58 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 5.0 mono TEA CI2AS / / 2:0 l.O 0 -. 0 lO.O 2 .0 30.0 $urfactant Concent r a t ion (g/lO0 ml) Figure 17. Comparison of irritations following 24 hr closed patch test on guinea pig skin using C12MAP monoTEA salt and C12AS dissolved at various concentrations. •2 3 C12 A 0 • ' 23456789 123456789 1 23456789 0•5[ C8MANamonoCIo123456789Namono 123456789 0•5[ CI2MANaMAC14mono123456789Namono 123456789 C16 MAP C18 MAP mono Na mono Na 123456789 123456789 Treatment Number Figure 18. Cumulative irritancy of series of monoalkyl phosphate monosodium salts at the 2.0% concentration as compared to typical anionic surfactants. AS: alkyl sulfate, ES: lauryl ether sulfate, AGS: lauryl acylglutamate.

SKIN IRRITATION BY ANIONIC SURFACTANTS 59 effect at 10 g/100ml. This finding indicates that this compound is at least 10 times less irritating than Q2AS. Comparison of cumulative irritancy by a homologous series of MAP monosodium salts (Figure 18) indicates that although Q0-compound invoked a slightly weak erythematous reaction after nine applications, all other MAP's exhibited no irritating effect. In contrast, most anionic surfactants showed relatively marked cumulative irritation under similar test conditions. When a cumulative irritation test was carried out at 4.0 g/100ml, a distinct skin response by MAP was observed, the magnitude, however, being similar to non-ionic surfactants such as C12-14EO (Figure 19). SOME POSSIBLE FACTORS INFLUENCING IRRITATION There are three different mechanisms (Figure 20) by which surfactants produce cutaneous irritation, depending upon the kinds of surfactant and how they come in contact with the skin. In the case of most ionic surfactants excepting soap, at a concentration less that cmc, inability to degenerate the keratin protein and ability to bind with keratin may result in abolishment of their penetration through horny layers (17,18). This view suggests that adequate surfactant concentrations to induce keratin protein denaturation, leading to the breakdown of the barrier function of the horny layers, are required for the elicitation of irritation. PROTEIN DENATURATION Since the degree of denaturation depends on unfolding of the secondary or tertiary structure, which is accompanied by the increased liberation of sulfhydryl groups, the amounts of the liberated sulfhydryl group (SH) from keratin powder following incubation with surfactant solution was measured by the mercury orange method (19). 5.0 ! •- 2.0 12345678 12345678 4.0 g/lOOml 12345678 12345678 E S A G S C1214EO CI2MAPmonoTEA Figure 19. Cumulative irritancy of Cx2MAP monoTEA salt at the 4.0% concentration as compared to ES, AGS and Cx2-14EO. ES: lauryl ether sulfate, AGS: lauryl acylglutamate, EO: alkyl polyoxyethlene.