Composite membranes based on macrocycle/polysiloxanes: preparation, characterization and electrochemical behaviour

Abstract

Composite membranes based on complexing macrocyclic compounds (crown ethers and cryptands) have been studied. These compounds were included into a polyorganosiloxanic matrix in an attempt to obtain new ion-selective materials. This macrocycle/polyorganosiloxane constitutes the active phase of the membrane which was prepared via the sol–gel method and then used to fill a fibrous support of borosilicate. Characterization, by chemical and thermal analysis, FTIR and NMR spectroscopies and scanning electron microscopy (SEM), reveals a homogeneous distribution of the entrapped macrocycles in the polyorganosiloxane network. Electrochemical impedance spectroscopy studies of the composite membranes have been carried out in order to evaluate their ionic transport properties when they are in contact with aqueous salt solutions. From the impedance spectra, the ionic resistance values of the membranes are obtained which vary according to the nature and concentration of the involved macrocyclic compound. The presence of these macrocycles produces a significant decrease in the ionic resistance compared with membranes without macrocyclic compounds. The ionic resistance of these systems is also strongly related to the nature of the electrolyte and to its concentration. Thus, cations exhibiting greater hydration energies show a higher ionic resistance. As an example, for a membrane doped with 12C4 crown ether the Ri values are 213, 128, 109 and 99 kΩ when the electrolyte consists of 10–2 mol I–1 solutions of LiCl, NaCl, KCl and CsCl, respectively. The selectivity of the membranes towards different ions, as well as the reversibility of the electrochemical response when the nature and concentration of the ions is changed, highlights the potential of these materials for sensor applications.