SKIN IRRITATION POTENTIAL OF MIXED SURFACTANT SYSTEMS

Virtually all current detergent formulations contain mixtures of surfa ctants: Our experience and test data on these formulations, which is i n agreement with that of many others, has shown that in use the formul ations exhibit lower acute irritation potential than predicted by simp le summation of the irritation potential of the individual actives. Us ing the criteria of the Dangerous Preparations Directive (EC Directive 88/379/EEC), many of these formulations classify as irritant in the n eat state, with consequent labelling requirements. Such classification is based on addition of irritant components giving a total concentrat ion which exceeds a nominal threshold. In this study, mixtures of surf actants were tested by application to a panel of 31 human volunteers f or up to 4 hr, using the technique established for the assessment of a cute skin irritation potential. The positive control, sodium dodecyl s ulfate (SDS) at 20%-concentration, gave an 84% positive response. Dime thyl dodecyl amido betaine (DDAB) at the same concentration gave a 94% response. However, a combination of 20% of each of these surfactants in the same panellists gave a response of only 44%-a significant reduc tion in the irritation potential. A further test conducted with a mixt ure of 10% SDS and 10% DDAB in a second panel gave a 31% positive resp onse compared with a 94% positive response to the 20% SDS control in t hat panel. These results clearly demonstrate that the acute irritation potential of mixed surfactants cannot be predicted by simple summatio n of the irritation potential of the component substances. Initial res ults of the mechanistic investigation indicate that the reduced irrita tion induced by the mixed surfactant:systems correlates with a reduced critical micelle concentration (CMC). However, the reduced CMC itself seems not to be responsible for the lowered irritation, since these e xperiments were conducted at concentrations well above the CMC. It is proposed that the critical event leading to skin irritation is binding to skin protein and that in mixed surfactant systems, the individual surfactants exhibit less affinity for this protein. (C) 1998 Elsevier Science Ltd. All rights reserved.