Selectivity of Polyelectrolyte Membranes

The major factors contributing to the selectivity of charged synthetic membranes are discussed. Transport through porous polyelectrolyte membranes, containing water-filled interstices, can be predicted approximately from Donnan equilibria, representing the interaction between polyelectrolyte and counter-ions by "ion-binding". A simple relation between permselectivity and maximal salt rejection in hyperfiltration is given for porous aqueous membranes. Therole ofthe streamingpotential, especially in the filtration of mixed electrolyte, is demonstrated. Dense polyelectrolyte membranes which swell only slightly in water, give much higher salt rejection than predicted for their permselectivity, according to the Donnan-based relation mentioned: they comprise a separate non-aqueous phase. Porous membranes, swollen with an organic solvent show very high permselectivity at low charge density. Ion-specificity can be imparted to them by specific complexing agents. It is shown that the change in the dissociation between fixed ionic site and mobile counter-ion by the ligand, may be more important than the change of ion-uptake by the membrane, from the mixed aqueous solution.