Enhancing the Anion-Transport Selectivity of Multilayer Polyelectrolyte Membranes by Templating with Cu²⁺

Alternating deposition of oppositely charged polyelectrolytes on porous supports is an attractive way to synthesize selective, ultrathin ion-separation membranes. This study reports the use of Cu 2+ as a template in poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) membranes to control fixed charge density and hence enhance anion-transport selectivity. Alternating deposition of PAA partially complexed with Cu 2+ and PAH on porous alumina supports followed by removal of Cu 2+ and deprotonation results in fixed −COO - sites in these films. Cu 2+ -templated PAA/PAH membranes show a 4-fold increase in Cl - /SO 4 2- transport selectivity compared to pure PAA/PAH membranes deposited under similar conditions. Cross-linking of these templated membranes further increases Cl - /SO 4 2- selectivity by an order of magnitude to a value of 610. Changing the top layer of cross-linked, Cu 2+ -templated membranes from negatively charged PAA to positively charged PAH decreases Cl - /SO 4 2- selectivity 10-fold, indicating that Donnan repulsion from a negatively charged surface is a major factor behind selectivity. Modeling studies suggest that selectivity is due to both Donnan exclusion and diffusivity differences among ions.