168 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 2 4 6 8 10 12 14 16 TIME OF 8LEACHING (HR) Figure 3. Kinetics of rinse-out of anionic dyes affinity between dye molecules and substrate. A binding site is referred to as any location inside the hair to which a dye molecule would attach. Since the internal structure of unbleached hair is highly crosslinked, it can be expected that the movement and penetration of large molecules inside the fiber are limited. But when the hair structure becomes a bit more open as a result of being bleached, this large molecule would be able to reach into some otherwise hindered positions. In other words, there is an increase in available binding sites. Since the hair samples were dyed to equilibrium, the rate at which the dye molecule gets into

DYE RINSE FROM HAIR 169 8O 2 4 6 8 10 12 14 16 TIME OF BLEACHINO (HR) Figure 4. Kinetics .of rinse-out of cationic dyes the sites is of no consequence. However, once they fall into these sites, they stay there as if they were trapped, either due to some steric con- straints surrounding them, or due to an increase in chemical affin,it¾. The increase in affinity can be a result of larger contact area between dye molecules and substrate, or, to a lesser degree, a modification of the chemical nature of the substrate, thus contributing a stronger interac- tive force between them. As long as the increase in site availability and affinity is large compared to the increase in molecular diffusion, the overall rate of dye rinse from bleached hair would slow down with the amount of bleaching. This behavior would continue until the molecular size is comparable to the internal "holes" of the bleached hair fiber so that site availability is no longer a factor. Because of the very open structure within the fiber and possibly the increasing damage to the cuticle layer, the diffusion rate would predominate therefore, when bleaching reaches a critical stage, the rate of dye rinse starts to increase.