Polypyrrole films have been characterized by simultaneous cyclic voltammetry driven force-displacement measurements. The aim was to clarify the role of cations in the electrolyte on the speed of response and on the strain of the film. The strain as a function of actuation frequency was studied in alkali metal chloride aqueous electrolytes. The intention was to test the hypothesis of the division of the inserted H 2 O molecules into categories: a smaller number strongly bound to the cation (corresponding to the inner solvation shell) and a larger number entering the film after the insertion of the cations because of forces related to osmotic pressure difference. The two processes have very different time constants: The solvated H 2 O molecules are associated directly with the cations, and are therefore inserted in a faster process, whereas the second type enters the film much more slowly. At higher frequencies, the strain depends almost exclusively on insertion of strongly solvated cations and therefore depends on the hydration number of the cations: Li + (hydration number~5.4) gives more strain than Na + (~4.4) and much more than Cs + (~0) as predicted by the model.

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Polypyrrole polymer films doped with the large, immobile dodecylbenzene sulfonate anions operating in alkali halide aqueous electrolytes has been used 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 molecules entering 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 primary membrane 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:Li þ : 5:5 À 5:3; Na þ : 4:5 À 4:3; K þ : 2:3 À 2:5;Rb þ : 0:9 À 0:8; Cs þ : $ 0; Mg 2þ : 10:4 À 10:6; Ca 2þ : 7:9 À 8:1; Sr 2þ : 5:7 À 6:1; Ba 2þ : 3:0 À 3:1; Y 3þ : 13:6 À 13:8; La 3þ : 9:0 À 9:1:The hydration number for all of these cations seems to follow the same simple relation.

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