Membranes composed of multilayer poly(sodium4-styrenesulfonate)/protonated poly(allylamine) films on porous supports exhibit high monvovalent/divalent cation selectivities. Remarkably, the diffusion dialysis K+/Mg2+ selectivity is = 500. However, in nanofiltrationthe selectivity is only 16, suggesting some convective coupling in ion transport. Under a series of MgCl2 concentration differences across the membrane, transmembrane potentials ssuggest Cl- transport numbers approaching 1. In nanofiltration, the high asymmetry of membrane permeabilities to Mg2+ and Cl- creates high streaming potentials that lead to negative rejections (the ion concentration in the permeate is larger than in the feed) as large as -200% for trace monovalent cations such as K+, Li+ and Cs+. Moreover, rejections become more negative as the mobility of the trace cation increases. Knowledge of single-ion permeabilities is vital for predicting the performance of polyelectrolyte films in the separation and purification of salts.

Fundamentals of selective ion transport through polyelectrolyte multilayer films

245th National Meeting (2013)

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