Electrochemical Study of the Binding Interaction Between Dihydroxybenzenesulfonates and Viologen-Linked Polypyrrole

Abstract

This article discusses the enhanced oxidation of dihydroxybenzenesulfonates at poly[N,N′-bis(3-pyrrol-1-ylpropyl)-4,4′-bipyridinium]chloride modified electrodes in aqueous electrolytes. Voltammetric measurements of the anion concentration in the film demonstrated that the electroactive anions were highly concentrated in the polycationic film from their aqueous solutions. 4,5-Dihydroxy-1,3-benzenedisulfonate gave a distribution constant of about 2.8×104. The distribution constant of electroactive organic species decreased in the order of dianion > monoanion > noncharged molecule. Moreover, the reduced positive charge of the polymer weakened its binding to the organic anions. Furthermore, the formal potential of the polymer shifted to more negative values with increased loading. Thus, the electroactive species are incorporated into the cationic polymer primarily by electrostatic attraction, although hydrophobic interactions contribute somewhat to the binding, particularly that of less highly charged species. When the concentration of 2,5-dihydroxybenzenesulfonate in the film increased from 0.3 to 1.2 M, the apparent diffusion coefficient for charge transport decreased to 1/3–1/4. This dependence indicates a primary role of the physical diffusion of the electroactive anion in charge transport through the film. A decrease in the anion mobility with increased loading arises probably from the ionic association between the incorporated redox anion and polymer bipyridinium group, which is due to a reduction in film permittivity induced by cation exchange.