Ionic equilibria in ion-exchange membranes: a comparison of pore model predictions with experimental results

Abstract

The predictions of three different pore models for the equilibrium electrolyte absorption in ion-exchange membranes are compared to experimental results from the literature for Nafion 120 and Nafion 117 membranes as well as to new experimental results for Nafion 117 and for a Permion radiation-grafted membrane. The pore models studied were: (1) the Poisson-Boltzmann equation approach assuming constant electrical permittivity and a bank of uniformly-charged pores, (2) a modified Poisson-Boltzmann equation allowing for a variable electrical permittivity and hydration effects, also for a bank of uniformly-charged pores, and (3) model (1) with the addition of a non-uniform charge distribution among the pores. The variable electrical permittivity model was able to provide reasonable predictions of electrolyte equilibria in Nafion for three different Nafion-electrolyte systems if the pore diameter was calculated from the Bragg spacing determined in X-ray diffraction studies. Whena smaller pore diameter estimated from transport coefficients was assumed, the constant electrical permittivity approach predicted the electrolyte sorption if 5% of the pores were assumed to be uncharged, and 10% of the imbibed water was assumed to be bound in the polymer phase and free of ions. A high level of electrolyte absorption by the Permion membrane at low concentrations indicates that almost 20% of the pore volume in this membrane may be uncharged or weakly charged, causing reduced coion exclusion.