Investigation of pH effect on cationic solute binding to keratin and partition to hair

In the process of hair treatment, various cationic actives contained in hair care products can be absorbed into hair fibre to modulate the physicochemical properties of hair such as colour, strength, style and volume. There have been very limited studies on the binding and partition properties of hair care actives to hair. This study aimed to investigate the pH effects on cationic solute absorption into hair and binding to keratin.

The keratin binding and hair partition properties of three cationic solutes (theophylline, nortriptyline and amitriptyline) have been measured at different pH using fluorescence spectroscopy and equilibrium absorption experiment. The binding constants, thermodynamic parameters and hair-water partition coefficients determined at different pH were compared and analysed.

Increasing the pH from 2.0 to 6.0 resulted in the net charge of hair keratin changed from positive to negative. As a consequence, the binding constants of the three cationic solutes with keratin increased with the increasing pH. This correlated with the variation of the electrostatic interaction between cationic solutes and keratin from repulsion to attraction. The positive ΔH and ΔS values indicated that hydrophobic interaction also played a major role in the binding of the three cationic solutes to keratin. There was a good correlation between solutes binding to keratin and hair-water partition of solutes.

It appears that solute binding to hair keratin is driven first by hydrophobic interaction and then by electrostatic interaction. The fitted thermodynamic parameters suggested that hydrophobic interaction dominates for the binding of the three cationic solutes to keratin. That binding of cationic solutes to keratin correlates with the partition of the solutes to hair could provide theoretical guidance for further developing mathematical models of hair partition and penetration properties.