Facilitated ion transport through polyelectrolyte multilayer films containing metal-binding ligands

Alternating layer-by-layer adsorption of poly[(N,Nʹ-dicarboxymethyl) allylamine] (PDCMAA) and protonated poly(allylamine) (PAH) at low pH yields thin films with abundant Cu2+-binding sites. When deposited on porous alumina substrates, (PDCMAA/PAH)3PDCMAA or (PDCMAA/PAH)4 films show average Cu2+/Mg2+ diffusion dialysis selectivities around 50. PDCMAA/PAH membranes also exhibit Cu2+/Ni2+ and Cu2+/Ca2+ selectivities >10. The high Cu2+/Mg2+ selectivity despite similar aqueous diffusion coefficients and equal charges for the two ions suggests a facilitated transport mechanism. For comparison, poly(styrene sulfonate)/PAH films with 7 bilayers show Cu2+/Mg2+ selectivities around 10, perhaps also due to facilitated transport via PAH. With PDCMAA/PAH membranes, Cu2+ flux increases nonlinearly with increasing CuCl2 concentrations in the feed. Sorption isotherms show that PDCMAA/PAH films contain both strong and weak binding sites, and the nonlinear increase in flux with increasing CuCl2 feed concentration likely represents hopping between weak binding sites, probably the amine groups of PAH. Strong binding of Cu2+ to PDCMAA may displace ionic cross-links in the film and create free amine groups for facilitated transport. Additionally, Cu2+ binding to the film suppresses Mg2+ transport, either through electrostatic exclusion or occupation of hopping sites.