Fundamentals of Selective Ion Transport Through Multilayer Polyelectrolyte Films

Membranes composed of multilayer poly(sodium 4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films on porous supports exhibit high monvovalent/divalent ion selectivities, particularly for cations. Remarkably, the diffusion dialysis K⁺/Mg²⁺ selectivity is >1000. However, the nanofiltration K⁺/Mg²⁺ selectivity is only 15 with these membranes, suggesting some convective coupling in ion transport. Membrane potentials with MgCl₂ solutions at different concentrations at the feed and permeate sides of the membrane show Cl^- transport numbers approaching 1. In nanofiltration, the high asymmetry of membrane permeabilities with respect to cations and anions (reflected by the high Cl^- transport number) creates high filtration potentials that give rise to large negative rejections of trace monovalent cations such as K⁺, Li⁺ and Cs⁺. When MgCl₂ is the dominant feed salt, concentrations of K⁺ and Cs⁺ in the permeate can reach nearly 3 times those in the feed. Under the experimental conditions, negative rejections increase in magnitude with both decreasing flow rate and increasing mobility of trace monovalent cation. Knowledge of single-ion permeabilities is vital for predicting the performance of these membranes in diffusion dialysis or nanofiltration with mixed salt solutions.