Професионални серии за коса- тема 153

The pH, or hydrogenionic potential indicates the acidity, alkalinity or neutrality of a given medium. The pH scale assumes the neutrality at 7.0, and varies from 1 to 14.A substance is considered acid above 7.0, and alkaline below 7.0. At the 7.0 pH, there is an equilibrium between H and OH at the center. The isoelectric point is defined as the moment of charge neutrality in a determined pH. The hair is extremely sensitive to the pH variation of the products applied in its surface. The pH of which a protein or particle has an equivalent number of total positive and negative charges is called the isoionic point. The pH at which a protein or particle does not migrate in an electric field is the isoeletric point.
Its isoelectric point is around a 3.67 pH. The isoionic point is around 5.6.The hair reaches neutrality of charges when the pH is around the isoeletric point. In lightened hair, the isoelectric point is reached at an even more acid pH. It occurs because the A-layer and the epicuticle are rich in cysteine and therefore there is the formation of cysteic acid. Free lipid which contains fatty acids is an important and essencial component of the surface of animal hairs. The freer lipid that is present in this surface layers the lower the isoeletric point of the keratin fibers. Free lipid is important to the adsorption of surfactants and other ingredients onto human hair. The longer the interval between shampoos, the more free lipid and the lower isoeletric point of the hair.
Any product applied on hair that has pH higher than 3.67 causes an increase in the negativity of the electric network of the hair, that is, an increase of static electricity and the repulsion between strands. The fiber surface bears a net negative charge because of its low isoeletric point. Positively charged ions are attracted to the negatively charged surface, thus helping to overcome the electrical barrier for anions.
When the hair is rinsed, the water pH is 7.0. Therefore an increase in negativity occurs and the micelle is repelled (washed out) by the negatively charged strands. The negative electrical net charge generated will tangle the hair and make it hard to comb, thus causing the frizz effect.Besides, in alkaline pH, hair has increased capacity to absorb water. Water penetrates the scales that open, hydrating the strand and breaking the hydrogen bonds of the keratin molecule.Keratin is a spiral, helical molecule that keeps itself in that shape due to chemical bonds between its atoms: Hydrogen, disulfide and ionic bonds and Van der Waals forces.Water causes hydrolysis, that is, temporarily breaks the hydrogen bonds and makes the molecule malleable and consequently, fragile to the hair strand due to lowering in elasticity and increase in plasticity, which means that the wet hair, if deformed, does not regain its original shape. When that hair is wet, the cuticle scales lift, leading to increased cuticle removal, cuticle fragmentation, and cracks to the fiber axis.