70 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS PARAMETERS OF COLOR FORMATION IN HAIR Metal catalysis. The rate of color development with DHI as the dye precursor depends on the nature of the metal ion used in the pretreatment: Cu(II), Fe(II), and Ni(II) are effective in promoting fast melanogenesis, whereas Mg(II), Ti(IV), and AI(III) give rise to much slower coloration. Usually when the melanin formation is fast, the color devel- oped in hair is dark grey to black. Slower melanogenesis leads to a color shift towards brown. Accordingly, grey hair pretreated with Cu(II) is dyed black, whereas the color is medium brown with a Ti(IV) pretreatment. Thus, by the use of the appropriate metal ion, some color modulation through the brown shades can be attained. We do not have satisfactory explanation at this time why and how the different metal ions lead to these color effects. Conversion of DHI to eumelanin by Cu(II) has been well documented (7,8) and this metal ion has been our primary choice in studying the melanization of DHI on hair. Also, salts of copper bind to keratin very readily (6) so that even a brief (5 min) treatment of hair with dilute solutions of copper salts (0.04 M at pH 9.0) deposits sufficient quantity of Cu(II) to dye the hair uniformly black after exposure to DHI. Under such conditions the,'quantity of Cu(II) taken up by the fiber is close to 1000 ppm. Longer treatment times, although leading to higher uptake of Cu(II), have vir- tually no effect on the attained color. Table I illustrates the effect of pretreatment of hair with different concentrations of cuptic salts on subsequent color development with DHI. The melanization of DHI is significant even for the lowest concentration of Cu(II) as judged by a large decrease in the L value. As the eumelanin pigment dispersion is relatively achromatic, its deposition should also affect the a and b values. However, the former is close to 0 for the untreated grey hair, and it remains virtually unchanged. The latter, however, does decrease substantially and even at intermediate concentrations of Cu(II) reaches the level typical for black hair. Color development. As a result of fiber-anchored catalysis, the conversion of DHI to mel- anin takes place preferentially in the hair. Unlike the auto-oxidation of DHI, which is favored by alkaline conditions, we find that the metal-induced melaninization process is less pH-sensitive (Table II). The DHI concentration required for effective coloring is in the range of 0.02 to 0.13 M. A study of the color intensity on bleached, blended grey, and brown hair as a function of the DHI concentration revealed that above 0.07 molar the L value for the dyed tresses reached a relatively constant value (Figure 3). The color develops quickly and there is little change in both the shade and intensity after ten minutes of treatment with DHI. However, one has to bear in mind the L Table I Color Parameters (L, a, and b) for Cu(II)-Catalyzed Melanization of DHI on Blended Grey Hair [Cu(II)]M L a b 0.000 33.4 -0.35 5.33 0.001 18.5 0.15 2.12 0.010 15.4 0.21 1.05 0.025 15.5 0.13 0.55 0.040 16.0 0.15 0.85 0.050 14.8 -0.10 0.95 0.100 14.7 -0.15 0.72 [DHI] = 0.07 M pH = 9.5 color development time = 15 minutes.

HAIR COLORING BY MELANIN PRECURSORS 71 Table II Color Parameter (L) for Cu(II)-Catalyzed Melanization of DHI on Blended Grey Hair at Various pH Values pH L 2.0 15.9 4.O 13.9 9.O 13.2 11.0 13.4 [Cu(II)] = 0.04 M, [DHI] = 0.07 M, color development time 15 minutes values denote merely the intensity of reflectance and these do not necessarily correlate with the quantity of coloring agent deposited within the hair. Nevertheless, the shade intensity imparted by the DHI system is comparable to that obtained from commercial oxidative dye formulations (Table III). It should be pointed out that although in the experiments reported here the color-forming reactions were performed promptly after the metal pretreatment, we have found that the catalyst efficacy is not subject to aging, i.e., the hair can be pretreated with metal salts days or weeks before exposure to DHI to impart the desired color range. PROPERT/ES OF CO•.OREr) Iqam Location of the melanin dye. Microscopic examination of the dyed hair reveals that the melanin is deposited primarily in the periphery of the hair fiber (Figure 4). This is not L 50 30 10 -• x x = bleached hair • o = 9rey hair 1 7 13 [DHI] X 10 -e molar Figure 3. A plot of L values for bleached, blended grey, and brown hair dyed with various concentrations of DHI.