Enhancement of the Ion-Transport Selectivity of Layered Polyelectrolyte Membranes through Cross-Linking and Hybridization

Layer-by-layer deposition of polyelectrolytes on porous supports is a convenient method for producing ultrathin membranes with moderate ion-transport selectivities. This study shows that the capping of poly(styrene sulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) membranes with a few layer pairs of poly(acrylic acid) (PAA)/PAH results in dramatic increases in anion-transport selectivity. Deposition of less than eight total layer pairs yields reliable Cl-/SO42- and Cl-/Fe(CN)63- selectivities up to 150 and 3000, respectively. Field-emission scanning electron microscope images show that the surface structure of PAA/PAH is altered by deposition on PSS/PAH, and we surmise that the PAA/PAH on these hybrid films is less strongly hydrated than the surface of pure PAA/PAH membranes. This would result in a higher surface charge density and more Donnan exclusion of multiply charged ions. Termination of membranes with PAH rather than PAA greatly reduces (in some cases reverses) Cl-/SO42- selectivities, demonstrating that selectivity is indeed largely due to Donnan exclusion at the membrane surface. Cross-linking of pure PAA/PAH membranes through heat-induced amidation can increase Cl-/SO42- selectivity as much as 4-fold. Because of their minimal thicknesses, hybrid polyelectrolyte films can be selective without greatly hindering the flux of monovalent ions. Thus, these films are attractive for applications such as water softening.