INTERACTION OF SURFACTANTS AND KERATINS 61 occur with the disulfide bonds of keratin when exposed to various reducing nucleophilic agents. Since the attacking species in most of these cases are negatively charged ions (e.g., bisulfite or thioglycollate ion), the presence of negatively charged detergents in the structure diminishes the rate of reactions with these attacking ions. On the other hand, cationic detergent absorbed into the keratin structures will, in fact, enhance the reactivity of the disulfide bonds with respect to nucleophilic agents and thus speed up the breaking of these important crosslinks. Again, Meichelbeck and Knittel confirmed these reactions by their experimental measurements (Figure 21). VIII. THE EFFECT OF FORMULATION INGREDIENTS ON THE UPTAKE OF SURFACTANTS BY KERATINOUS SUBSTANCES From the foregoing sections it is evident that during cosmetic treatments involving the use of surfactant-containing products, certain amounts of the surfactant molecules 70 60 40 30 Figure 22. SLS ALONE SLS + 0.1% POLYMER SURFACTANT/POLYMER wt. I t APPEARANCE 1:8 1:4 1'2 1'1 2:1 c t ppsp c -4 -3 -2 -1 LOG SURFACTANT CONCENTRATION (m/I) Surface tension, concentration curves of Na Lauryl sulfate in water at 25øC, with and without 0.1% Polymer JR. (Reproduced with permission from reference 19.)

62 JOURNAl, OF THE SOCIETY OF COSMETIC CHEMISTS penetrate the keratinous tissues (skin, hair and nails) affecting the properties of these tissues sometimes in a beneficial way, sometimes in a detrimental way. It is also known that many of the surfactants penetrate beyond the stratum corneum and interact with the vital tissues of the skin causing, on occasion, irritation and even sensitization. The art of product formulation is therefore to find a surfactant composition capable of achieving maximal functional effects (e.g., cleaning power) with minimal penetration of detergents into the human tissues. On the whole, the penetration of the surfactants can be regarded as being proportional to the number of monomeric (nonmicellar) surfactant molecules in the system. The functional effects of detergents will, on the other hand, generally depend in a more complicated way on the surfactant activity. As an example, let's consider a product that contains a polymer capable of complexing with surfactants (e.g., PolymerJ.R.). The cleaning ability of the surfactant system (e.g., of the shampoo) will depend on its surface activity. Goddard et al. (19) demonstrated that the presence of Polymer J.R. in a surfactant solution will enhance surface activity (Figure 22). On the other hand, other experimental data also indicate that the presence of Polymer J.R. reduces the irritation potential of surfactants (20), presumably owning to complexation of the attacking free surfactant molecules. These results show, in a very vivid fashion, that given suitable formulation technology a high degree of functional efficacy can be achieved at the same time as the irritation potential of a surfactant can be diminished. To quote another example for the importance of formulation technology in achieving the desired product attributes, let us consider Poret's (10) results with mixtures of sodium lauryl sulfate and capry/monoglyceride. Poret measured the surface tensions and the depositions into stratum corneum from solutions containing these surface active materials as functions of the ratios of the constituents and their concentrations. 70' dyn cm-•30 20 10 3 Figure 23. The surface tension of surfactant solutions as a function of concentrations. Curve No. 1--pure SDS Curve No. 2--pure MCG Curve No. 3--mixture of SDS with various proportions of MCG. (Reproduced with permission from reference 10.) i i i IlL 10-s 10 -4 10 -3 10 -2 10 -• C mole I -•