Interaction between cations in hydrophobic solvent-saturated filters containing fixed negative charges.

Previous investigations have shown that Millipore filters bearing fixed negatively charged sites saturated with organic solvents behaved like cation exchangers. In this study the mobilities of some cations in bromobenzene-saturated filters were determined at various membrane compositions. Appreciable interaction between counterions in the membrane was demonstrated; e.g., Na(+) was about four times more mobile in a Na(+) membrane than in a K(+) one. The qualitative nature of the interaction could be summarized as follows: the bringing together of rapid and slow counterions in the membrane made the fast ion faster and the slow ion slower. Possible mechanisms for such an interaction are discussed. A quantitative estimation of K and Na mobilities and selectivity factors at various K-Na fractions in the membrane was made. The above parameters were used to compute the potential difference in a K-Na bi-ionic cell. The calculated bi-ionic potential was found to agree reasonably well with the measured value.