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 rum, 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.