Hydrophobic and Ionic Exchange of Tris(2,2′-bipyridine)ruthenium(II), Methylviologen, and Sulfonatopropylviologen in Nafion Films

The present paper concerns the hydrophobic interactions of Nafion layers with tris(2,2′-bipyridine)-ruthenium(II) (Ru(bpy)₃²⁺) and methylviologen (MV²⁺) and the dynamics of solute adsorption. The distribution of the solutes between the layer and the bulk has been studied by spectrophotometric and electrochemical methods. The standard free energy of the hydrophobic binding of Ru(bpy)₃²⁺ to Nafion is comparable to that of hydrogen bonding. The free energies of hydrophobic interactions with layers are similar to those of water solutions of Nafion. The binding becomes weaker upon increasing the viologen concentration. The viologen depletion in the bulk, due to adsorption by the layer, has been found to obey a first-order rate law, being proportional to the viologen concentration in the bulk and to the surface area of the layer. The half-life for 1 cm² Nafion is about 3 h. The rate does not depend on the thickness of the layer. The results are rationalized in terms of an activation barrier for crossing the Nafion-water interface. The Nafion layer adsorbs up to 1.67 M MV²⁺ (local concentration inside the layer). This value is considerably higher than reported before. Practically all the MV²⁺ in the layer is electrochemically active. Aging and heating affect the properties of the Nafion. CdS and TiO₂ colloidal particles, incorporated in the layer during the preparation, have a small or no effect on the rate of diffusion inside the Nafion and no effect on the rate of the adsorption.