Polypyrrole membranes containing chelating ligands: synthesis, characterisation and transport studies
Conducting polymer membranes consisting of polypyrrole (PPy) doped with either 8-hydroxyquinoline-5-sulfonic acid (HQS) or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonic acid (bathocuproinedisulfonic acid, (BCS)) were prepared electrochemically using aqueous solutions of pyrrole and the respective chelating ligands. The ratio of pyrrole to dopant in the resulting PPy/HQS and PPy/BCS free-standing membranes was found to be 9:1 and 6:1, respectively, whilst the conductivities were comparable (1.13 and 3.40Scm-1, respectively). Cyclic voltammetry studies using electrodes coated with thin films indicated that both polymers undergo reversible oxidation/reduction processes in aqueous solution that are influenced by the nature of the supporting electrolyte. Electrochemically controlled transport of metal ions was demonstrated using laminated membranes (either PPy/BCS or PPy/HQS was deposited onto platinum sputter-coated polyvinylidene filters). Transport of Co2+, Ni2+ and Zn2+ was demonstrated for the first time across PPy/BCS/Pt/PVDF membranes, which also proved permeable to K+, Mg2+, Ca2+, Mn2+, Fe3+ and Cu2+. By contrast PPy/HQS/Pt/PVDF membranes proved permeable to a smaller number of metal ions under the same conditions, with only K+, Co2+ and Cu2+ transported to a significant extent. The flux of copper across membranes containing PPy/BCS was approximately 10 times greater than that observed using membranes containing PPy/HQS, and 50 times greater than for polypyrrole membranes containing other types of dopants.