54 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS O: L /%: a I I I Number of Treatment Figure 8. The color stability to shampoo treatment. II with their oxidation-reduction potential. In solutions it is possible to oxidize PPDA only in the case where silver ions are held in the pores, these ions having the highest potential of all metal ions we used. Interestingly, only the smaller-pore zeolites are able 'to oxidize the intermediate. In addition, silver ion content in the zeolite also affects the rate of the oxidation reac- tion. Consequently, it is speculated from these results that silver ions catalyze the oxidation reaction. However, PPDA was little oxidized when only silver ion solution was used in this system without zeolite. This may be explained by the results of later examination. Figure 12 illustrates the relationship between the reaction rate and silver ion concentration in the solution. It is shown that a high concentration of silver ions decreases the oxidation reaction in spite of its producing much more final dye. Conse- quently, in the case where silver ion solution was used, too many silver ions inhibit the progress of the reaction. We speculate that zeolite does not function as a catalyst in the oxidation reaction, and it is necessary to supply silver ions into the solution in limited amounts throughout the reaction. 400 200 / ..o/ø/ , ! I 8.0 10.0 Figure 9. The effect of pH of the solvent on the elution of silver ions from zeolite. Ag-zeolite containing 2.5 wt% silver was used.

HAIR COLORING 55 ß 200 o o / /0 /0 /0 /0 CI r=o. 995 offO/O p o. o• 50 1 oo 150 Na Cone. (•mol/•) Figure 10. The effect of sodium concentration in the solvent on the elution of silver ions from zeolite. Ag-zeolite containing 2.5 wt% silver was suspended in Carmody buffer at pH 9.8. Silver ions held in zeolite were shown to elute into the solution by exchanging with sodium ions in buffer solution. In hair coloring systems, ammonia ions were dissolved instead of sodium ions. The NH 4+ ions may work the same as Na + ions so that their ion radius is about 0. 100 nm, close to the Na + radius of 0.095 nm. Silver ions can elute into the solution by exchanging ions with NH 4+ ions and cata- lyzing the PPDA oxidation. On application of this system to actual hair coloring, its coloring effect was seen on a level with commercial products. The coloring process is completed in about 30 min. The hair coloring was affected by many factors: PPDA concentration, treatment time, content of Ag-zeolite, and hamamelis extract concentration. Using the excess Ag-zeolite reduced the coloring effect as shown in Figure 5. This phenomenon is considered to relate to silver ion elution. It has been shown (Figure 11) that excess Ag-zeolite decreased the elution of silver ions. It may be explained as follows: Excess Ag-zeolite is not able to suspend well in the solution and it results in a 800 ," 400 o ! I 0.5 1 .0 Ag-Zeolite Content (g/4mt) Figure 11. The effect of Ag-zeolite content on the elution of silver ions from zeolite. Ag-zeolite containing 2.5 wt% silver was suspended in Carmody buffer at pH 9.8.