INTERACTION OF SURFACTANTS AND KERATINS 55 uptake Q x 10 6 M/g 1200 IOO0 /x /x I ! • 10-3 10 -2 10 -1 Final concentration (SDS) mole/I Figure 15. Adsorption isotherm of sodium dodecyl sulfate in isolated stratum comeurn. (Data reproduction reference 10.) disappearance of the maximum at higher temperature can be most probably attributed to a melting of the crystalline regions susceptible to detergent absorption. It is only fair to point out that alternative interpretations of the various experimental data from that outlined in the previous paragraph is also possible. In particular, it has been suggested that the effects observed by Spei can also be explained by assuming that the detergents penetrate the matrix and not the microfibrils of the hair. (I am grateful to one of the referees for pointing out this fact.) Furthermore, it is also important to keep in mind that Spei obtained most of his X-ray data on mohair, which is a low sulfur-containing keratin and, hence, contains only relatively few cross-links. Owing to the higher cross-link density and the thicker cuticule, the penetration of detergents into human hair is a slower process. The effects observed by Spei on mohair can only be perceived in human hair after prolonged reduction of the disuIfide bonds (12). Nevertheless, it is the author's view that the processes outlined by Spei most probably play an important role in the interaction of surfactants with human hair, especially since most hair fibers on heads are considerably weathered and, therefore, much more susceptible to surfactant penetration than the virgin human hair fibers which were used in Spei's experiments. The effects of the detergents on the mechanical properties of keratin fibers have been investigated by Zahn et al. TM who found that the presence of detergent in wool and hair fibers brought about a diminution of 25% Stretch Index (i.e., the work required to stretch the fibers by 25% of their original length) (Figure 16), and that the higher the detergent uptake, the larger was the drop in the value of the 25% Stretch Index, i.e., the less stiff the fibers became. Similar findings were obtained in stratum comeurn by Putterman et al. (15). Zahn's results also confirmed that a proportionality existed

56 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 25 I: 2 STRAIN (%) Figure 16. Typical strain-stress curve of keratin. The areas under the curves represent the 25% Stretch Indices, i.e., the work required to stretch the fiber to 25% strain. Abscissa: strain coordinate: stress. (Reproduced with permission from reference 14.) between the amount of detergent taken up and the reduction in the stretching work of the fibers. Furthermore, the experimental results also revealed that hexyl sulfate was more efficient than dodecyl sulfate in reducing the stretching work of wool fibers suggesting that the former compound, owing to its smaller size, penetrated the crystalline regions of the fiber with greater ease than the latter and, thus, exercised a stronger effect on the tensile properties of the wool fibers (Figure 17). It was also observed that considerable time was required before equilibrium values of the 25% Stretch Index were achieved (Figure 18). In some cases, treatment times of a few hundred hours were required for the 25% Stretch Index to reach a steady value. The above results further confirmed the conclusions reached by the X-ray diffraction and thermodynamic absorption measurements, i.e., that detergent molecules penetrate the crystalline regions of keratin and, by doing so, strongly affect the tensile moduli of these materials. The long time effects are characteristic of processes that involve conformational changes of polymer, i.e., crystalline-amorphous transitions. VI. THE COSMETIC IMPLICATIONS OF UNEVEN DISTRIBUTION OF SURFACTANTS IN KERATIN The two observations, (a) that surfactant molecules penetrate into the crystalline regions of the keratin fibers (e.g., hair) and sheets (e.g., stratum corneum and nails) strongly affecting their mechanical properties and (b) that the distributions of the