ION AND PORE FLUID TRANSPORT-PROPERTIES OF A NAFION(R) MEMBRANE SEPARATING 2 ELECTROLYTE-SOLUTIONS .2. KINETICS OF THE RU(2,2-BIPYRIDINE)(3)(2+) ION TRANSFER

Ion exchange and impedance measurements were used to investigate the i on and pore fluid transport properties of the as-received Nafion(R) me mbrane (Nafion(R) 117, 0.181 mm in thick) in the presence of the Ru(bp y)(3)(2+) cation (bpy = 2,2'-bipyridine). Very slow transport of Ru(bp y)(3)(2+) across the membrane is reflected in both the ion exchange ra te and the impedance of the membrane located between two electrolyte s olutions. The initial rate of exchange of proton by Ru(bpy)(3)(2+) in the H form Nafion(R) corresponds to a diffusion coefficient of 7.3 x 1 0(-14) cm(2) s(-1). At higher Ru(bpy)(3)(2+) loadings, the ion transpo rt rate decreases, so that the 100% limit is not reached in a reasonab le period of time. Impedance behaviour can be understood on the basis of the theoretical model previously developed and applied to the trans port of the alkali metal cations and proton. However, a coupling betwe en the kinetic and transport impedances is manifested here by an incre ased Warburg coefficient of the membrane impedance, which is predicted to correspond to the low ratio of rates of the pore fluid flow inside the membrane and the interfacial ion transfer. The latter process is characterized by an apparent rate constant of about 10(-3) cm s(-1). I on and pore fluid transport properties of Nafion(R) are likely to be r elated to its morphology, e.g. to the diameter of pores. An analysis o f the membrane capacitance suggests that the equilibrium potential dif ference across the Nafion(R) membrane is concentrated in the compact l ayer at the membrane solution interfaces, which can be represented as the surface dipole formed by the ionized fixed sites on the polymer su rface and the counterions from the solution phase.