REACTION OF PPD AND BENZENE DERIVATIVES 431 anisole sulfate, employed in current hair dye formulations, were chosen as model cou- plers. Seven pH values in the range of 7.5–10.5 were investigated. MATERIALS AND METHODS MATERIALS Technical grade p-phenylendiamine (PPD), resorcinol (RES), 2-methylresorcinol (MeRES), 3-aminophenol (AP), 2-amino-4-(2-hydroxyethyl-amino)anisole sulphate (AHA), and perhydrol were kindly provided as a gift by Tocco Magico (Italy) and utilized without further purifi cation. Perhydrol, stabilized with phosphoric acid, was opportunely diluted to obtain hydrogen peroxide at 20 volumes. TLC was performed on preparative 20 × 20-cm, 1-mm, Silica gel 60 F254 plates from Merck (Germany). METHODS The commercial virgin wool tissue, cut in 5 × 20-cm strips, was washed with a non-ionic detergent aqueous solution, rinsed with water, and dried before the coloration. Each strip was dipped in 100 ml of a solution formed by 50 ml of hydrogen peroxide (20 volumes, 6%) and 50 ml of a solution containing the p-phenylendiamine-coupler (1:1 ratio) and the ammonium hydroxide needed to obtain the fi nal required pH value. After 30 minutes the strip was removed, carefully rinsed with tap water, and dried at 45°C. In each experiment the reagents’ concentration (precursor and coupler) was 2 × 10−2 M. Experiments were carried out at pH 7.5, 8.0, 8.5, 9.0, 9.5, 10.0, and 10.5. pH values were monitored at room temperature with a Metrohm pH-meter 744. The colorimetric measurements were performed with a colorimeter X-Rite SP62 model, with an integration sphere to determine the color refl ectance. The L, a*, and b* values reported are related to the illuminant D65 at 10 degrees. Each reported value is the mean of ten measurements, performed randomly on the surface of the colored strips, with standard deviation never exceeding 1.2%. In these conditions the colorimetric data of virgin wool were: L = 95.12, a* = 0.05, and b* = 6.73. TLC experiments were carried out by spotting about 0.5 ml of the aqueous solution of the performed reaction on a strip of 5 cm and eluting with CHCl3:EtOAc:EtOH 7:2:1. For the weathering experiments, two wool strips were colored with the color mixture obtained with each couple of reagents at the stated pH. One of these was directly exposed to atmo- s pheric agents the other one was weathered, avoiding a direct exposure to sunlight in order to distinguish the color degradation induced in the presence or in the absence of direct sunlight. The weathering experiments were carried out for 24 days. At days 0, 4, 7, 11, 14, 18, and 24 the color of each strip was measured. All the experiments were performed in duplicate. RESULTS AND DISCUSSION Preliminary experiments showed that for each pair of precursor-coupler, the color refl ectance produced on the wool strips was constant (R∞) when the reagents’ concentration was ≥1 × 10−2 M. As a consequence, a 2 × 10−2 M concentration of the two reagents was employed in all the reported experiments. The values of the colorimetric parameters (L, a*, b*), from each pair of p-phenylendiamine-coupler at different pH values, are reported in Table I.

JOURNAL OF COSMETIC SCIENCE 432 The colorimetric data for each coupler, even if in different way, show that by increasing the pH of the medium, in the pH range of 8.5–9.0 a considerable color variation takes place. In order to achieve an estimate of the presumably different distributions of colored pigment formed by these reactions above and below this pH range, the reaction solutions of PPD both alone and with each coupler at pH 7.5 and 9.5 were analyzed by preparative thin-layer chromatography. In the absence of a coupler and at pH 7.5, the best conditions for the Bandrowski’s base formation (2), the TLC from PPD alone shows the presence of several colored bands. The main band contains an intense dark brown component (Rf = 0) that, according to the literature, is a high-molecular-weight material, and an intense dark blue band (Rf = 0.15) that the literature data assign to Bandrowski’s base (10). The L, a*, and b* values of the color formed on wool by PPD under the same conditions are 15.98, 1.24, and −4.04, respectively, and may be considered in agreement with the TLC data. The reaction between resorcinol (RES) and PPD, according to Corbett (6), gives a very unstable magenta hydroxyindoaniline 3 that may lead to a brown polymer (polymeric hydroxyindoaniline) and to a dark green molecule 5 that, furthermore, may be trans- formed to low-molecular-weight brownish intermediates (8). From the data reported in Table I, in the pH range of 7.5–8.5, the pigment formed on the wool shows a small L value, with values of a* and b* very near to zero (very dark brown color). For pH values 9.0 a strong increase of b* takes place, while the value of a* even if positive, is always small. The pigment formed in the range of pH 9.0–10.5 is a brown that moves to light brown. TLC of the system PPD-RES at pH 7.5 shows many other minor products having Rf 0.5, a strong dark brown component with Rf = 0, a trace of a narrow dark blue band (Bandrowski’s base), and a large moderate brown/light greenish band with Rf = 0.2–0.3. At pH 9.5 the brown/light greenish band is more intense, while the component with Rf = 0 has almost disappeared. The colorimetric data appear consistent with the Table I L, a*, b* of the Colors Due to the Reaction of p-Phenylendiamine with Different Couplers as a Function of pH Colorimetric Parameters pH 7.5 8 8.5 9 9.5 10 10.5 RES L 17.31 16.08 17.04 28.61 36.78 41.4 45.13 a* 1.24 1.03 1.43 3.64 3.49 2.52 2.12 b* 2.72 1.96 2.99 11.91 15.44 18.04 17.56 MeRES L 17.51 18.18 20.51 30.28 43.15 49.01 48.41 a* 5.21 5.5 6.8 9.24 8.98 8.8 9.1 b* 5.39 5.97 8.02 12.7 13.36 16.85 18.49 AP L 17.22 20.47 20.72 21.74 28.05 28.08 27.05 a* 1.24 1.22 1.32 2.36 5.39 7.77 8.48 b* 3.74 5.3 5.39 5.77 4.44 1.67 0.78 AHA L 24.77 23.09 19.63 18.39 13.36 13.8 15.86 a* 5.43 5.44 5.39 6.61 4.46 3.95 3.82 b* −11.65 −11.58 −11.36 −14.38 −9.13 −9.98 −11