Redox behavior of polyaniline as influenced by aromatic sulphonate anions: cyclic voltammetry and molecular modeling

We have investigated the influence of aromatic sulphonate anions on the redox behavior of polyaniline (PANI) by doping PANI, synthesized from sulphuric acid, with p-toluene sulphonic acid (pTSA), naphthalene sulphonic acid (NSA) and 5-sulphosalicylic acid (5-SSA). The contribution arising exclusively due to the anions was evaluated by keeping the concentration and the pH of the dopant solution constant at a value of 0.1 M and 2 respectively. The three aromatic sulphonate anions influenced the redox behavior of PANI differently. For instance, in the presence of NSA anions, PANI prefers to be in the protonated leucoemeraldine form, whereas at the same pH, PANI in the presence of sulphate, 5-SSA or pTSA anions, prefers to be in the unprotonated leucoemeraldine form. Interestingly, the presence of NSA gave highest stability to PANI against degradation due to hydrolysis. In order to rationalise these observations, molecular modeling (MM) studies were conducted. Using the molecular mechanics method, the energy of interaction between the dopant anion and the various forms of the polymer, namely, the leuco, emeraldine base, emeraldine salt and pernigraniline were estimated in order to take into account any factors other than just the basicity of the anion (e.g. non-covalent bonding interactions) that could contribute to the redox behavior of PANI. We have tried, for the first time, to correlate the redox peak positions with the interaction energies. The results of MM studies on this system are in good agreement with our experiments. (C) 2001 Elsevier Science B.V. All rights reserved.