Surface charge of hair

There are many products for hair: shampoos for dry, damaged, greasy or colored hair as well as conditioners. Why and how hair care products work is related to the surface charge of the hair — a parameter that can be measured.

Hair is a complex keratin fiber, which basically consists of three layers: the medulla, the cortex, and the cuticle. To understand the effect of hair care products, the surface charge of the hair has to be taken into closer consideration. Untreated human hair has a strongly negative surface charge. Carboxyl groups of glutamine and aspartic acid and sulfonic acid groups in the hair are responsible for this property. If a hair is treated with shampoo, conditioner, dye, or bleach, the charge of the hair surface changes.

Most shampoos contain anionic surfactants, which are detergents with a negative charge. These surfactants ensure the reduction of the hair surface’s negative charge. This also reduces the water-repellent properties of the hair, so the shampoo can be better absorbed while the hair is being washed. After the shampoo is rinsed, the lower negative charge of the hair surface compared to the native hair remains. This value shift indicates that shampoo residues remain in the hair after rinsing.

Conditioners contain cationic compounds such as polypeptides or functionalized silicones. The negatively charged hair fiber absorbs cationic compounds very strongly, reversing the charge of the surface. Using a conditioner causes the surface charge to become positive in just 20 seconds. As a consequence, conditioners can usually be absorbed by the hair much faster than shampoo. After another 60 seconds, the surface charge is stable again. Once the conditioner is washed out, the charge returns to negative just as quickly as it previously became positive, but does not reach the original negative value of the native hair. This indicates only partial desorption of the conditioner.

Bleaching destroys the natural hair structure. The stronger it’s bleached, the greater the destruction of the natural hair texture. The cystine bonds are broken and the remaining sulfide groups are oxidized. Water penetrates into the more open, hydrophilic structure of bleached hair and leads to a lower negative surface charge of the hair. This knowledge is the basis for the development of shampoos and hair care products specially designed for bleached or dyed hair.

To measure the surface charge of hair, the so-called zeta potential is determined. The zeta potential of untreated hair is negative at neutral pH and thus represents the starting point for any form of hair care. A reversal of the surface charge, as when using a conditioner, begins at the so-called isoelectric point. This point indicates the pH value at which the zeta potential is 0 mV.

This takes advantage of the fact that macroscopic materials such as hair develop a charge on their surface when in contact with liquids, which can be either negative, positive or neutral. The charge behavior is an indicator of all changes to the material surface and is thus an important parameter for the characterization and development of suitable products.
The charge behavior and its changes are described using the zeta potential.

Hair samples and their interaction with various care products can be followed in real time by means of the zeta potential analysis. These findings can be incorporated directly into the development of appropriate hair care products for different hair types.

In the cosmetics industry, for example, the importance of producing suitable care products that take into account the complexity of the hair structure has increased dramatically, in part because of rapidly increasing competition. By now it is thus just as important to understand the effect of different hair treatments, as to adapt also to changes in hair structure by permanent treatments such as dyeing or bleaching and to develop suitable products.

The SurPASS™ 3 from Anton Paar can measure the surface charge of samples, and can therefore be used for the analysis of hair surfaces to create ideal hair care products.