EQCM studies of polypyrrole films. 1. Exposure to aqueous sodium tosylate solutions under thermodynamically permselective conditions

We describe electrochemical quartz crystal microbalance (EQCM) studies of polypyrrole films exposed to aqueous sodium tosylate solutions. The films are thermodynamically permselective in both doped and undoped redox states only for electrolyte concentrations at or below 0.1 mol dm^-3. Under kinetically controlled conditions, permselectivity failure occurs transiently (even in dilute electrolyte) for the undoped form. These phenomena result in complex, non-monotonic film mass changes during redox cycling. This is a consequence of competing counter and co-ion transfer mechanisms for instantaneously maintaining electroneutrality, along with solvent transfer that attempts to maintain a constant film volume. The observed responses are a function of time scale; initial transfer of the faster moving cation ultimately gives way to the thermodynamically favored transfer of the slower moving anion. The interconversion of the variously solvent and salt populated forms of the two film redox states can be visualized by an extension of the scheme-of-cubes model, represented as a prism. This new prism visualization helps rationalize the participation of the competing elementary steps and gives insight into film history and experimental time scale effects.