A Study of the Causes of Variation in the Acid Dye-Combining Capacity of Human Hair

Abstract

The capacity for human hair to combine with acid dye is rapidly and precisely determined by a modification of Maclaren's method for measuring the equilibrium uptake of Orange II from formic acid solution. The uptake determined in this manner provides a reliable estimate of the number of "free" basic sites in keratin fibers, which is shown to be a good index of chemical alteration produced by current bleaching products. A collection of hair samples with a known history of prior chemical treatment, from 30 female Caucasians, was examined by this method. Although significant differences in equilibrium dye pickup were found among 20 samples that had not been permanent-waved or bleached, the range was from only 0.67 to 0.77 mmoles/g. Permanent-waved samples were also in this range. Bleached hair, however, showed a marked reduction in Orange II pickup, the range being 0.30 to 0.63 mmoles/g. These data, together with amino acid analysis data and results from differential infrared spectroscopy, indicate that a primary reaction during the bleaching of hair with current bleach products is the conversion of cystine residues to cysteic acid residues. Distal ends of undamaged hair (unbleached, unwaved, undyed) showed a significantly lower uptake of Orange II than proximal ends, which is presumably a result of weathering.