Coupling between proton binding and redox potential in electrochemically active macromolecules. The example of Polyaniline

Abstract

In this work the coupling between the redox potential and the extent of proton binding in electrochemically active polymers is investigated for the specific case of Polyaniline (Pani). To this purpose the degree of oxidation of the polymer was measured as a function of the external applied potential. The degree of oxidation was measured spectrophotometrically by changing the external potential applied to the Pani films in of different values. The knowledge of the oxidation degree for the different applied potentials allows determining the apparent formal redox potential. For Pani, in the absence of interactions between the redox centers, the apparent formal redox potential should be independent of the oxidation degree and it should decrease linearly with the with a slope of 0.059 V, at room temperature. The values of the apparent formal redox potential, experimentally obtained, show that the dependence is not as expected. This fact implies interactions between the redox centers and some sort of coupling between the redox potential and the state of binding of the polymer. It is possible to estimate the interaction energy between the redox centers and also the acid dissociation constants of both, the oxidized and reduced forms, by employing a statistical mechanic model. The values obtained for both dissociation constants agree well with some of those reported in the literature. The application of the model also allows explaining the observed change of the apparent formal redox potential and the electrolyte pH.