Charge and dielectric characterization of nanofiltration membranes by impedance spectroscopy

The conductance and capacitance of a nanofiltration membrane have been obtained by impedance spectroscopy analysis in a wide range of concentrations of KCl aqueous solutions. The permittivity and conductivity of the electrolyte–membrane system have been determined by means of dielectric analysis and modeling the system with an equivalent circuit that corresponds to the relaxation times experimentally observed. The volumetric charge in the membrane has been assessed by solving the equilibrium partitioning equations in the membrane that include Donnan, steric and dielectric effects. The mean transport number has been evaluated by the membrane potential method. It has been proved that the effects of confinement within the narrow pores of nanofiltration pores on the dielectric phenomena arising in such narrow pores are substantial and have to be taken into account in transport models. In summary by using impedance spectroscopy and membrane potential measurements it has been possible to get the electric and dielectric parameters that should allow the available nanofiltration models to be used as predictive tools.