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 Cu2+ 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 Cu2+ and PAH on porous alumina supports followed by removal of Cu2+ and deprotonation results in fixed -COO- sites in these films. Cu2+-templated PAA/PAH membranes show a 4-fold increase in Cl-/SO42- transport selectivity compared to pure PAA/PAH membranes deposited under similar conditions. Cross-linking of these templated membranes further increases Cl-/SO42- selectivity by an order of magnitude to a value of 610. Changing the top layer of cross-linked, Cu2+-templated membranes from negatively charged PAA to positively charged PAH decreases Cl-/SO42- 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.