Ion and Water Transport in Ion Exchange Membranes

Measurements have been made of the electrical conductivity of cation‐ and anion‐permeable ion exchange membranes (Nepton CR‐61 and AR‐110) in a number of ionic forms at 10°, 25°, and 40°C and the corresponding Arrhenius activation energies calculated. Conductivity measurements have also been made on free solutions chemically analogous to the membrane systems and on membranes containing two mobile ions. Conductivities in the membrane are much lower than in free solution and the activation energies somewhat higher. In the case of mobile cations, but not anions, there is a very strong charge dependence. The observed conductivity phenomena are interpreted in terms of a general structural retardation of movement as a consequence of the enhanced water structure in the internal solution, and a more specific relaxation arising from the interaction of the mobile ions with the total coulombic field of their environment. Water transport measurements indicate that the ions moving in the membrane carry with them their primary hydration sphere, a sizable fraction of their total coulombic hydration atmosphere, and, especially in the case of larger species, entrained (or electro‐osmotic) water as well.