Permeation of neutral, cationic, and anionic electrode reactants through a polycationic polymer film as a function of electrolyte concentration

The permeability of 15-100-nm-thick films of polycationic poly(tris(4-methyl-4′-vinyl-2,2′-bipyridine)ruthenium), poly-[Ru(vbPy)₃]²⁺, to ferrocene, ferrocenium, tetracyanoquinodimethane (TCNQ), and TCNQ^- was measured in 0.1-10^-5 M Et₄NClO₄ electrolyte in acetonitrile with microelectrodes and twin-electrode thin-layer cells. Measurements were conducted with microelectrodes to mitigate potential gradient effects of extremely dilute electrolytes on the quantitative limiting current and E_1/2 measurements from which permeabilities were extracted. The permeabilities vary with electrolyte concentration, allowing separation of the contributions of partition and diffusion of the electroactive permeant into and within the polymer, respectively. Permeability is independent of, decreases, and increases with decreasing electrolyte concentration for the neutrals ferrocene and TCNQ, cationic ferrocenium, and anionic TCNQ^-, respectively. The main factor changing permeability with electrolyte concentration is the permeant partition coefficient into the polymer.