BINDING OF CETYLPYRIDINIUM CATION BY POLY(ACRYLIC ACID). EFFECT OF POLYMER CHARGE DENSITY1
Ksenija Kogej* and Jože Škerjanc
Faculty of Chemistry and Chemical Technology, University of Ljubljana, 1000 Ljubljana, Slovenia
he effect of polymer charge density on the binding of cetypyridinium cation (CP+) to poly(acrylic acid) (HPA) has been investigated by using a potentiometric technique based on surfactant cation-selective membrane electrode. The amount of binding, b, has been determined at constant polyelectrolyte concentration (mp = 5 ´ 10-4 monomol/kg) and at five degrees of ionization of the acid (a = 0.0, 0.27, 0.53, 0.76, and 1.0) in aqueous solutions without added simple electrolyte at 25 °C. It has been observed that appreciable binding of CP+ cation to PA- anion starts at the total detergent concentration about 1 ´ 10-5 mol/kg which is well below the critical micellization concentration, cmc, of this surfactant. The free surfactant concentration at this point is still lower, about 3 ´ 10-7 mol/kg. A surprising finding is that the onset of binding is independent on the degree of ionization, a, of the acid. On the contrary, the plateau region which is reached for b values below 1 depends considerably on a. In this range, b values increase with increasing degree of ionization. Above the plateau, at free surfactant concentrations higher than 5 ´ 10-4 mol/kg, b again increases sharply and even reaches values higher than 1. This has been attributed to the formation of the second layer of bound surfactant.