Proton Binding to Polyaniline Film

M. INÉS FLORIT; WALDEMAR A. MARMISOLLÉ; JULIANA SCOTTO; DIONISIO POSADAS

Szeged - Hódmezovásárhely

Workshop; 7th WEEM 2012. International Workshop on the Electrochemistry of Electractive Materials; 2012

Electrochemically active macromolecules are substances that can be oxidized and reduced in a reversible way. They can be characterized by its redox potential, its state of binding, its state of deformation and the extension of its screening. The redox potential is related to the possibility of the macromolecules to transfer electrons to a suitable redox couple in the same solution or to an electrode submitted to a suitable potential. The state of binding refers to the amount of bound ions, generally protons, on different sites of the macromolecules. The state of deformation refers to the state of tension that appears in the macromolecules, as a consequence of the conformational deformation or the interaction between charged positions of the macromolecules. Finally, the screen refers to the weakening of the electrostatics interactions between the fixed charged sites of the macromolecule due to the ionic atmosphere surrounding them. Previously, it was demonstrated that in electrochemically active macromolecules there are coupling effects among the state of tension, the state of binding, the extension of screening and the redox potential [1], [2]. That is, if one of these states changes, all the others will change too. Thus, if the binding species are protons, changing the pH of the solution in contact with the macromolecule makes the redox potential, the state of binding and the tension in the macromolecule to change. Moreover, in the case of Pani, the redox potential should depend directly on the amount of oxidized and reduced polymer and also on the proton activity in the solution, because protons participate in the redox reaction. However, as protons also participate in a binding equilibrium with the macromolecule, the redox potential should also indirectly depend on the solution pH through the coupling effects mentioned above. In this work was investigated the coupling between the redox potential and the extent of proton binding in electrochemically active polymers, for the particular case of Polyaniline (Pani). To this purpose, the oxidation degree was determined spectrophotometrically as a function of the applied potential to the Pani film in solutions of different pH values. Also, it was developed a simple statistical mechanic model that allows linking the redox potential with the oxidation degree and the state of binding that depends on the solution pH. The model allows the analysis of the experimental results to determine the proton binding constants of the reduced and the oxidized forms of the polymer, as well as other parameters, such as the interaction energy between the redox centres, as a function of the solution pH.