two series of quaternary ammonium UV-B sunscreens, derived from salicylic acid (S) and from p-methoxycinnamic acid (C), and containing N-alkyl chains of different length (C(2) to C(16)) were prepared and submitted to substantivity tests on animal (wool) keratin, and to microbiological tests. A direct correlation between substantivity and antimicrobial/antifungal activity was observed in all cases, the compounds bearing C(12) alkyl chains displaying peak values. The C-derivatives, when compared with the S-derivatives, showed relatively higher substantivity and sunscreen index (SI) values, coupled with lower antibacterial activity. The substantivity-microbiological activity correlations and the particular behaviour of the C(12) derivatives are briefly discussed in the light of the existing literature data. On the basis of the experimental results, possible guidelines for the development of substantive and non-antibacterial (and possibly, non-irritant) quaternary ammonium sunscreens are indicated.