Preparation and properties of a pyrrole-based ion-gate membrane as studied by the EQCM

Polypyrrole ion-gate membranes were produced by electrochemical polymerization of pyrrole onto a gold-covered quartz crystal from aqueous sodium tosylate solutions under varying experimental conditions with the aim of finding the optimum conditions for the use of such a membrane for the release of anionic drugs. The variables studied were the potential of polymerization, pH, temperature and the relative permittivity of the solution. The n values determined for the polymerization reaction were about 2.24 and they were found to be quite independent of the polymerization conditions used. The effect of temperature and the relative permittivity of the solvent on the doping levels and the ion exchange properties was of minor importance. The cathodic reduction of the ion-gate membrane, however, showed that if mainly anions are to be released from the membrane the polymerization potential has to be above 900 mV (on the calomel electrode scale) and the pH of the electrolyte solution should be neutral or acidic. If this potential is kept below 800 mV and the pH of the solution in which reduction of the membrane is performed is alkaline, cations should be the main species moving. Impedance measurements and a separate determination of the apparent activation energy (6.8 kJ mol−1) confirmed that, in the oxidized state of the membrane, ion transport within the film is the main rate-limiting process.