894 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS in the fiber structure. It was thought that the contribution of this fac- tor to the total swelling characteristics of the bleached hair could be as- certained by specific oxidation of cystinc in brown hair with peracetic acid (without attacking the melanin) or by control bleaching runs on white hair. Table VIII correlates the swelling data with the extent of disulfide bond breakdown. The swelling was measured at pH 7 using the liquid retention method with the reduction-oxidation cycle. Table VIII Swelling of Oxidized Hair Liquid Retention (%) Cystinc White Hair, Brown Hair, Brown Hair, Oxidized, Oxidized with Oxidized with Oxidized with % of Original H202 H20• Peracetic Acid 0 54.5 50.2 50.2 10 63.2 63.6 64.0 13 68.8 70.5 67.0 15 78.5 97.8 77.2 At the same levels of disulfide bond oxidation the bleached brown hair appears to be more damaged than its white counterpart. This difference could be due to the breakdown of melanin, a contention supported by the fact that the damage in brown hair can be lowered when the disinte- gration of pigment is prevented (peracetic acid oxidation). It is, however, obvious that the main source of damage resides in the destruction of disulfide bonds which not only opens the structure of hair but provides additional hydrophilic centers in the form of cysteic acid residues. The introduction of these residues significantly alters the swelling characteristic of hair as illustrated in Fig. 5. A novel feature of the swelling behavior is its unusual dependence upon pH. A pre- cipitous increase in swelling of bleached hair occurs in the pH region 5-7.5, while no such change is observed in case of untreated hair. This phenomenon can be best explained by accepting the charge rearrange- ment mechanism postulated for oxidized wool by Thompson and O'Donnell (31). The strongly acidic c¾steic acid residues are, in the course of their formation, being cmnpensated for by the ionized basic groups of the keratin. As a result, the carboxylic groups of the acidic side chains are being expelled from their salt links and remain essen- tially un-ionized at pH 4 and below. This decrease in their acidity

HAIR BLEACHING 895 .• 46 Figure 5. Effect of pH on swelling of bleached hair (in the intact fiber their pK = 2) is brought about by the growth in negative charge density following the formation of the cysteic acid resi- dues. Above pH 5 the displaced carboxylic groups begin to titrate this is reflected in an increase in swelling which reaches a maximum value as complete ionization is approached. The available evidence thus strongly suggests that the ionization of the carboxylic groups leading to increased swelling contributes to the resultant fiber damage. The ionization effect can be convincingly demonstrated on the behavior of a hair tress which was exposed for a short time (10 min) to the oxidizing action of a dilute solution of peracetic acid (pH 3.5). When the treated tress was evaluated for feel and combing, it was indistinguishable from an intact tress. However, when the oxidized tress was subsequently im- mersed for a few minutes in pH 9 buffer, rinsed, and rated again, its tactile and combing properties were similar to those of bleached tresses. Apparently, sufficient etching of the keratin took place in the buffer to change the surface characteristic of the oxidized fibers.