•'• J. Soc Cosmet. Chem., 28, 2713-2 84 (May 1977) Wettability of keratin fiber surfaces ':??:it Y' K. KAMATH, C. J. DANSIZER, and H.-D. WEIGMANN ii!:i!:!i•!.:.i Textile Research Institute, Princeton, New Jersey. ::. !53!•:i?!i•... Received July 19, 1976. Presented Ninth IFSCC Congress,June 1976, The WETTING of HAIR FIBERS by WATER has been measured using the Wilhelmy balance technique developed at Textile Research Institute Princeton, N.J. specifically for FIBROUS MATERIALS. The data for selected and treated hair samples suggest cause-effect relationships between WATER WETTABILITY of the FIBER SURFACE and MECHANICAL, WEATHERING, and CHEMICAL FIBER DAMAGE. Critical surface tension of the hair fiber surface has been determined with water-butanol mixtures using the same technique. Furthermore, dispersion and nondispersion contributions to the surface free energy of the fiber have been evaluated by measuring wettabilities against a polar and a nonpolar liquid. The results indi- cate that the molecular processes occurring at the interface between the keratin fiber surface and a liquid have considerable effect on the surface free energy of the fibers. INTRODUCTION Human hair is an important member of the keratin fiber group. Keratins are structural proteins, which occur widely in the vertebrate epidermis and its appendages. Unlike man-made fibers, human hair is cellular in structure, consisting of a central core called the cortex covered by a sheath of several layers of flattened cuticle cells. The cuticle cell itself consists of various layers, the endocuticle, exocuticle, and the a-layer, proceeding from the inside to the outside in that order. The thin outermost layer that forms a sheath around the cuticle cell is known as the epicuticle and is hydrophobic, whereas, the cortex is hydrophilic. Although, no definitive information is available about the composition of the epicutJ. cle of hair fibers, King and Bradbury [1] have found that the epicuticle obtained from Merino wool consists of 7 8 per cent protein, 5 per cent lipid, and 4 per cent ash. Values may be of a similar order for hair. The hydro- phobicity of the fiber surface may in part be due to the lipid content of the epicuticle. The characteristic toughness and the insolubility of keratins in water is attributable to the presence of the sulfur-containing amino acid, cystine, which acts 'as a crosslinking agent. From the work of Wolfram and Lindemann [2] and Swift and Bews [3], it can be seen that the cuticle of hair contains more cystine than the cortex. Histochemical observations of cuticle cross sections by the latter authors show that most of the cystine is concentrated in the exocuticle and especially in the a-layers. This renders these outer :. 273