Electrochemical Properties and Selectivity of Bilayer Ion-Exchange Membranes in Ternary Solutions of Strong Electrolytes

Abstract Bilayer ion-exchange membranes consisting of a thick heterogeneous membrane layer and a thin homogeneous membrane layer are studied theoretically and experimentally for their electrochemical properties such as overall and partial current–voltage curves, external and internal diffusion-limited currents, effective transference numbers, and specific selectivity for counterions and coions to be separated. For a diffusion layer (I)/modifying layer (II)/substrate membrane (III)/diffusion layer (IV) system in ternary solutions of electrolytes, we develop and verify a four-layer mathematical model with quasi-equilibrium boundary conditions. For the conditions of small current densities and limiting currents, we obtain analytical expressions for calculations of the limiting current, effective transference numbers, and specific selectivity coefficients. A numerical approach to calculating the specific selectivity coefficients is proposed. The mathematical model is verified for two electromembrane systems: one system is an anisotropic bilayer membrane with its MK-41 and MF-4SK polymer matrices having the same type of charge in a mixed CaCl 2 + NaCl solution, and in another one its MA-41 and MF-4SK matrices have opposite types of charge in a mixed Na 2 SO 4 + NaNO 3 solution. The transport parameters of both the modifying film (MF-4SK) and initial membranes (MK-40 and MA-41) are determined in single-component and mixed electrolyte solutions in separate experiments.