HENNA AND LAWSONE SORPTION BY WOOL 365 1ø/o 1Cm PH 5.7 80 40 350 450 (rim) Figure 2. Electronic spectra of solutions of lawsone at various pH values. suggested that because the process involves diffusion, the quantity of dye extracted should be proportional to the square root of extraction time. Plots of absorbance against the square root of extraction time were rectilinear, but slopes obtained with different experimental conditions could not be compared because the amount of dye released increased with the amount initially absorbed. Comparisons were therefore made by plotting the observed absorbance (A t) divided by the absorbance at 140 minutes (Aoo) against root time. A typical plot, showing the effect of temperature on the substantivity of lawsone is shown in Figure 3. The points for all temperatures fall on one straight line, indicating that substantivity is not changed by changing temperature. A similar plot was obtained with henna, and with the effect of dyeing time and of pH on the substantivities of henna and lawsone. None of the graphs passed through the origin this is said to be due to a surface barrier on the hair, the epicuticle (1,7), but experi- mental conditions, such as incomplete wetting of the sample during the first extraction, must have made a significant contribution. Forestier (1) used Eq. 1 to calculate the diffusion coefficient (D) of lawsone in human hair. r represents the radius of the fiber and t is time. Linear regression of the 25 re- sults in - _ (1) Aoo r

366 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS At/A :o 1.0 0.6 0.2 2 6 •0 ,/Time (min) Figure 3. Effect of dyeing temperature on the substantivity of lawsone. x, 15 ø (0.429) i,, 30 ø (0.649) O, 40 ø (0.752) O, 50øC (0.763). A•o values are shown in parenthesis. Figure 3 gave a slope of 0.116 min- •/2. Substitution in Eq. 1, assuming a fiber diameter of 20 p•m, yields a diffusion coefficient of 9 x 10-•1 cm 2 sec-1, in good agreement with those observed by Forestier. The situation is not as simple as suggested by Figure 3. Felts were dried after extraction and examined with the tintometer and the colorimeter. Three general changes were