About the species formed during the electrochemical half oxidation of polyaniline: Polaron-bipolaron equilibrium

The UV-Vis spectra of polyaniline (Pani) films were measured in the range 200 nm-900 nm, at constant potential as well as sweeping the potential. Both free standing films and Pani films deposited on ITO substrates were employed. These measurements were carried out at different electrolyte pHs, covering the range −0.6 < pH < 3.0. The experimental results were analyzed considering two potential regions: that corresponding to the occurrence of the faradaic reaction; and that corresponding to the purely capacitive response. The oxidation of the leucoemeraldine form to the emeraldine form leads to the formation of two species that could be associated to bipolarons and a polaron lattice. The experimental data show that these two species are in chemical (not electrochemical) equilibrium with each other. It is shown that bipolarons convert into a polaron lattice and that this conversion is favoured by the external applied potential, in order to satisfy the charge requirements at the polymer fibril/internal solution interface. In this way, the nature of the capacitance of Pani can be explained.