j. Soc. Cosmet. Chem., 30, 41-64 (January/February 1979) The interaction between surfactants and keratinous tissues M. M. BREUER, Gillette Research Institute, 1413 Research Boulevard, Rockville, MD 20850. Received August 7, 1978. Presented at Annual Scientific Seminar, Society of Cosmetic Chemists, May 1978, Chicago, Illinois. Synopsis During cosmetic treatments, SURFACTANTS penetrate into KERATINOUS TISSUES (hair, skin and nails). Whereas some of these surfactant molecules migrate to the vital tissues, a considerable fraction remains bound to the keratin. The extent of binding depends both on the nature of the head group and the length of the hydrophobic tail of the detergent molecules. In addition to entering the amorphous region of the keratin, some of the detergents also penetrate into the crystalline microfibrils and change their structures affecting their tensile properties. Owing to an uneven distribution of detergent molecules in the tissues, an anisotropy of the elastic moduli will occur, resulting in considerable internal stresses which, in turn, might lead to a deterioration of hair, skin and nails. The chemical behavior of keratins is also influenced by the presence of absorbed detergent in their structures. Depending on the detergents and the conditions, these effects can be either protective or detrimental. The deposition of detergent molecules into keratin can be enhanced or diminished by the inclusion of appropriate ingredients into the product formulae. I. INTRODUCTION Surfactants come into daily contact with human keratinous tissues (hair, skin and nails). As a result of these exposures, a fraction of the surfactant molecules will reach, after penetrating and migrating through the keratinous tissues, the vital layers of the skin (e.g., migrate through the stratum corneum or nails and penetrate the epidermis and the dermis) and will eventually enter the blood stream. The bulk of the penetrating molecules, however, will remain bound to the keratin structure itself. The various physicochemical factors governing the penetration and migra?ion of surfactants in skin and the biological responses which the presence of these surfactants in the living tissues evoke, have been extensively studied and reported in the past (1). On the other hand, no critical literature review exists that surveys the binding of surfactants to keratins and the effects which the presence of the surfactants in the keratinous tissues exert on the physical and the chemical properties of hair, skin and nails. This void in our knowledge is regrettable, especially as many of the important cosmetic attributes of skin, hair and nail depend on the physical and chemical integrity of their constituent 41

42 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS keratins. The present communication aims to fill this gap and intends to review critically the current knowledge, as it appears in the published literature, on these subjects. In attempting this task, it is necessary to make the assumption that the experimental data obtained on keratins from different origins (e.g., hair, wool, mohair, stratum comeurn, etc.) are interchangeable and, thus, can be pooled. In view of the known chemical differences between the various keratins, this assumption cannot be regarded as being completely correct. Nevertheless, it is the author's view that the errors committed by the pooling of data obtained from keratins of diverse origins affects only the quantitative aspects of the conclusions and does not influence the essential features of our understanding regarding the effects of surfactants on the cosmetically important properties of keratins. II. THE UPTAKE OF SURFACTANTS BY KERATINS Essentially the magnitudes of two quantities determine the extent of a binding of surfactants to a keratinous tissue: noo the maximum nuber of molecules that the keratin in question can accommodate, and A G, the free energy change (i.e., the affinity) that 1'5 E 1.0 E m 0'5 I 2 3 6 I I I i pH Figure l. Uptake of acids by wool keratin. Acids: O Hydrochloric, ß dichloroacetic, A Monochloracet- ic, formic, [] glycolic, ß acetic, X propionic, V benzoic, ß p-nitrophenol. (Reproduced with permission from reference 3.)