Transport phenomena accompanying redox switching in polythionine films immersed in aqueous acetic acid solutions

The transport of neutral molecules in electroactive polymer films is a problem of considerable importance and is addressed here for thin (ca. 10 nm thick) polythionine films. In weak acid media, pH < PK_HA, both solvent and undissociated weak acid are present in the polymer. Raising the pH above pK_HA progressively removes HA. Using the quartz crystal microbalance, we determined the film weight changes accompanying the redox switching process. At all pH's studied, a mass decrease accompanies reduction. The magnitude of this change is consistent with the expulsion of one water molecule per redox site where, at low pH, HA within the film acts as the sole source of counterion, A^-. At higher pH, counterions must increasingly be supplied by the bathing solution, with the result that the mass change becomes less negative. These results are interpreted from a purely thermodynamic viewpoint, and then we discuss a coordination model as a relevant, special case.