The adsorption behavior of cationic surfactant onto human hair fibers

Quaternary ammonium surfactants are important ingredients that are frequently formulated into hair care products to modify the properties of hair surface. The adsorption kinetics, isotherms and association structures of cationic surfactants on hair surface, however, are not fully understood due to the heterogeneous nature of human hair fibers. In this work, a quaternary ammonium of surfactant, dimethylpabamidopropyl laurdimonium tosylate (DDABDT) was chosen as a probe to investigate the adsorption behavior of cationic surfactant on cuticles of scalp hair. The results reveal that the adsorption kinetics fit to a pseudo-second-order kinetic model and the adsorption isotherms fit to the Freundlich adsorption model. With the increase of DDABDT adsorption, the wettability of hair fibers changes from hydrophobic to hydrophilic. The association structure could be monolayer or bilayer depending on the initial concentration of the surfactant. In the monolayer structure, the ‘anchor’ surfactant molecules are believed to adsorb vertically on the surface of hair fibers through electrostatic interaction. In the bilayer structure, the second layer molecules may then pile up on top of the first layer with charged groups orienting outward. The thickness of DDABDT film on hair fibers treated with 5 × 10−4 mol/l DDABDT solution is measured to be 5.42 nm on average with a force–distance method. This figure is very close to the two times of the theoretical molecular size of the DDABDT molecule.