Cooperative Nature of Complex Formation in Mixed Polyelectrolyte−Surfactant Systems

The binding of the cationic surfactant dodecylpyridinum chloride (DoPyCl) to different preformed anionic charged polyelectrolyte complexes consisting of cationic copolymers with varying charge density and poly(acrylic acid, sodium salt) (PAA) has been investigated by using a potentiometric technique based on a surfactant ion solid-state electrode. The solid membrane used in the electrode consists of poly(vinyl chloride) plasticized by bis(2-ethylhexyl) phthalate. Binding isotherms are analyzed in terms of the nearest neighbor interaction model by Zimm and Bragg and for systems of reduced polyanion availability of the Scatchard plot, which indicates cooperative interactions at very low surfactant concentrations (∼0.01 mM). The binding constants decrease by decreasing the charge density of the cationic copolymer used. For selected systems the titration data have been complemented with static light scattering measurements.