Discrimination between monovalent and divalent cations by hydrophobic solvent-saturated membranes containing fixed negative charges

Summary Cellulose acetate-nitrate filters were saturated with hydrophobic solvent and interposed between various aqueous solutions. The membranes thus formed are cation permselective. The discrimination between a monovalent cation such as K⁺ and the alkaline earth group divalent cations is very sharp. The discrimination ratio is at least a few thousand times in favor of the monovalent cation. A major part of this discrimination is caused by the very low mobility of the divalent cation within the membrane compared with that of the monovalent cation. The remainder of the discrimination is caused by the selectivity of the membranes which prefer monovalent to divalent cations. There is a clear discrepancy between Ba⁺⁺ diffusibility and mobility within, the membrane. This implies that Ba⁺⁺ may move within the hydrophobic membrane as a neutral complex. Some similarity with natural biological membranes is indicated.