Water transport in cation exchange membranes

Abstract

The time variation in emf caused by pressure differences has been used to find transference coefficients of water as well as water permeabilities in ion exchange membranes. Previous experimental methods have been corrected and facilitated. The cation exchange membrane CR61 AZL 386 was investigated. Equilibrium solutions contained 10⁻⁴ kmol-m⁻³ < c_KCl < 10⁻¹ kmol-m⁻³, or constant concentration of chloride, c_Cl- = 0.03 kmol-m⁻³ and different mole fractions of H⁺ and K⁺. We report here that the transference coefficient of water, t_w, is constant and close to 11 for c_KCl < 10⁻¹ kmol-m⁻³ when the membrane is in the K⁺-form. When the membrane is in the H⁺-form, the transference coefficient of water is close to 2. A new result is that the number of water molecules carried by each ion may be taken as constant also when both cations are present in the membrane. The water permeability, L_p, increases with increasing water activity, and is larger in the HM membrane than in the KM membrane. While the total water content of the membrane is the same in both cases, a larger number of water molecules is available for diffusion in the HM-membrane. Results are derived from a new theoretical basis avoiding the use of zeta potential, but rather emphasizing operationally defined thermodynamic variables. Hydrodynamic equations are not needed to describe transport in these membranes.