Membranes composed of multilayer poly(4-styrenesul- fonate) (PSS)/protonated poly(allylamine) (PAH) fi lms on porous alumina supports exhibit high monovalent/divalent cation selectivities. Remarkably, the di ff usion dialysis K + /Mg 2+ selectivity is >350. However, in nano fi ltration this selectivity is only 16, suggesting some convective ion transport through fi lm imperfections. Under MgCl 2 concentration gradients across either (PSS/PAH) 4 - or (PSS/ PAH) 4 PSS-coated alumina, transmembrane potentials indicate Mg 2+ transference numbers approaching 0. The low Mg 2+ transference numbers with both polycation- and polyanion-terminated fi lms likely stem from exclusion of Mg 2+ due to its large size or hydration energy. However, these high anion/cation selectivities decrease as the solution ionic strength increases. In nano fi ltration, the high asymmetry of membrane permeabilities to Mg 2+ and Cl − creates transmembrane di ff usion potentials that lead to negative rejections (the ion concentration in the permeate is larger than in the feed) as low as − 200% for trace monovalent cations such as K + and Cs + . Moreover, rejection becomes more negative as the mobility of the trace cation increases. Knowledge of single-ion permeabilities is vital for predicting the performance of polyelectrolyte fi lms in the separation and puri fi cation of mixed salts.

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