Transport of simple electrolyte solutions through ion-exchange membranes - the capillary model

The transport of aqueous binary electrolyte solutions (NaCl, HCl) through the cation- and anion-exchange membranes (Nation, Neosepta) has been described in terms of the capillary model based on the thermodynamics of irreversible processes. The equivalent radius of pores has been calculated from the electroosmotic number of water. The L-ik coefficients have been calculated for each membrane/electrolyte system basing on the experimental equilibrium properties of the membrane (IEC, water content) and on the transport coefficients of ions in a free solution. Having Lik the electrical conductivity, the permeability coefficient of electrolyte, the osmotic and hydrodynamic permeability coefficients of solvent have been calculated and compared with the experimental values. The best agreement has been found for the classical Boltzmann equation and those transport coefficients, which are dependent on counterions, the poorest-for the permeability coefficient of electrolyte, which depends on coions. Other tested distributions of ions inside the pores (the modified Boltzmann equation and the homogeneous distribution) have yielded poor results. The negative osmosis for the anion-exchange membrane AM1/HCl and the relatively small transport number of Cl- counterions can be explained by assuming a high mobility of the H+ coions in that membrane. The change of pore geometry from the cylinder to infinite slit does not change the results substantially. (C) 2002 Elsevier Science B.V. All rights reserved.