INTERACTION OF SURFACTANTS AND KERATINS 57 x x Hexylsulfate Dodecylsulfot• ctyl- 4 6 10 '2 2 4 6 10 4 2 4 6 10 ø Ck C) 2 C)o Cm • Conoent•t[on(•oL/L) Figure 17. Percentage reduction of the value of the 25% Stretch Index of keratin fibers exposed to various concentrations of surfactants (Reproduced with permission from reference 14.) surfactant in these tissues are highly asymmetrical, have important consequences as far as the cosmetic attributes of these tissues are concerned. To illustrate this point, let us consider for instance the case of a layered structure (e.g., stratum corneum) where each layer contains a different amount of surfactant (Figure 19). Depending on the detergent content, the elastic modulus of each layer will be affected to a different Figure 18. x x x OctyLsuLfate,( 2 10 'i X •-.• X X X X x/ DodecyLsuLfote('1MoL/L)2.10 0 / 0 ß _ •odecyLsuLfat•5 1• • MoL/L) 4 6 10 ø 2 4 6 10 • 2 4 6 10 2 4 6 ß •te•t•ent •e •oots) •e[cent•e [educdon o• the 25• •t•etch Index o• kerntin •be[s •s • •unctJon o• time when immersed into solutions of surfactants. Abscissa: sufactant concentration. Coordinate percentage loss of wool. (Reproduced with permission from reference 14.)

58 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Strain c c c c M M M M Strain Shearing stress 01/• 02/• 03 • 04 M2 M3M4 Figure 19. Schematic representation of events in stratum corneum containing an uneven distribution of detergents in the various cell layers. c, and M, represent the detergent concentrations and the toodull of the i-th layer, respectively. degree: the layer nearest to the surface and containing most detergent will have the lowest elastic modulus, whereas each subsequent layer containing less and less detergent molecules will have an increasingly higher elastic modulus. If a structure of this sort is now exposed to a uniform longitudinal stress, the differences of moduli which exist between the layers will induce considerably different strains in each layer, with the net result that large shearing stresses will arise at the layer boundaries (Figure 19). Skin chapping and nail breakage as consequences of detergent exposure probably originate from this kind of physicochemical effect. It is also highly probable that materials facilitating the diffusion of materials in skin (e.g., urea, water) will bring about a more uniform distribution of the surfactants in skin and, therefore, will ameliorate the effects of detergent-induced shearing stresses. VII. THE EFFECT OF DETERGENTS ON THE CHEMICAL REACTIVITY OF KERATINS A variety of chemical reactions can affect keratin structure. Among these, surfactants can influence especially three important processes: the degradation of the main polypeptide chains, the reduction of the disulfide bonds and the lanthionine and lysoalanine formation from disulfide bridges. As is the case with proteins in general, keratin polypeptide chains can be broken by acid hydrolysis. Depending on the pH and the nature of the charges they carry, ionic surfactant molecules, absorbed in the keratin structure, will influence these processes by either attracting or repulsing hydrogen or hydroxyl ions. The effective concentrations of these attacking species near the peptide or disulfide bonds will be, therefore, augmented or depleted. These