Ion Exchange Membranes in Aqueous, Methanolic and Ethanolic Electrolyte Solutions. Membrane Characterization and Bipolar Membrane Modelling

by Frank Sarfert

Ion exchange membranes serve as the selective barrier in (electro-)dialytic separation processes. To date they are applied almost exclusively to aqueous electrolyte systems. In principal, however, ion exchange membranes also should be applicable to non aqueous solutions. In order to clarify how and why the membrane behavior in non aqueous electrolyte systems deviates from aqueous systems, equilibrium and transport properties of commercial monopolar and bipolar membranes are determined experimentally for aqueous, methanolic and ethanolic sodium perchlorate solutions in this study. Based on these results the dependence of the membrane performance on solution characteristics such as the relative permittivity, the solute concentration and the ionic mobility and on membrane specifics such as the type of functional groups and the membrane morphology is discussed. Thus, evidence is found for the importance of ion pair formation in the solution as well as in the membrane phase. Both effects lead to a more or less pronounced degradation of the membrane performance.Further insights into the stationary and dynamic behavior of ion exchange membranes is obtained from a detailed, physically meaningful and dynamic bipolar membrane model. It is characterized by the consideration of four ionic species as well as solvent and electric potential, the existence of a space charge region at the cation/anion exchange layer interface, the description of non-idealities by means of solution and membrane phase activity coefficients and the solvent dissociation according to the chemical reaction model. Thus, a model is developed which is able to reproduce all observed experimental trends for both aqueous and non aqueous solutions.