Ion mobility and partition determine the counter-ion selectivity of ion exchange membranes

Abstract

Ion (perm)selectivity and conductivity are the two most essential properties of an ion exchange membrane, yet no quantitative relation between them has been suggested. In this work, the selectivity between two different counter-ions is correlated to the membrane conductivity. We show that the counter-ion selectivity measured by conventional electrodialysis (ED) can be expressed by the product of two parameters: (a) the mobility ratio between these two different counter-ions in the membrane and (b) their partition coefficient between the solution and the membrane. This is reminiscent of the classical solution-diffusion model. Via the counter-ion mobility in the membrane, the selectivity could be simply expressed with the membrane conductivity and dimensional swelling degree at pure counter-ion forms and at mixed counter-ion form when the membrane has been equilibrated with 1:1 equivalence ratio of the two counter-ions in the solution. This correlation is validated experimentally for the ion selectivity of K + /Na + in two commercial hydrocarbon-based cation exchange membranes (CEMs). For K + /Na + in a commercial perfluorosulfonic CEM, and for Mg 2+ /Na + in all the three types of CEMs, the correlation could predict the counter-ion partition very well; but there is an underestimation of the K + /Na + and Mg 2+ /Na + mobility ratios afforded by this correlation, which might be due to simplification of the cation activity coefficients in CEMs. This work offers a convenient method to decouple experimentally the effect of partition and mobility in controlling the membrane selectivity, and also proposes a new perspective to study the selectivity as well as conductivity of ion exchange membranes.

Ionic Mobility in Ion-Exchange Membranes

“…Selectivity is determined by the ratio of the mobility of different ions or molecules in the membrane. At the same time, the rate of target component transfer, determined by its mobility, is a no less important parameter [ 59 , 60 , 61 ]. It is this parameter that determines the performance of different electromembrane processes [ 62 , 63 ].…”

“…This shows the importance of investigation of ionic and molecular mobility in ion-exchange membranes [ 60 , 64 ]. Impedance spectroscopy is undoubtedly the main method for studying ionic mobility in solids [ 65 , 66 , 67 , 68 ], but it usually provides information on the total conductivity.…”

Cation Exchange Membranes Coated with Polyethyleneimine and Crown Ether to Improve Monovalent Cation Electrodialytic Selectivity

“…As shown in Figure 12 , M-PEI-12C4-0.50 displays increased E SEC and decreased η values in comparison with M-12C4-0.50-PEI, which is attributed to the reduction in membrane conductivity (lower IECs and WUs shown in Figure 5 ). A thin rigid coating on membrane surface affects the microstructure of membrane; thus, partial ionic transport is retarded [ 25 , 50 ]. R LiCl and R MgCl2 of M-PEI-12C4-0.50 are respectively higher than those of M-12C4-0.50-PEI (see in Figure 7 ), respectively, which manifests that a more compact membrane structure for M-PEI-12C4-0.50 suppresses the migration of Li + and Mg 2+ ions.…”

Lactic Acid and Salt Separation Using Membrane Technology

“…Two working electrodes were used to apply the voltage inducing electric current, while two reference electrodes recorded the voltage drop across the membrane, using Haber–Luggin capillaries to bring these close to the membrane surface. Further details of this technique are provided in Roghmans et al and Luo et al [ 30 , 31 ].…”