REDUCTION-INDUCED HAIR SURFACE MODIFICATION 9 after which a decrease is observed. These results obtained by single-fi ber wettability scan- ning correspond well with the results of the microfl uorometric studies. “Relipidation” or “refatting” of the reduced hair surface with CETAB. In earlier oxidation stud- ies, we had shown that after scission of the 18-MEA, the newly formed acid groups (acidifi cation) on the scale faces can become “relipidized” or “refatted” by electrostatic bonding with a cationic conditioning molecule during subsequent treatment. In other words, the cationic conditioning molecule is used to replace the “lost” lipids on the scale faces. The same approach was used for reduced hair. Reduced hair fi bers will also adsorb the low-molecular-weight cationic CETAB because of the negative charges generated on the hair surface during the reduction reaction. Therefore, microfl uorometry was again used to detect the resulting changes in the surface chemistry of the cuticula caused by reduction-induced delipidation and the subsequent relipidation/refatting with CETAB. Hair fi bers that had been reduced for various times were treated for 15 minutes with a 0.5% aqueous CETAB solution, rinsed for 30 seconds in running water, and air-dried overnight at room temperature. The hair was then tagged for 60 seconds with a 0.020% aqueous Rhodamine B solution, rinsed for 15 seconds, and blow-dried at a moderate temperature. Microfl uorometric scans were then carried out along the length of an appropriate number of these hair fi bers. Comparisons between the interfi ber averages of the fl uorescence intensities of reduced/RB-tagged and reduced/ CETAB-treated/RB-tagged hair segments are shown in Figure 6. The averaged fl uorescence intensity of the CETAB-treated controls and that of the CETAB- treated long-term-reduced hair segments are rather similar, suggesting that long-term- reduced hair treated with CETAB behaves in a manner similar to unaltered control hair with an intact lipid layer. This indicates a similar hydrophobic surface chemistry of the “long-term-reduced, CETAB-treated” hair stemming from the presence of a lipid-mono- layer-like structure from CETAB molecules on the scale faces. Short-term-reduced/ CETAB-treated hair segments still show a high fl uorescence intensity however, it is lower than the fl uorescence of the comparable reduced hair segments without the CETAB treatment. To explain the results of Figure 6, we again need to consider the chemistry of reduction reactions as explained in the equations above. Reduction of hair by ammonium thiogly- colate involves these two reversible reactions and produces mainly -SH groups, a small Figure 5. Effects of oxidation (alkaline H2O2) and reduction (NH4OH thioglycolate) on hair fi ber surface wettability (on the outer β-layer of the exposed scale faces).
JOURNAL OF COSMETIC SCIENCE 10 amount of x - SO groups from air-oxidized -SH groups, and the mixed disulfi de with -C-S-S-CH2-COO- groups. It is shown again that the concentration of the mixed disulfi de is highest at short reaction times (equation 1), and decreases progressively with reaction time. This is clearly seen in Figure 6. When treated with CETAB, the cationic molecules will adsorb by salt links at the (air- oxidized -SH) - 3 SO and −C−S−S−CH 2 −COO- groups of the reduced hair surface. The schematic representation in Figure 7 shows that what we envision might occur to the Figure 6. Interfi ber averages of fl uorescence intensities of “reduced” and “reduced and then CETAB-treated” hair segments, subsequently tagged with the cationic RB. The lower fl uorescence intensity of “reduced (de- lipidized),” then CETAB-treated scale faces indicates a decrease in available sulfonic acid groups due to “re- lipidation reaction/refatting” with the cationic CETAB molecule. Figure 7. Schematic of what might occur to the outer β-layer of the exposed scale faces during reduction (delipidation) and subsequently during a “relipidation/refatting” treatment with the cationic conditioning molecule CETAB.
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