Chapter 5 173 Table 1 Diffusion constants penetration and waving efficacy pH 5 min 5 min 15 min 15 min D (x1010 cm2/sec) Penetration (μm) Waving Penetration (μm) Waving 7 3.0 ± 0.6 4.63 0 13.9 12.1 9 23.6 ± 3.0 23.2 48 Complete 79.1 In a separate paper, Kozuhara and Hori studied hair reduction by L-cysteine (CYS), and found it both less efficient and less damaging than thioglycolate.51 A 15-min treatment with cysteine penetrated 9.3 μm into the hair and yielded a waving efficiency of 44% compared to complete penetration and 79% waving efficiency with thioglycolate at pH 9.0 after 15 min. Salce et al.40 reasoned that adding the disulfide form of the reducing agent (R-S-S-R) would reduce the extent of the reaction at equilibrium according to the Le Chatelier-Brown principle, based on the equilibrium constant for the reaction with mono- thiol reducing agents. They used amino acid analysis to investigate the reaction of hair with ATG and glyceryl monothioglycolate. The authors reported that adding dithiodiglycolic acid (DTDG) to ammonium thioglycolate or diglycerol dithioglycolate to glycerol monothioglycolate reduced both the extent and rates of the reactions. Manuszak et al.32 used the SFTK method to study the effect of DTDG on the reaction of hair with ammonium thioglycolate. We did not see a statistically significant effect on reaction kinetics. However, addition of DTDG was found to reduce the amount of damage to the hair as judged by tensile measurements after treatment. Ogawa et al. also found DTDG to have no effect on reduction rate with ATG.52 DTDG is sometimes added to perm formulations to produce “safety perms.”23 Manuzak et al.42 investigated the correlation between SFTK data and amino acid analysis with cysteamine and ATG, concluding that stress supporting bonds are broken at a faster rate than non- stress supporting bonds, but the general correlation between the two methods was good. Fit to pseudo first order kinetics was also good for both methods allowing measurement of the half-life of