SORPTION-DESORPTION OF IONS BY DYNAMIC ELECTROKINETIC AND PERMEABILITY ANALYSIS OF FIBER PLUGS

Sorption/desorption of ions on human hair was investigated by using a new instrument which can perform simultaneous measurements of the stre aming potential, conductivity, and permeability of the fiber plugs. Th e apparatus allows, first, the surface of a newly formed plug of fiber s to be characterized and then the dynamics of change in its ionic cha racter after treatment with solutions of surfactants and/or polymers, and during rinsing with the test solution, to be followed. The sensiti vity of this technique is demonstrated by the analysis of anionic and cationic surfactants as well as cationic polymers. Anionic surfactants such as sodium, ammonium, diethanolammonium, and triethanolammonium l auryl sulfates were shown to bind to hair transiently by surface adsor ption and, possibly, penetration into the bulk of the fiber. Their aff inity to hair was found to be related to the nature of the counterion as judged from the time dependence in electrokinetic parameters. Catio nic surfactants were found to bind to the fiber surface, resulting in reversal of the sign of the streaming potential and reduction of the p lug conductivity. The former effect was also temporary, and the initia lly high and positive zeta potential was shown to gradually decrease a s a result of the removal of the cationic species from the fiber surfa ce during rinsing with the test solution. The treatment of the hair wi th cationic polymers produced more durable surface modifications, with the extent of change in the electrostatic character of the surface be ing related to the charge density of the adsorbed polymer.