Determination of Ion Populations and Solvent Content as Functions of Redox State and pH in Polvaniline

The piezoelectric quartz crystal microbalance (QCM) is used to investigate the ion populations and solvent content of thin films of polyaniline (PA) on electrode surfaces as functions of redox state and pH. The data are shown to be consistent with a model for PA in which the initial oxidation at a potential of ca. 0.2 V vs. SCE to give the conductive form of the polymer creates charged sites by oxidation of amine moieties along the polymer chains. Higher potentials cause further (pH dependent) oxidation with cumulative removal of exactly one electron per ring to produce a polymer containing imine groups which can hydrolyze to form quinone and quinone/imine types of structures. The initial oxidation process is accompanied by proton expulsion at low pH values, indicating partial protonation of the PA amine nitrogens in the reduced form with loss of these protons on oxidation. In addition to the proton expulsion, anion insertion also takes place during oxidation under all conditions that were studied. Data from such measurements as a function of pH allow estimation of the pKa values of the reduced (insulating) and oxidized (conducting) forms of PA, which are given as ca. -0.3 and 3, respectively. Experiments in strong (10 M) acid solutions reveal changes in solvent content during oxidation which are observed to influence the electrochemical response of PA. The ability to measure both electrochemical and gravimetric data simultaneously is shown to greatly constrain possible models for systems such as PA, leading to less ambiguity in their description.