INTERACTION OF SURFACTANTS AND KERATINS 47 s-s k•-u.s-s -s s• e .s ,r• s-'. •6• 66• 66• Figure 6 Model proposed for the keratin structure keratin consists of three ordered components--one microfibrillar component and two matrix components. (Reproduced with permission from reference 7.) which come into contact with each other (8). The binding energies of detergents to proteins, therefore, can be expected to increase linearly with the length of the hydrophobic chain, as observed experimentally in the case of bovine serum albumin (Figure 5). With keratin, however, an additional factor also affects the binding characteristics of surfactants. Keratins, as mentioned before, are highly cross-linked polypeptide gels. To penetrate into a cross-linked structure of this type, the surfactant molecule has to pry apart the constituent chains, i.e., to overcome the elastic energy of the polypeptide network. The work required to accomplish this will be proportional to the volume of the penetrant molecule. Consequently, the larger the surfactant molecule, the more difficult will be the penetration of the keratin structure and the interaction with the various binding sites attached to the protein chains in the interior of the tissues. The differences between the respective free energies of binding of surfactant molecules to BSA and to keratin are measures of the additional thermody- namic work that is required for the surfactant molecule to reach the binding site in the keratin structure. This model also accounts for the turndown of the free-energy- vs.-carbon-chain-length curve in the case of keratin for molecules with aliphatic chains longer than C•2 (Figure 5). III. THE ROLE OF CHARGE IN BINDING OF SURFACTANTS TO KERATINS As mentioned before, it is an experimental observation that charged compounds penetrate keratinous tissues to a much lesser extent than do uncharged molecules of similar sizes. The presence of cross-links in keratin structure is unlikely to account for this negative effect of charge. To explain it, therefore, we have to consider the

48 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS / / / / / / D-- 80 / / / / / / Plane surface Circular cavity d=d I [a} [b] Figure 7. Schematic representation of ions at protein-water interfaces. (a) Ion at a plane interface. (b) Ion in a circular cavity. D, D', d and d' denote the dielectic constant of water and protein and the distances of the floating and fixed ion mirror images from {he interface, respectively. (Reproduced with permission from reference 3.) //// D=80 dmd I {a) {b] Figure 8. Schematic representation of ions interacting with a fixed charged site situated at a protein-water interface. (a) Interaction of a monovalent dye with a charged site. (b) Interaction of a divalent dye with a single charged site. (Reproduced with permission from reference 3.)