14 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS performed at low pH followed by a prolonged exposure of the fibers to a high-pH medium. Similar reactions were reported to occur between certain azo dyes containing disulfide links and the model compounds cysteine and thioglycolic acid, and wool ker- atin (11). However, an alternative explanation of the observed high substantivity of the hydrolyzed isothiuronium dyes could be their immobilization in the fiber structure as a result of the change in their water solubility. Such an explanation was proposed to account for the washfastness of a number of isothiuronium-containing dyes in wool staining experiments (9,10). The relatively high substantivity of the isothiuronium-containing dye to reduced hair, observed for the treatments involving the application of dyes after the reducing step and prior to oxidation, and in simultaneous treatments of hair with a reducer and a dye followed by oxidation with H202, could be explained by the reactions presented in Scheme V. The reaction [1] between the thiol and the isothiuronium group was not Scheme V. + ,•NH2 X-- (1) R•H 2CH •--S• •/' + K--SH •NH2 uahydrolyzed •NH2 X , R•H=CH=--SH + K--S• • •NH 2 (2) R--CH2CH2--SH + K--SH hydrolyzed H=02 • R•H2CH2--S--S--K (3) 2 R•H2CH2--SH H202 • R•H=CH=--S--S•H=CH2--R (5) K--S- H=O= • K--SO 3- (7) R•H2CH=--Br + K--SH • R•H=CH•S--K vhere R = or CH3 X- +1 CHa(CH2) 14CH2--N-- I CH3 and K = Keratin Protein

DYES AND SURFACTANTS FOR HAIR 15 previously reported in the literature, but it is thought to be possible because of the strong nucleophilic character of the thiol group. This results in attachment of the iso- thiuronium group to the keratin protein and the insolubilization of the dye in the fiber. Insoluble products can also be formed as a result of the dimerization of the thiol-substi- tuted dye. Covalent attachment of the dye to keratin can occur during the oxidation step (reaction 2). Also, oxidation of the dye to the sulfonic acid derivative, and the well known reactions of reduced keratin, are included in the scheme as viable processes (reactions 4-6). The relation between the surfactant structure, its mechanism of hair ad(b)sorption, and conditioning was investigated by Finkelstein et al. (20). They found that the sorption of the octadecyltrimethyl ammonium iodide proceeded much faster, and produced a more pronounced conditioning effect, than the dodecyltrimethylammonium chloride. In ad- dition to this, they noted that the desorption, observed after long time intervals in the adsorption experiments, was evident only in the case of a longer-chain quat. As an explanation, it was suggested that while dodecyttrimethyl ammonium chloride pene- trates deeply into the fiber structure and thus does not modify the fiber surface, the octadecyl derivative can rearrange from its initial distribution into surface hemimi- celles. These desorption and surface rearrangement processes, leading to a conditioning effect, were demonstrated for dodecyltrimethylammonium chloride-treated hair by sub- jecting it to prolonged heating in the moist state at 56øC. The data presented in this paper indicate that 0.5-hour exposure of hair to unreactive surfactants such as stearalkonium chloride and hydroxyl- or bromide-containing quats results in their penetration into the fiber bulk and the formation of surface deposits. This is supported by both the quantitative absorption data as well as the observed reduction in combing work. Shampooing should remove some of the surface surfactant deposits and result in a significant increase in combing work. This indeed occurs for the fibers treated with the surfactant solutions for very short periods of time during which only adsorption can take place. In the case of 0.5-hour treatments, the conditioning effect is still evident after four and even eight shampooings, suggesting that, similar to the effect described by Finkelstein et al. (20), desorption and surface rearrangement processes after shampooing may take place. It should also be mentioned that the ad(b)sorbed cationic surfactants can possibly interact with anionic detergents present in the shampoo (21). The precipitation of the cationic surfactant-anionic surfactant com- plexes on the fiber surface can potentially contribute to the observed conditioning ef- fects. The durability of the unhydrolyzed isothiuronium-containing surfactants on unmodi- fled hair might be attributed to both electrostatic interactions, increased by the pres- ence of the additional positively charged groups and Van der Waals forces. The adsorp- tion and desorption data, which showed relatively small rates and extents of deposition of the isothiuronium-containing quat into hair, suggest that this compound does not readily penetrate the bulk of the hair and possibly remains confined to the outermost layers of the fiber structure. The increased size of the isothiuronium-containing surfac- tant molecules might also contribute to such a distribution of the quat in the fiber. The data presented in Table III suggest that thiol/disulfide exchange reaction (Scheme IV) might be an important factor contributing to the high washfastness of the treat- ments performed with both single and double isothiuronium quats at pH 10.7 on