Experimental observations on transport phenomena accompanying redox switching in polythionine films immersed in strong acid solutions

Redox switching of polythionine films in strong acid solutions results in film mass changes which are not in agreement with those predicted by a simple permselective mechanism. Notably, the results depend not only upon the specific counterion and its concentration in the bathing electrolyte, but also on solvent transfer. While solvent transfer is clearly indicated by the data, inclusion of this transfer is insufficient to account for the experimental mass changes. A previous thermodynamic treatment is applied to allow specifically for ion aggregation in the film. Film mass changes, measured with the quartz crystal microbalance, are successfully described in terms of solvent (water) transfer across the polymer/solution interface, and the movement of counterion (A^-) and hydronium ion (H₃O⁺) to form ion pairs (H₃O⁺A^-) and electrostatically bound net neutral polymer/counterion aggregates in the polymer phase. Free ions in the polymer film do not contribute significantly to the overall mass change. Permselective behavior, a subset of our treatment, does not apply to the polythionine system in strong acid solutions.