REDUCTION-INDUCED HAIR SURFACE MODIFICATION 5 down the 18-MEA-containing lipid layer of the exposed cuticle surface. Again, the intact hydrophobic lipid domains on the exposed scale faces resist rapid adsorption of the cationic fl uorochrome (indicated by low levels of fl uorescence intensity of the scale faces). However, removal of the 18-MEA lipid layer from the scale faces leads to rapid adsorption of the cationic Rhodamine B, which is indicated by the high fl uorescence intensity of the scale faces. We have shown (9) that cuticular damage increases from root to tip end, even in unaltered hair (indicated by increases in fl uorescence intensity) therefore, studies of pro- gressive reduction were again carried out along the same hair fi bers. This eliminates additional problems due to fi ber-to-fi ber variation. Adjacent sections of relatively short segments of the root portion of the same hair fi bers were exposed for increasing times to reduction with ammonium thioglycolate, (Figure 2). Again, the tagging time of the reduced hair segments was kept short to limit the tagging process to adsorption and to prevent dye diffusion into the cuticula. Increased delipida- tion of the hair surface is indicated by high fl uorescence intensity, as a result of dye uptake by acid functionalities on the scale faces. Figure 1. (a) Interfi ber averages of fl uorescence intensities of unexposed and progressively UV-exposed seg- ments of 30 different hair fi bers. (b) Interfi ber averages of fl uorescence intensities of unexposed and progres- sively chemically oxidized segments of 30 different hair fi bers. Figure 2. Designating specifi c fi ber sections to reduction with ammonium thioglycolate.
JOURNAL OF COSMETIC SCIENCE 6 Figure 3 compares the averaged fl uorescence intensities of interfi ber averages between progressively reduced and progressively bleached segments of 30 different hair fi bers. A total of 1200 fl uorescence intensity data points were measured along each of the six seg- ments that were scanned for each of the 30 individual fi bers of each set. As shown in Figure 3, the fl uorescence intensity (which is proportional to dye uptake) of hair segments exposed to progressive reduction with ammonium thioglycolate increases much more rapidly during short treatment times than was observed in chemically oxidized hair. Dam- age to the scale faces occurs at a much faster rate during reduction than during chemical oxidation (see Figure 4) and photochemical oxidative processes (see Figure 2a). Under the experimental conditions used in this study, a larger number of disulfi de groups is con- verted much more rapidly to R-SH by reduction, than to SOx by oxidative processes. Also, in the case of reduction, thioester cleavage (on the exposed scale faces) takes place along with cystine disulfi de reduction (in the bulk of the hair fi ber). The concept is as follows: Cystine disulfi de reduction reaction (in the bulk of the hair fi ber). The chemistry of reduction reactions, which occur in the bulk of the hair fi ber, involves a two-step reaction of cystine disulfi de reduction, and in the case of reduction of hair by ammonium thioglycolate, the two reversible reactions shown below in equations 1 and 2 are involved. In the fi rst step, a mixed disulfi de is formed as the intermediate, having a carboxyl group which is also negatively charged and capable of adsorbing the cationic fl uorochrome RB. Sharply increasing FI right from the beginning suggests that these reactions occur initially at a faster rate (probably because of higher concentration of reactants). Then the mixed disul- fi de is used in the second step, and therefore its concentration goes down as the reaction progresses. Contrary to the commonly held notion that mixed disulfi de is unstable, sig- nifi cant amounts of the mixed disulfi de remain in the bulk of the reduced hair. This is why reduced hair swells more than oxidized hair. K S S K RSH K S S R KSH R (1) K S S R RSH R S S R KSH R (2) Thioester cleavage (on the exposed scale faces). On the surface of the exposed scale faces, how- ever, although reactions of similar nucleophilic substitutions occur, their outcome is dif- ferent. This is shown by the two reactions shown in equations 3 and 4 below. The results obtained in this study strongly suggest that ammonium thioglycolate reacts with the Figure 3. Comparisons of interfi ber averages of fl uorescence intensities (FI%) between progressively bleached and progressively reduced segments of 30 different hair fi bers.
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