THEORY OF EQUILIBRIUM POTENTIAL INVOLVING ORDER-DISORDER TRANSITION AT POLYANILINE-COATED ELECTRODES

The alternate conjugation of two dimeric benzenoid-quinoid (IS) and quinoid-quinoid (2S) segments of polyaniline is known to form a stable phase with good electric conduction although the phase is in an intermediate redox state. A number of 1S- and 2S-segments were arranged theoretically on latent sites in a series, according to the order-disorder theory of binary alloys in the Bragg-Williams' approximation. The internal energy of the arrangement was expressed by the pair potentials between like segments and unlike segments multiplied by the numbers of pairs arranged randomly. The Helmholtz energy was evaluated as a function of an order parameter and the molar fraction of each species. It provided electrochemical potentials for 1S and 2S, from which the expression for the electrode potential was derived when both species were in electrochemical equilibrium. For small values of the pair potential difference, u, the molar fraction vs. potential curve was Nernstian and hence sigmoidal. With an increase in u, the curve consisted of two sigmoids. In the potential domain between the two sigmoids, the polymer had a highly ordered and alternate arrangement of 1S and 2S. The structure may be responsible for the appearance of the electric conduction. Reversible linear sweep voltammograms which were obtained by differentiating the curve showed two peaks for large values of u, demonstrating observed CV curves. In the potential domain between the two peaks, the arrangement was in order, whereas it was in disorder in the other domain.