Investigation of pH Effect on Cationic Solute Binding to Keratin and Partition to Hair

The study investigated the effect of pH on the binding of three cationic solutes (theophylline, nortriptyline, and amitriptyline) to hair keratin. Using fluorescence spectroscopy and equilibrium absorption experiments, it was found that increasing the pH from 2.0 to 6.0 changed the net charge of hair keratin from positive to negative, enhancing the binding constants of the solutes due to a shift from electrostatic repulsion to attraction. The positive ΔH and ΔS values indicated that hydrophobic interactions were significant in the binding process. The study concluded that solute binding to hair keratin is primarily driven by hydrophobic interactions, followed by electrostatic interactions, and that this binding correlates well with the partition of solutes to hair, providing a basis for developing mathematical models of hair partition and penetration properties.