306 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.5 5a Calculation using Scheme I 0.4- 0.3- 0.0 I I 0.30 0.25 0.20 0.15 0.10 0.05 0.00 5b, Calculations using Scheme II I 20 I I I 0 10 20 30 40 50 60 70 80 90 100 0 40 60 80 100 Time/min. Time/min. Figure 5. Comparison of calculated indo-aniline concentrations using Scheme I (Figure 5a) and Scheme II (Figure 5b), with the corresponding experimental data obtained using three set of concentrations of hydrogen peroxide. [PPD] = 1.9 X 10 -4 M [2,6-DMP] = 2.9 X 10 -4 M pH = 9.2 Experimental data: A, [H202] = 5.3 X 10 -3 M El, [H202] = 3.5 X 10 -2 M C), [H202] = 7.0 x 10 -2 M. Calculated values/Scheme I: A, ... - [q, ------' ¸, .. .. Calculated values/Scheme II: A, ........ I-1, '0 ..... During the PPD oxidation process, the reaction between PBDI and PPD is known to produce Bandrowski's base. The kinetics and mechanism of Bandrowski's base formation have been extensively studied by J. F. Corbett (5-7). However, including this reaction in our scheme (Scheme I) does not show any significant effect in reducing the difference between the experimental and calculation values shown in Figure 5a. In other words, self-reaction of PPD is insignificant under the experimental conditions used. Modification of Scheme I is made by assuming that a reactive intermediate instead of PBDI is formed in the rate determining-step (Scheme II). While this reactive interme- diate is reacting with hydrogen peroxide to form products other than the indo-aniline (reaction 2'), it is also rapidly converted to PBDI (reaction 2"). Once PBDI is formed, the dominant path will be the coupling reaction between PBDI and 2,6-DMP. The intermediate formed in Scheme II is likely to be a cationic radical formed by the loss of one electron from PPD. Although little is known about this cationic radical, there are reports that cationic radicals are formed from the oxidation of substituted PPDs in aqueous solution (8,9). Both rate constants of reaction 2', k2' , and of reaction 2", k2", are unknown. However, the value of k2" can be estimated from experimental data of indo-aniline formation obtained using low [H202] where reaction 2' is assumed to be negligible. Therefore, only the rate constant for reaction 2' is unknown and is used as an adjustable parameter in fitting experimental data. It is assumed that reaction 2' follows the same kinetic law as that of the reaction between PBDI and hydrogen peroxide. Simulations of indo-aniline formation with k 2' as fitting parameter are shown in Fig- ure 5b (lines) and are compared with the corresponding experimental data. Agree- ment between the predicted values (lines in Figure 5b) and our experimental observa- tions (symbols in Figure 5b), over tenfold variation of [8202] is considered excel-

INDO-ANILINE DYE FORMATION 307 .Scheme II H202 RDS kl Intermediate (1) H202 ) Undesired Products (2') -I- • /C• I•1 • Oxidant ) 1(3 k:• H•N v v 'O Fast Intermediate-- (2" (3) lent since only one parameter (k2') is used to fit three sets of experimental data. The rate constant of the reaction between the intermediate formed in Scheme II and hydrogen peroxide (k 2' in Scheme II) is •300 times higher than the rate constant of the reaction between PBDI and hydrogen peroxide (k 2 in Scheme I). Therefore, reaction 2' is mainly responsible for the loss of reacted PPD to products other than the indo-aniline and is responsible for the observed decreases in indo-aniline formation at higher concentrations of hydrogen peroxide (shown in Figure 2). From Scheme II, it is clear that the conversion efficiency of PPD to the indo-aniline is essentially governed by the ratio of k2"/(k 2' [H202] ). At low [H202] , the rate of reaction 2' is less significant than that of reaction 2". Most of the reacted PPD are predominantly converted to the indo-aniline. As [H202] is increased, increasingly the reacted PPD is consumed through reaction 2', and the efficiency of PPD to the indo- aniline is decreased. In conclusion, the formation of a more reactive intermediate other than p-benzoquinone diimine is suggested in our kinetic modeling of the indo-aniline formation process in aqueous peroxide solution. The reduced conversion efficiency of PPD to indo-aniline at higher [H202] can be explained by an increasing consumption of the reactive interme- diate by hydrogen peroxide. ACKNOWLEDGMENTS We would like to thank Dr. K. Brown and Dr. J. F. Corbett for useful suggestions and