36 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ß Reaction of the imine with the coupler to give a diphenylamine ß Oxidation of the diphenylamine to the dye. With hexacyanoferrate, the first and last steps are fast and the kinetics of the slower coupling reaction can be evaluated. For all phenolic couplers studied, the reaction mechanisms followed a similar pathway. With H202 as oxidant, color formation is many times slower than the coupling reaction, suggesting that the initial oxidation is rate-controlling. However, indo dye decomposition by H20• is a competitive process, especially in ot-naphthol and 4-amino-2-hydroxytoluene reactions, and thus accurate coupling rate constants are difficult to determine. Much slower dye decomposition is observed for the couple PPD/2,6-dimethylphenol, and some rate data have been ob- tained. Even so, the overall scheme is not simple since the reaction rate is relatively independent of primary intermediate concentration and proportional to [H•O•] ø'5 (6). A possible mechanism involves rate-determining formation of reactive intermediate from peroxide followed by a series of faster oxidative and coupling steps, i.e., slow H202 k 2X X + PPD ,Imine Imine + Coupler , Leuco dye Leuco dye -Dye. Analysis of this scheme gives the rate equation d [Dye]/dt = k [H•O2] ø'5, which satisfies the experimental data. Thus, although this mechanism predicts an increase in reaction rate with increased peroxide concentration, in dyeing practice this effect must be moderated by the increased dye decomposition rate at higher peroxide concentration. Also, this mechanism predicts no effect of dye concentration, whereas this is the main factor observed in the dyeouts. Presumably, this may reflect a change in mechanism in, or at the surface of the substrate, or may reflect the importance of diffusion processes during dyeouts. The mechanistic change could involve an alternate pathway for H•O• decomposition, or a change in rate-determining step so that imine formation becomes rate-controlling. The nature of intermediate X would aid in distinguishing between the possibilities. However, as yet we have no identification. Finally, the rate of reaction shows a moderate increase with solution pH as is observed during the dyeouts. This clearly points to a rate-determining step that is not coupling since the coupling rate decreases rapidly with pH over the range 9-11 for these couplers (1). CONCLUSIONS Using reflectance measurements to quantitate the amount of adsorbed dye in hair or wool appears to be a valid technique. It is clear that the major factor controlling dye deposition from oxidation dye solutions is the concentration of the dye precursors. This presumably has a strong effect on both dye diffusion and on its rate of formation. In addition, the indo dye formed in sobation does contribute to coloration if it is suffi- ciently stable to the excess H•O•. Other factors having some effect are iolution pH and dyeing time. Surfactants reduce dye deposition probably by slowing diffusion. Peroxide concentration over the range 1-11% has little effect, but this is a large molar excess

OXIDATIVE DYEING OF KERATIN 37 over dye concentration and, therefore, any real effects may be masked. Alternatively, there may be a balance between dye formation and dye loss by excess H20 2. Solution kinetics and mechanisms predict a dependence on peroxide and not on dye concentration. These differences may reflect the importance of diffusion processes with substrates or a change in mechanism at the substrate surface. REFERENCES (1) J. F.Corbett, The role of meta difunctional benzene derivatives in oxidation hair dyeing. I. Reaction with p-diamines, J. Soc. Cosmet. Chem., 24, 103-134 (1973). (2) K. C. Brown and J. F. Corbett, The role of meta difunctional benzene derivatives in oxidative hair dyeing. II. Reactions with p-aminophenols, J.Soc. Cosmet. Chem., 30, 191-211 (1979). (3) K. C. Brown, Hair colorants, J. Soc. Cosmet. Chem., 33, 375-383 (1982). (4) W. G. Cochran and G. M. Cox, Experimental Designs, 2nd Ed., 0ohn Wiley, 1957), pp 335-375. (5) P. Kubelka and F. Munk, Ein Beltrag zur Optik der Farbanstriche, Z. Tech. Physik, 12, 593-601 (1931). F. W. Billmeyer and M. Saltzman, Principles of Color Technology, 2nd Ed., (John Wiley, 1981), p 140. (6) K. C. Brown and A. Chan, unpublished data.