EQCM studies of polypyrrole films. Part 2. Exposure to aqueous sodium tosylate solutions under thermodynamically non-permselective conditions

Abstract

We describe electrochemical quartz crystal microbalance studies of polypyrrole films exposed to concentrated aqueous sodium tosylate solutions, in which the polymer is non-permselective in doped and undoped redox states at equilibrium. Then, sources and sinks of salt within the film open up new mechanistic possibilities for film redox switching that are not seen under thermodynamically permselective (low electrolyte concentration) conditions. In particular, sodium co-ion transfer (as opposed to tosylate counter-ion transfer in the opposite direction) is an expedient means to satisfy electroneutrality on short time scales for redox switching in either direction. Film mass responses under potentiodynamic (cyclic voltammetric) conditions depend markedly on experimental time scale (potential scan rates); these are generally non-monotonic as a result of, first, the competing mechanisms for maintaining electroneutrality and, second, slower neutral species transfers. Furthermore, shortening of the time scale can lead to a progressive freezing out of the solvent and salt transfer processes. The complex pattern of redox driven ion, solvent and salt transfer (involving both time scale and history effects) is visualized through an extension of the scheme-of-cubes model.