888 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tion by allowing measurements to be made at sufficiently low dilutions for Van't Hoff's law to be valid. Melanin solubilized by ammoniacal hydrogen peroxide was dialyzed for 24 hours prior to the measurements. The molecular weight was calculated from Burk and Greenberg's equation (27): M = CdRT/lOOP where C is the concentration of polymer in grams per 100 ml of solvent, d is the density of the solvent, R is the gas constant, T is the temperature, and P is the osmotic pressure. The equation reduces to C M = 2.527 X 10 .5 dff after inserting the numerical values for the constants. A melanin con- centration of 0.310 g per 100 ml of salt solution gave rise to an osmotic pressure of 6.86 cm of water. The molecular weight calculated from these data yielded the value of 11,400. A value of the same order, viz., M z 15,000, was also obtained from the molecular weight determination using the thin-layer gel-filtration technique. Free Radical Content--Samples of melanin were examined in a Varian X-band esr spectrometer. Both the intact and solubilized mela- nin gave rise to virtually identical structureless absorption, with line widths o1: the order of 6 gauss and g values of 2.003. The spin density was determined for both of the samples by comparison with a known DPPH (o6cr-diphenyl-/:t-picryl-hydrazyl) standard and gave a value of 10 •9 spins per gram. The most important point emerging from this brief study is that the free radical character of the melanin is not affected by the solubiliza- tion process. This means that these radicals are extremely stable and do not rely for their existence and stability on a physical trapping mechanism. Decolorization--The solubilization of melanin by H.,O• is only the first step in the reaction sequence. Prolonged treatment results in bleaching or decolorization of the intensely dark solution. Although the high efficacy of hydrogen peroxide (as compared with other oxidants) for the solubilization of melanin was clearly established in this investi- gation, it did not connote its superiority in the bleaching step. Con- sequently, the effect of a number of oxidizing agents on the color change of the aqueous solutions of solubilized melanin was assessed. The reac-

HAIR BLEACHING 889 Table IV Effect of Oxidizing Agent on the Bleaching of Solubilized Melanin Oxidizing Agent Bleaching Ability Conditions, pH (NH4)2S.•O8 None 1-10 KI O 3 None 1-7 K2Cr•O7 None 1-7 NaC104 None 1-7 12 None 5.2 H202 q- 10 NaOCI + q- 7 KMnO4 q- q- q- 3 CHaCOOOH q- q- 7-8 tion was carried out at room temperature. In each case, excess of the oxidant was present in the system. The results are given in Table IV. The most surprising finding was the high decolorization efficacy of the permanganate, particularly in view of its inability to react with the intact melanin. Are the cross links which are broken during the solubilization important to the preservation of color? Or does the solubilizing action of peroxide sensitize the melanin polymer, e.g., by generation of labile, peroxide-type structural elements? Approximately 0.03 meq of KMnO4 was required to bleach 1 mg of soluble melanin to a pale yellow color. Assuming an average unit weight of the melanin as 145, 1 mole equivalent of KMnO4 is utilized for 2 melanin units. The contribution of peroxy anion species to the bleaching process can again be readily seen in the case of peracetic acid. In slightly acidic media this reagent is specific for oxidative cleavage of the disulfide bonds in hair but has little effect on the melanin. The latter is, however, readily attacked under alkaline conditions and a maximum decoloriza- tion effect was observed in the pH range close to the pK value (8.2) of the peracid (28). It is worth pointing out that while the disintegration of the pigment granules and their solubilization are the necessary prerequisites for bleaching, these processes, by themselves, are not likely to affect the color of hair significantly. At best, the conversion of pigment particles into soluble melanin dye might bring about a slight change in hue. The decolorization step, on the other hand, although contingent upon the former, is more readily perceived and thus may be considered ot• greater practical importance. From the experimental evidence ob- tained so far, it is impossible to elucidate the precise chemical nature