A perfluorocarboxylic acid membrane was chemically plated with gold bends under electric stimuli in water. When exchanged with alkali metal ions as counter ions, the displacement is rapid, though charge-specific displacement, or 'pumping' efficiency, is small. The fast response is thought to be linked with the small radius of the hydrated ion relative to the hydrophilic channel size in the membrane. In contrast, the charge-specific displacement with alkyl ammonium ions increases, while the rate decreases, systematically with molecular size. The effects are interpreted in terms of an 'ion-pumping' model, in which the size of the hydrated ion relative to that of the hydrophilic channel is of paramount importance. A good 'pumping' ion inevitably effects only a slow displacement.