Water content of ion-exchange membranes: Measurement technique and influence on the ion mobility

Ion-exchange membranes (IEMs) are essential components of several electrochemical water technologies where they promote the transport of certain ionic species over others. A characteristic property of IEMs concerns their ionic charge density (ICD) which is a key parameter for modeling ion transport. In literature, significant variations in ICD for similar membranes are reported. We analyzed the sources of variations of this property and traced those back to the water content measurement. In this manuscript, we developed a new technique for measuring the water content, i.e., via stacking layers of membranes. This technique reduces the impact of the surface water film on the water content measurement. Using this technique, we measured the water content of CEMs at different counter-ion forms and analyzed the contribution of the ionic hydration shells inside the membrane. The relative change in the measured water content for the studied membranes at different counter-ions (K+, Mg2+, or Ca2+) was below 23 % of the Na+ value. Furthermore, we examined the relation between the water volume fraction and the membrane tortuosity, where we compared the theoretical predictions of ion mobility based on the Mackie and Meares theory to the values calculated based on the membrane resistance measurements.