Characterization of damaged hair and repair agent adsorption behavior by zeta potential analysis

The zeta potential of virgin and damaged hair determined by the streaming potential method was employed to characterize the hair surface and to track the adsorption behavior of a cationic polymer used as a repair component.

Today, hair washing has become a more functional action to protect hair and to add smoothness, in addition to the purpose of merely washing to remove sebum and dust adhering to the scalp. Therefore, the formulation of hair care products is complex and includes not only surfactants, which are cleansing ingredients, but also compounds that moisturize hair and support damage repair. Furthermore, the processes of bleaching and dyeing apply oxidative stress and cause additional damage to the hair fiber.

Upon exposure to water in the washing cycle, the hair-water interface gets charged according to the hair fibers’ surface functional groups. Hair is a complex keratin fiber composed of various fibrous proteins. The surface of hair is negatively charged in water at neutral pH due to the presence of acidic amino acids, which are some of the constituent amino acids, and sulfone groups formed by the oxidation of cysteine-derived S-S bonds [1].

It is therefore important to determine the zeta potential, which is closely related to the surface condition of hair in water, in order to evaluate the surface characteristics of hair and to understand its interaction with ingredients of hair care formulations.

There are different methods for determining the zeta potential. The measurement of the electrophoretic mobility by the electrophoretic light scattering method (ELS) is widely used for the zeta potential analysis of a dispersion system. However, with ELS it is not possible to directly measure a flat plate or fibrous sample that does not disperse.

In such a case, the zeta potential is determined by the streaming potential method where a streaming potential or a streaming current is generated by passing an electrolyte solution along the surface of a solid material.

The charge characteristics of the hair surface has already been evaluated by the streaming potential method with the instrument SurPASS, and the adsorption of hair care agents on virgin hair has been investigated [2]. In this report, we employed the successor instrument SurPASS 3, which offers a higher sample throughput and an easier operation, for the zeta potential analysis of damaged hair and evaluated the adsorption behavior of hair care agents.

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