Transport numbers of ion-exchange membranes in ternary mixtures of KCl, H2O and ethanol relevant for electrodialysis and desalination processes

Understanding the transport phenomena in electrolyte mixtures involved in electrochemical processes is of utmost importance for the modelling and improvement of battery technology, fuel cells, ion-exchange membrane processes, and much more. In this work, we derive relations and present methods for the analysis of electric potential measurements of concentration cells and ion-exchange membrane permselectivity with ternary mixtures of KCl, H2O and ethanol (EtOH). Measurements of electric potentials across the Selemion CMVN (cation-exchange) and AMVN (anion-exchange) membranes in this ternary mixture suggest that the presence of EtOH affects the membrane permselectivity, but that the transference of EtOH is zero; i.e., ta=0. Instead, the decrease in membrane permselectivity can be explained by a varying water transference coefficient, tw. In the absence of EtOH we find tw≈4 in both membranes, and that the decrease in tw is proportional to the average thermodynamic activity of EtOH in mixtures adjacent to the membranes. The membranes were also found to be charge selective, as quantified by the ionic transport numbers; i.e., tK+=1 for the Selemion CMVN and tK+=0 for the Selemion AMVN. The presented method can readily be extended and leveraged to determine transference coefficients in other electrochemical systems. •New measurements of ion-exchange membrane electric potentials in KCl/H2O/EtOH.•Permselectivity is explained via non-eq. thermodyn. and transport numbers.•Permselectivity measurements indicate no transference of EtOH.•The membrane permselectivity varies due to co-transport of water with ions.•The co-transport of water decreases with increased EtOH concentration.