Study on the diffusion behavior of thioglycolic acid and L-cysteine in human hair with a microspectrophotometer

In order to investigate the penetration of thioglycolic acid (TG) and L-cysteine (CYS), into human hair, cross-sectional samples of white human hair treated with TG and CYS were prepared. In particular, a method for analyzing the diffusion behavior of TG and CYS in human hair was developed. The TG parts and CYS parts in the cross-sectional samples were dyed with methylene blue and the cross sections were examined at a wavelength of 664 run (lambda(max) of methylene blue) with a microspectrophotometer. In our method, the penetration of TG in human hair clearly increased by increasing the treatment time and by raising pH. On the other hand, the penetration of CYS in human hair almost unchanged by increasing the treatment time, and by raising pH. Also, the diffusion pattern of TG showed Fickian type characteristics. While, the diffusion pattern of CYS was Non-Fickian. By calculating the diffusion coefficient from the TG concentration profile, the apparent diffusion coefficient of TG at pH 9.0 was found to be the order of 10(-9) cm(2)/s.