INTERPRETATION OF TRANSPORT-COEFFICIENTS IN NAFION USING A PARALLEL PORE MODEL

Onsager coupling coefficients for 0.1-1.0M NaCl transport in a perfluorosulfonate membrane (Nafion 120) were predicted to within 20% of experimentally determined values by a charged pore model. The pore solution properties were assumed to be constant and equal to the bulk solution properties. The model parameters (pore radius, fixed charge concentration, and a tortuosity factor) were determined from the measured ion exchange capacity, membrane water absorption, and two of the Onsager coefficients (membrane conductivity and permeability). These values for the membrane parameters also gave accurate predictions of the negative ion partition coefficient if some nonuniformity of the surface charge distribution among the pores was assumed. This nonuniformity did not significantly change the predictions of the transport coefficients. A pore model which allowed for a variable dielectric constant in the pore due to ion solvation and alignment of solvent dipoles in the high electric field near the pore wall could not accurately predict all of the transport coefficients using a single value for the tortuosity factor. This failure occurred when the solution viscosity and ion diffusivity in the pore were assumed constant and when these properties were assumed to decrease in a stepwise manner in the region near the pore wall. The simpler model which assumes constant solution properties in the pore provides an adequate means of predicting the transport coefficients in a Nafion membrane.