Layer-by-layer adsorption of poly[(N,N'-dicarboxymethyl) allylamine] (PDCMAA)/poly(allylamine hydrochloride) (PAH) films at low pH yields a thin film with abundant Cu2+-binding sites. When deposited on a porous alumina substrate, (PDCMAA/PAH)n films show average Cu2+/Mg2+ diffusion dialysis selectivities of 50 and 80 for PAH-capped and PDCMAA-capped films, respectively. PDCMAA/PAH membranes also exhibit Cu2+/Ni2+ and Cu2+/Ca2+ selectivities. The high Cu2+/Mg2+ selectivity despite similar aqueous diffusion coefficients and equal charge for the two ions suggests a facilitated transport mechanism. In contrast, PAA/PAH and PSS/PAH films show Cu2+/Mg2+ selectivities <10. With PDCMAA/PAH films, Cu2+ flux increases nonlinearly with increasing CuCl2 concentration in the feed. Sorption isotherms show that PDCMAA/PAH films contain both strong and weak binding sites, and the nonlinear increases in flux with increasing feed concentration likely represent hopping between weak binding sites, probably the amine groups of PAH. Strong binding of Cu2+ to PDCMAA binding sites may displace ionic cross-links in the film and free amine groups for facilitated transport. Additionally, Cu2+ binding to the film suppresses Mg2+ transport, either through electrostatic exclusion or removal of hopping sites.
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