High Selectivities among Monovalent Cations in Dialysis through Cation-Exchange Membranes Coated with Polyelectrolyte Multilayers

Cation-exchange membranes allow preferential passage of cations over anions, but they show minimal selectivity among cations, which limits their use in ion separations. Recent studies show that modification of cation-exchange membranes with polyelectrolyte multilayers leads to exceptional monovalent/divalent cation electrodialysis selectivities, but no studies report high selectivity among monovalent ions. This work demonstrates that adsorption of protonated poly­(allylamine) (PAH)/poly­(4-styrenesulfonate) (PSS) multilayers on Nafion membranes leads to high K+/Li+ selectivities in Donnan dialysis, where K+ and Li+ ions in a source phase pass through the membrane and exchange with Na+ ions in a receiving phase. Addition of 0.01 M HNO3 to a source phase containing 0.01 M KNO3 and 0.01 M LiNO3 increases the K+/Li+ selectivity from 8 to ∼60 through (PAH/PSS)5PAH-coated Nafion membranes, primarily because of a ≥fivefold increase in K+ flux. These selectivities are much larger than the ratio of 1.9 for the aqueous diffusion coefficients of K+ and Li+, and uncoated Nafion membranes give a K+/Li+ selectivity