Shifts in Redox Formal Potentials Accompanying the Incorporation of Cationic Complexes in Perfluoro Polycarboxylate and Polysulfonate Coatings on Graphite Electrodes

Abstract

The formal potentials of several redox couples incorporated in coatings of a perfluoropolycarboxylate on graphite electrodes were measured and compared with the formal potentials of the same couples in homogeneous solution. The differences observed agreed with those calculated from the Nernst equation with the independently measured incorporation coefficients for both halves of the redox couples. The dependences of the shifts in formal potentials on the nature of the incorporating complex ion, the ionic strength, and the temperature were determined and indicated that the incorporation equilibrium is governed by electrostatic and hydrophobic interactions that act in opposite directions. The incorporation of most cation s examined was driven by large increases in entropy which overcame the usually unfavorable enthalpy changes.Redox reactants incorporated in polyelectrolyte coatings on electrode surfaces will exhibit formal potentials that are shifted from their values in solution whenever the two oxidation states of the reactant have different equilibrium constants for incorporation by the polyelectrolyte. This situation has been recognized in earlier reports (1, 2) .and in a recent study this aspect of the electrochemistry of redox polymer coatings was examined in detail (3). In continuing studies of the electrochemical responses obtained from redox reactants at electrodes coated with Nation, I, and the re- coatings. Both the magnitude of the shift in formal potential and its sign were influenced strongly by the nature of the ligands coordinated to the metal center of the redox couples. It was the purpose of the experiments described here to document the behavioral differences observed with several redox couples in both polyelectrolytes and to attempt to understand their origins. The results indicate that both electrostatic and hydrophobic interactions of incorporated electroactive ions with the polyelectrolyte combine to determine the direction of shifts in formal potentials. The temperature dependences of the shifts in formal potential were also measured. They pointed to large entropic contributions to the differences in the equilibrium incorporation of the oxidized and reduced halves of the redox couples. ExperimentalMaterials.--Nafion I coatings were prepared as previously described (4) by evaporation of aliquots of a solution of the polymer supplied by E. I. du Pont de Nemours and Company several years ago. Polymer II was supplied in the form of a fine suspension of the methyl ester by Asahi Glass Company, Tokyo. To prepare coatings, aliquots of the suspension were evaporated on freshly cleaved electrodes. The resulting deposit was exposed to 1M NaOH for 5 min to convert the ester to the carboxylate, and the electrode was then washed thoroughly with water. The extent and uniformity of the de-esterification were not examined in detail, but it was established that increasing the time that the coatings were exposed to the 1M NaOH produced no changes in their behavior. Electrodes were prepared from pyrol...

Voltammetry in supercritical carbon dioxide at platinum microdisk electrodes coated with perfluorinated ion-exchange membranes

“…contrast to the usually straightforward voltammetry of this compound (37). Such behavior has been previously reported (29,38,39) and was attributed to interactions of ionic redox couples with polyelectrolyte films (40). In this case it appears that the amine group of DHBA is the cause of the observed behavior.…”

Electrochemical and Spectroelectrochemical Studies of Phenothiazine Dyes Immobilized in Nafion Film

“…The hydrophilic and hydrophobic domains in the Nafion film are in close proximity, 1a-c and hence both hydrophilic and hydrophobic interactions play an important role in determining the relative stability of the electroactive ions in Nafion film . It has been found that the interaction of the incorporated molecules with the hydrophobic region of the Nafion film influenced the formal potentials of metal complexes . In the case of phenothiazine dyes, the hydrophobicity of the reduced dye molecules played a major role in determining the electrochemical properties of these molecules in Nafion film.…”

Influence of Polycation Composition on Electrochemical Film Formation

“…The smaller (apparent) diffusion coefficients derived from Levich plots confirm slower diffusion/reduced availability of the ferrocyanide upon polymer addition (Supporting Information Figure S11). More importantly, the voltammograms shift towards more negative potentials upon addition of the polyelectrolytes, due to the preferred interaction of the quaternized units with the ferri- and not the ferrocyanide (Figure 2b) [35]. Hence, the oxidation of ferro- to ferricyanide strengthens the interaction pattern of these two oppositely charged components in solutions of hexacyanoferrates and the corresponding quaternized polymeric sites.…”