Changing the Shape of Hair 170 dithiothreitol, the reduction reaction appears to proceed by a different mechanism.29-31 A kinetic model that fits reduction under these conditions is the moving boundary model. In this model, diffusion into the hair is initially slow compared to the rate of the reaction. However, as soon as some reaction takes place the diffusion constant increases dramatically due to swelling of the hair as disulfide bonds break. This leads to a moving boundary or “sharp front” of reduction proceeding through the hair. The moving boundary concept is supported by the micrographs in Figure 11 where the hair is stained with silver methenemine to show where reduction has occurred. Treatments were for 5 min at pH 9.3 with either 0.5M ammonium thioglycolate (B) or 0.15M dihydrolipoic acid (C) (A) is untreated. In Figure 11B the reduction reaction has occurred more or less uniformly throughout the hair. The moving boundary or “sharp front” of reducing agent can be clearly seen 11C. Inside of the boundary, the concentration of reducing agent is assumed to be zero, and outside the boundary the concentration is C 0 and reduction is assumed to be occurring by a pseudo first order mechanism. With these assumptions chemical stress relaxation under moving boundary conditions can be fit to the following equation:29 Eq. 7 Figure 10 Fit of SFTK data to pseudo first order kinetic model at pH 9.0 1. 0.16 M NaTG, k = 4.1 x 10-4 M-1*sec-1 2. 0.32 M NaTG, k = 4.3 x 10-4 M-1*sec-1 Modified from Wickett29 with permission of the Society of Cosmetic Chemists