Isoelectric point of hair

These facts play an important factor in the dyeing behavior. The isoelectric point of hair is at a pH of ca. 5, whereas untreated collagen has its isoeletric point at a pH of ca. 6, which varies with the kind of tannage. For pure chrome suede it is close to pH 7 or about pH 6 after slightly anionic retanning. Consequently, for good dye fixation the acidification at the end of a dyeing is around pH 3 for fur hair and ca. pH 4 for chrome-tanned suede. 

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. 

Using shennpoos containing anionic surfactants leaves the hair difficult to comb while wet. It also results in a static charge build-up or fly away when the heiir is combed dry. As will be discussed later, the isoelectric point of hair is approximately 3.67 and... 

In dyeing the hair and the suede portion a number of factors must be allowed for. The keratin of the hair contains basically the same amino acids as the collagen of the skin but in a different ratio. The keratin of the hair includes cysteine, which cross-links the polypeptide chain and imparts stability. The collagen of the skin does not have these substances, and the cross-links are made by the tanning agent. On the other hand, only L-hydroxyproline can be found in the collagen. As a result the thermal stability is different, and in addition the isoelectric points of the two polypeptides diverge. 

Mild nonionic shampoos free of ionics, allow washing of hair without destroying or adversely affecting the disulfide bonds of file keratin without changing file isoelectric point of file 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. 

Wilkerson [118] found the isoelectric point of a single hair sample to be pH 3.67 by measuring the electrophoretic mobility of hair particles in buffer... 

Similar isoelectric points for hair and wool fiber are to be expected because chemical compositions of the cuticle are similar and because both fibers show similar dye-staining characteristics. Cuticle from both fibers stains more readily with cationic dyes than with anionic dyes [96], whereas the cortex stains readily to anionic dyes [121]. 

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. 

Untreated hair comprises proteins that exhibit an isoelectric point near pH 3.67 [111], As a result, despite its hydrophobic surface, hair carries a negative charge at the normal pH levels of hair care products. Loss and damage of the surface lipid layer also reduce the hydrophobicity of the hair. This combination of negative charge and hydrophobicity affects the types of soils that bind to hair as well as the ease with which different soils can be removed from the fiber surface. 

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. 

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. 

Since ionic bonds are the most important primary valence bonds for binding to hair under low-temperature conditions in an aqueous or aqueous alcohol system, the net charge at the fiber surface is critical to polymer hair interactions. Wilkerson has shown that unaltered human hair has an isoelectric point near pH 3.67 [24] therefore, the surface of hair bears a net negative charge at all pH values above its isoelectric point. Since most cosmetic hair treatments are above this pH, cationics are attracted to hair more readily than anionics, and polycationics are more substantive to hair than polyanionics. 

Cationic ingredients in general are highly substantive to hair because of hair s low isoelectric point, which is approximately pH 3.67 [24] in cosmetically unaltered hair, and even lower in bleached hair. Therefore, at any pH above the isoelectric, the surface of hair bears a net negative charge, and positively charged (cationic) ingredients are attracted to it. 

The use of protein hydrolysates and quaternized derivatives in shampoo formulations has been demonstrated to reduce the loss of tensile strength caused by anionic tensides on the hair fibers (124,125). This protective effect is higher for quaternized derivatives than for parent hydrolysates. A wheat protein hydrolysate was found to be more effective than a collagen hydrolysate. Since the two tested products had similar isoelectric point and average molecular size, the difference could be explained by the higher hydrophobicity of the vegetable derivative. 

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