A Novel Method for the Determination of Membrane Hydration Numbers of Cations in Conducting Polymers.

Polypyrrole polymer films doped with the large, immobile dodecy lbenzene
sulfonate anions operating in alkali halide aqueous electroly tes has beenused as a novel physico-chemical environment to develop a more direct way of obtaining reliable values for the hydration numbers of cations. Simultaneous cyclic voltammetry and electrochemical quartz crystal microbalance technique was used to determine the amount of charge inserted and the total mass change during the reduction process in a polypyrrole film. From these values, the number of water molecules accompanying each cation was evaluated. The number of water moleculesentering the polymer during the initial part of the first reduction was found to be constant and independent of the concentration of the electrolyte below ∼1 M. This well-defined value can be considered as the primarymembrane hydration number of the cation involved in the reduction process. The goal was to investigate both the effects of cation size and of cation charge. The membrane hydration number values obtained by this simple and direct method for a number of cations are: The hydration number for all of these cations seems to follow the same simple relation.