REDUCTION-INDUCED HAIR SURFACE MODIFICATION 7 thioester group on the surface of the exposed cuticle face by a nucleophilic displacement (RS¯ or OH¯), releasing 18-MEA (or other lipids) as shown in equation 3: ( O || (or other lipid) — RS −) →C^^^^^^^^^→K ↑ S O || C^^^^^^^^^K– K– – O S – S – S-R +18-MEA — (3) The reaction is similar to the one involving a cystine disulfi de functionality involving two steps, the second step being: ( ) S n K S R K SH R S S R RS R (4) Figure 4. (a) Schematic of the appearance of the F-layer (lipid domains) on the scale faces during the initial stages of reduction. (b) Schematic of the appearance of the F-layer (lipid domains) on the scale faces after completion of the second step.

JOURNAL OF COSMETIC SCIENCE 8 The fi rst step (equation 3) is unlikely to be a reversible reaction the second step (equation 4) is probably reversible. In the initial stages (fi ve minutes of reduction), the F-layer (lipid layer on the scale faces) will appear as shown in Figure 4a. After completion of the second step of reduction, the fi nal appearance of the F-layer (lipid domains) on the scale faces will appear as shown in Figure 4b. It is important to point out again that this surface (exposed scale face) delipidation reaction has nothing to do with the reduction taking place in the bulk of the hair fi ber by the conventional reversible two-step reaction mechanism. The hair that has undergone these reduction reactions will have mainly -SH groups, and a small amount of - x SO groups (air-oxidized -SH groups), and in the case of reduction with ammonium thioglycolate, hair will also have the mixed disulfi de K-S-S-R, which is a -C-S-S-CH2-COO¯ group (depending on the pH). Also, it should be noted that at short reaction times (equation 1), the concentration of the mixed disulfi de is high and decreases progressively with treatment time. Since the pH of the Rhodamine tagging solution is ~5.8, it is unlikely that the -SH group ionizes to -S¯ to form salt linkages with the quaternary nitrogen of the Rhodamine B. However, Rhodamine B can easily form a salt link with the -COO¯ of the mixed disulfi de. This is the cause of such high fl uores- cence intensity at short reduction times when the concentration of the mixed disulfi de with the -C-S-S-CH2-COO¯ group is the highest. Wettability as a measure of surface damage caused by reduction. We wanted to substantiate the fact that chemical oxidation shows progressive damage to the cuticle surface as a function of increasing treatment time, while in the case of the reduction reaction, the fl uorescence intensity reaches a maximum by fi ve minutes of reduction treatment and gradually decreases during longer exposure times. Therefore, we used an alternate technique, which is well-suited to detect and characterize changes in the surface chemistry of the scale faces, namely, wettability scanning using the Wilhelmy technique (using our TRI/scan apparatus) (8). In this case, work of adhesion, which is a measure of surface wettability, was used to express changes in the surface chem- istry of progressively oxidized and reduced hair fi bers. The magnitude of the work of adhesion should refl ect the increase in surface energy caused by oxidation and reduction processes, involving the removal of the lipid and creation of acid functionalities in the hair surface. Work of adhesion is given by the expression LV ı (1+cosș) (5) where cos θ is the contact angle and σLV is the surface tension of the wetting liquid. Studies at TRI using this technique, using water as the test liquid, have shown that the “work of adhesion” for the reduced hair increases much more rapidly and to a greater extent than that of oxidized hair (Figure 5). The results obtained in these earlier studies, using single-fi ber wettability scanning, support the results obtained in our current research quite well. As can be seen in Figure 5, the progressively increasing hydrophilic nature of the scale faces, stemming from increased lipid scission by hydrolysis and pro- gressive formation of acid groups on the scale faces, increases the work of adhesion of the hair fi ber surface during chemical oxidation with hydrogen peroxide. However, in the case of reduction, a maximum of hydrophilicity is reached by six minutes of treatment,