Simultaneous monitoring of the transport of anions and cations across polypyrrole based composite membranes

A mechanism for the macroscopic charge balance during the transport of anions and cations across polypyrrole based composite membranes is proposed. For the mechanism to be studied, anions and cations were monitored simultaneously across PPy based composite membranes, which are known to have cation exchange (PPy(PSS)), anion exchange (PPy(ClO 4)) and mixed ion exchange properties (PPy(pTS)). Even though none of the membranes were found to be completely permselective, the flux of cations was higher than that of anions across the PPy(PSS) composite membrane, while the flux of anions was higher than that of cations across the PPy(ClO 4) composite membrane. Distinct changes in pH of the receiving solution were also observed. These were a decrease in pH when a predominantly anion exchanging polypyrrole composite membrane was used, and an increase in pH when a membrane that maintains charge balance principally by cation exchange was used. When membranes which display approximately equal permeability towards anions and cations were used the pH of the receiving solution was ca. 6-8. There was only a negligible flux of Ca 2+ across the PPy(PSS) membrane in the transport experiments carried out with the source solution consisting of either Ca(NO 3) 2 or an equimolar mixture of KNO 3 and Ca(NO 3) 2. The PPy(PSS) composite membrane was impermeable towards NO 3 - ions when the source solution was Ca(NO 3) 2 but permeability towards NO 3 - was observed when the source solution was either KNO 3 or an equimolar mixture of KNO 3 and Ca(NO 3) 2. Copyright 2011 Elsevier Ltd. All rights reserved

Electrochemically controlled ion transport across polypyrrole/multi-walled carbon nanotube composite membranes

Polypyrrole (PPy) films doped with acid-treated multi-walled carbon nanotubes (CNTs) were prepared by galvanostatic polymerisation of pyrrole (Py) dissolved in aqueous dispersions of CNTs, which served as the background electrolyte. Morphological characterization of PPy(CNT) films showed a random deposition of the polymer on the surface of the support material, with the former creating a porous, interconnecting three-dimensional network structure. The porous nature of PPy(CNT) films favours movement of large amounts of water in and out of the films during redox cycling, which also causes extensive volume changes. Raman spectroscopy, the electrochemical quartz crystal microbalance method and scanning electron microscopy were used to explore the properties of PPy(CNT) films. In addition, the factors influencing the electrochemically controlled transport of metal ions across composite membranes composed of polypyrrole doped with acid-treated multi-walled carbon nanotubes (PPy(CNT)) were studied. The factors examined were: The polymerization conditions (current density and time), the type of support material on which the polypyrrole films were deposited, and the use of single layer or bilayer type polypyrrole films. The flux of metal ions across the composite membranes was able to be electrochemically controlled when NucleoporeR track-etched membranes were used as the support material, while the flux was controlled by the concentration gradient present when polyvinylidene difluoride (PVDF) was used as the support material for PPy(CNT) films. However, electrochemically controlled movement of metal ions across PVDF-supported membranes was achieved when a bilayer type PPy film (PPy(pTS)/PPy(CNT)) was used (pTS = toluene-4-sulfonic acid). Increasing the thickness of the PPy(CNT) layer in the composite membrane was found to enhance the membranes permeability towards K+. Copyright 2011 Elsevier B.V. All rights reserved