J. Cosmet. Sci., 60, 429–436 (July/August 2009) 429 Address all correspondence to M. Scalzo. Colorimetric investigation of the reaction between p-phenylendiamine and meta-substituted derivatives of benzene on a model support M. SCALZO, M. STRATI, M. A. CASADEI, F. CERRETO, and S. CESA, Dipartimento di Chimica e Tecnologie del Farmaco, Università di Roma “La Sapienza,” Piazzale Aldo Moro 5, 00185 Rome (M.Sc., M.A.C., F.C., S.C.), and Tocco Magico, Via dei Monti Tiburtini 770, 00157 Rome (M.St.), Italy. Accepted for publication February 11, 2009. Synopsis The aim of this work was the employment of colorimetric techniques in the analysis of the color formed, on a proteic substrate, by the reaction between p-phenylendiamine and some meta-substituted benzene deriva- tives in the presence of hydrogen peroxide and in media at different pH values. In particular we investigated the chromatic variations that take place on the substrate in dependence on different reaction conditions. The obtained results show that for each couple of reagents the colorimetric data, namely the refl ectance of the formed color, change considerably with the pH of the reaction medium and demonstrate how this parameter can be considered a good descriptor of the composition of the formed pigment. INTRODUCTION The reaction of p-phenylendiamine (PPD, precursor) with meta-derivatives of benzene, having amino and/or hydroxy groups as substituents (couplers), is widely applied in oxi- dative hair dyeing. The general pathway of these reactions has been explained by Corbett (1–7) (Figure 1), who has pointed out that it is not unique, is affected by the reagents’ concentration, and is strongly sensitive to the pH of the solution. The p-quinone di-imine 1, or rather its conjugated acid 2, is the key species of the whole process of dye formation. According to Corbett, at pH 8 and using ferricyanide as oxi- dant, the oxidation of p-phenylendiamine (PPD) to the di-imine 1 is quantitative and very fast (1). Even if the reaction kinetics may vary with different couplers, the indo dye 3 is the main product because in these conditions the self coupling of p-phenylendiamine, which yields the Bandrowski’s base 4, may be considered negligible. In contrast, with hydrogen peroxide, the rate of formation of the di-imine 1 is slow and the indo dye 3 can react with the unoxidized p-phenylendiamine, leading to molecule 5, which is considered

JOURNAL OF COSMETIC SCIENCE 430 the major colored species formed through the process (6). Because the rate of formation of the Bandrowski’s base 4 is slower than that of the reaction of p-phenylendiamine with the couplers, the literature data exclude the fact that in these conditions, even if more favor- able, this compound could be formed (2–5). The permanent hair-coloring process employs hydrogen peroxide as an oxidant and in- volves the in-fi ber formation of indo dyes. According to Corbett, the hair could infl uence the relative diffusion rates of the reagents inside the fi ber (7). For this reason it has been concluded that knowledge of the kind of colored molecules produced by these reactions in aqueous solution could not be easily extended to hair dyeing (7). On the other hand, the quantization of the colored species formed in the hair appears diffi cult because of the complexity of the reactions. In fact, in some cases the species 3 and 5 may also be trans- formed to high-molecular-weight colored molecules (6,8). For these reasons, the predic- tion of the chromatic outcome performed by a mixture of precursors and couplers in the hair dyeing process is, at present, an unsolved question. Since the solution of this problem is related to the overall characteristics of the color ob- tained at the end of the dyeing process, colorimetric analysis could be a useful tool in this investigation. In fact, the refl ectance of the dyed support can be considered a useful indi- cation of the colored pigments produced in the dyeing process. In this respect, investiga- tion of the color refl ectance variations produced on a support by a couple precursor–coupler under different reaction conditions could be a new and useful approach to the problem. Our study on the color and on the color variations induced on a model substrate by the reaction of p-phenylendiamine with different couplers at different pH values in the pres- ence of hydrogen peroxide constitutes a fi rst step in this direction. Virgin wool, being of a proteic nature, was considered suitable as a model substrate for human hair. Some dif- ferences exist between hair and wool, but we consider this aspect subordinate to providing homogeneous samples without complicating pigments. In addition, wool fabric has been used by others as a model support for human hair for predicting the tanning reaction of dyes (9). Resorcinol, 2-methylresorcinol, 3-aminophenol, and 2-amino-4-(2-hydroxyethylamino) Figure 1. General pathway of reactions of p-phenylendiamine with meta-derivatives of benzene.