Mass transfer in Nation membrane systems: Effects of ionic size and charge on selectivity

The interdiffusion of metal ion-hydrogen ion couples in Nation membrane systems has been measured by a rotating diffusion cell (RDC) technique. At low metal concentrations, the transport rate is controlled by boundary layer diffusion and the metal flux follows the order Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺ for alkali metals, and Ca²⁺ < Sr²⁺< Ba²⁺ for alkaline earth metals. At high metal concentrations, the transport process is controlled by membrane diffusion and the order of the flux is Li⁺ <Cs⁺ <Rb⁺ <Na⁺ <K⁺ for alkali metals, Ca²⁺<Ba²⁺<Sr²⁺ for alkali earth metals, and Y⁺ <Ca²⁺ < Na⁺ for ions of similar ionic size (0.93-0.99 Å) but different charge. The results are interpreted in terms of ionic hydration coupled with charge and size effects.