Membrane‐Properties of Polymer Films: The Acid‐Doping Reaction of Polyaniline

Abstract

Abstract The protonation/deprotonation reaction of polyaniline in the emeraldine oxidation state (PANI‐E) is discussed on the basis of membrane‐equilibrium considerations. The important role of the partitioning of H + between the polymer and the bathing solution is pointed out. The concentration of (free) H + in the polymer phase is used as a parameter in formulating the dissociation equilibrium of the Brønsted acid (PANI‐E)H + . It is shown that the p K a value determined for this dissociation equilibrium, p K a ≈ 5, is practically constant for all experimental conditions considered in this work. However, the pH of the bathing solution which leads to a defined protonation state of the PANI‐E may vary widely depending on the partitioning equilibrium for H + . This fact is demonstrated experimentally, and the experimental findings are shown to agree with theoretical predictions based on estimated and experimentally determined Donnan potentials.