Isoionic point of hair

The condition of the hair also affects the uptake and the diffusion of creme rinse and shampoo ingredients. A rule of thumb is that diffusion is faster into altered or damaged hair than into unaltered hair. Bleaching also lowers both the isoelectric and the isoionic points of hair, thereby attracting more cationic surfactant to the hair. Thus, the use of bridging agents is even more important to the adsorption to bleached hair than to chemically unaltered hair. 

For longer-term interactions, if the pH of the surrounding solution is below the isoionic point of hair, the hair will pick up acid, and if it is above its isoionic point, it will attract hydroxide ions more readily. For short-term and surface interactions, the isoelectric point is more important than the isoionic point. The isoionic point becomes more important to whole-fiber treatments such as perms and bleaches. 

The minimum in swelling is consistent with the observation of Steinhardt and Harris [106]. In the absence of added electrolyte, there is no combination of wool hber with mineral acid or alkah from pH 5 to 10. This is in the vicinity of the isoionic point of hair. The large increase in swelhng above pH 10 is largely due to ionization of diacidic amino acid residues in the hair and partly due to keratin hydrolysis. The increase in swehing from pH 3 to 1 is due to the combination of acid with the dibasic amino acids. Brener and Prichard [57] attribute the decrease in swelhng below pH 1 to an irreversible structural change. 

Hydrolysis of peptide and amide linkages is also a possible complication in an alkaline medium. Hydrolysis of the amide groups of the residues of aspartic and glutamic acids will increase the ratio of acidic to basic groups in the fibers, conceivably altering the isoelectric and/or isoionic points of the hair. 

TTie isoionic point of human hair may be evaluated from titration data in the presence of salt (see Figure 5-12) or buffers. Allowing thoroughly rinsed hair to equilibrate in deionized water and determining the pH of the resultant solution may also approximate it. 

The isoionic point of human hair is close to that of wool fiber (generally near pH 6.0), and it varies among hair of different individuals. Freytag [117] has found isoionic points from pH 5.6 to 6.2 by following the pH changes of hair in buffer solutions, and an isoionic point of pH 5.8 1.0 was found for unaltered hair from nine different individuals, in a study by Robbins. 

Because bleaching increases the ratio of acidic to basic amino acids [122], the isoionic point should decrease with increasing oxidation. One might also anticipate a similar decrease in the isoelectric point of hair with bleaching because the A-layer of the cuticle cells is rich in cystine. 

The pH at which a protein or particle has an equivalent number of total positive and negative charges as determined by proton exchange is the isoionic point. The pH at which a protein or a particle does not migrate in an electric field is called the isoelectric point. The isoionic point is a whole fiber property of hair and is reflected in the equilibrium acid-base properties of the total fiber the isoelectric point is related to the acid-base properties of the fiber surface. 

The effect of increasing chloride ion concentration in hydrochloric acid solution is to produce a greater uptake of acid by the fibers at any given pH below the isoionic point. Steinhardt and Harris [104] have demonstrated this effect for wool fiber, and Robbins has demonstrated it for human hair (see Table 5-19 and Figure 5-12). 

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