Reaction entropy measurements for transition-metal ions bound to Nafion-coated electrode surfaces

Abstract

Reaction entropies, ΔS⁰_(rc), have been determined for a series of transition-metal ions electrostatically bound into Nafion films. Complexes with hydrophobic, "shielding" ligands such as Ru(2,2'-bipyridine)₃^(3+/2+) and Co(2,2'-bipyridine)₃^(3+/2+) yield similar ΔS⁰_(rc) values in Nafion films and in aqueous solution, indicating that the electrochemically active probes reside in a predominantly waterlike local environment in the polymer film. Complexes containing aquo and ammine ligands yield markedly lower ΔS⁰_(rc) values in Nafion than in aqueous media (µ = 0.1 M). The ΔS⁰_(rc) values for these Nafion-bound systems are similar to values measured for the Ru(NH₃)₆^(3+/2+) system in concentrated aqueous salt solutions (5 M LiCl). The ΔS⁰_(rc) measurements imply that the outer-sphere electron-transfer work terms for polymer bound ions may be lower than those observed in typical aqueous solutions. Differential free energies, enthalpies, and entropies of transfer for the redox reaction M(III) + e⁻ → M(II) from aqueous media to Nafion have also been determined.

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