294 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.0 0.9 0.8 0.7 0.6 0.5 0.4. 0.3 0.2 0.1 0.0 0.0 ß ß ©© 0.5 1.0 1.5 2.0 2.5 5.0 t/t0. 5 Figure 13. Reduced-time plot obtained when using fine Caucasian hair in conjunction with ATG solution (pH 9, 0.42 M), demonstrating the presence of a strong sigmoidal behavior. Data are again compared to the contracting area model. Table II Difference in Reduction Times Between Japanese Hair and Fine Caucasian Hair for Some Common Reducing Agents (pH 9, 0.42 M, 25øC) Hair type ATG GMT Cysteamine Japanese hair 3.25 min 29.5 min 5.4 min Fine Caucasian hair 8.25 min 34.7 min 16.8 min 9, 0.42 M) are shown in Table II. It can be seen in each case that reaction with fine hair gives rise to considerably longer reaction times. Therefore, it may be concluded that one underlying reason for the poor reactivity of "difficult to perm" hair is probably the extra time that is required to break a suitable number of disulfide bonds in order to give a good curl strength. It is observed that changing the properties of the reducing solution can also change the kinetic behavior by which the reaction proceeds. Wickett (1) has described how reduc- tion with DTT (dithiothreitol) at pH 7 and 8 gives rise to a diffusion-controlled process, while at pH 10 and above the reduction is reaction-controlled. Figure 14 shows reduced- time data from the treatment of Japanese hair with 0.42 M cysteamine as a function of pH. It can be seen that from pH 6-9 we observe the same kinetic behavior as shown in Figure 9 however, at higher values we observe a change in the pathway to the type of behavior seen when using ATG as a reducing agent (Figure 7). Therefore, from the microscopy data we may postulate that here we are observing a change from reaction- controlled to diffusion-controlled conditions. Table III shows the average half times as a function of pH.

KINETICS OF HAIR REDUCTION 295 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.5 0.2 0.1 0.0 0.0 TYPE 1 • -- TYPE 2 •m- • - •/• - _ ø• 0 • _ t• o pH6 - a• © pH7 _ 2i•' v pH8 d• v pH9 _ r• u pill0 _ 0.5 1.0 1.5 2.0 2.5 3.0 5.5 4,0 t/to. 5 Figure 14. Reduced-time plots for Japanese hair treated with 0.42 M cysteamine as a function of pH. Table III Half Times for Reduction of Japanese Hair With 0.42 M cysteamine as a Function of pH pH of Reducing solution Half time (min) 6 63 7 37.5 8 29.5 9 7.6 10 4.9 Table IV Effect of Cysteamine Concentration on the Rate and Mechanism of Reaction When Treating Japanese Hair With Cysteamine at pH 9 Concentration Half time (to. 5) Kinetic behavior 0.3 M 10.5 min Reaction-controlled 0.42 M 5.4 min Reaction-controlled 0.6 M 5.5 min Diffusion-controlled Table IV shows results obtained from our laboratory, indicating the effect of varying cysteamine concentration when treating Japanese hair at pH 9. It is observed that at 0.3 M the reduction process is reaction-controlled, and as such an increase in concen- tration to 0.42 M results in shorter half times. However, further increasing the con- centration of the reducing solution does not result in any change in the half time, indicating that the process is no longer reaction-controlled. Performing the reduced- time analysis on the experimental data also confirms the presence of a change in kinetic behavior from the one shown previously for reaction with cysteamine to one that more resembles the diffusion-controlled behavior that was seen with ATG.