INTERACTION OF SURFACTANTS AND KERATINS 53 28, 3 •, Figure 13. Schematic representation of the sequence of events that occur when surfactants penetrate keratin. For details see text. (Reproduced with permission from reference 7.) of papers, Spei et al. investigated extensively the interaction between keratins and surfactants using low angle x-ray diffraction (11-13). From Spei's work the following sequence of events appears to take place when keratin is exposed to detergents. The first surfactant molecules which enter the keratin structure penetrate its noncrystalline regions, i.e., the matrix, and by doing so, they disturb its long-range order. This is demonstrated by the changes of the spacing at 28.3 • that is characteristic for the structural repeat units of the matrix (Figures 13a and b). When the detergent concentration of the solution in contact with the keratin is further increased, the additional surfactant molecules gradually find their way into the more organized and crystalline microfibrillar regions of the keratin structure (Figures 13c and d). Again, this second sotage of penetration is evidenced by the changes of the characteristic spacings at 39.6 A reflections. In addition to Spei's X-ray data, results of other experimental techniques also indicate that surfactant molecules do penetrate the crystalline regions of the keratin structure. The absorption isotherms of surfactants, both on stratum corneum and on wool fibers, reveal maxima near the critical micelie concentrations (Figures 14 and 15). Since, according to thermodynamics, in a three-component system a downturn of the absorption isotherms is only possible if structural changes occur in the absorbant substrate, the results shown in Figures 14 and 15 also support Spei's

54 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.25F k- 0.2 0.15 0,1 005 ß \ ß ß o i I I I I I I II I 0 20 40 60 80 I00 120 300 c (Moles/litre x i04) s Figure 14. Isotherms for the adsorption of SDS by wool at 40øC (0) and 60øC. (0) cf and c, are the concentrations of SDS in the wool and in the solution, respectively. (Reproduced with permission from reference 21.) interpretation, i.e., that surfactant molecules enter and disrupt the crystalline regions of the keratin structure. Furthermore, the drop in the uptake curve, both for wool and for stratum corneum, coincides with the concentrations where Spei has observed the penetration of the first detergent molecules into the crystalline structures. It is also interesting that, when the isotherms are measured at higher temperatures, the maxima disappear and the curves show a gradual increase towards a plateau (Figure 14). The