Fundamental study of the ion permeability of ultrathin, layered polyelectrolyte films

Layered polyelectrolyte films are attractive as coatings and membranes because of their ease of synthesis and versatility. The feasibility of utilizing these films, however, will depend on their permeability and stability under relevant conditions. Cyclic voltammetry and electrochemical impedance spectroscopy studies presented in this thesis show that the permeability of poly(allylamine hydrochloride) (PAH)/poly(styrene sulfonate) (PSS) coatings depends on the number of bilayers in the film, deposition conditions, and solution pH. Unfortunately, even the most passivating films are relatively permeable and unstable at high pH. To overcome this problem we employ PAH/poly(acrylic acid) (PAA) films which, when heated, form a nylon-like coating that dramatically decreases film permeability while increasing stability.; Ultrathin films such as layered polyelectrolytes are attractive membrane materials because they potentially offer both high flux and high selectivity. Synthesis of such membranes is challenging, however, as it requires defect-free deposition of a film at the surface of a porous support. Scanning electron microscopy shows that adsorption of layered polyelectrolyte films on porous alumina supports naturally results in a defect-free membrane. Ion transport through these membranes is primarily controlled by interactions between the ions and the fixed charge present in the film, which results in high selectivity between anions of different valence.; Layered polyelectrolyte films offer several means for controlling ion-flux through these membranes. Since Donnan interactions at the film/solution interface greatly affect ion-transport, capping the films with an additional anionic polyelectrolyte layer enhances the monovalent/divalent anion selectivity of these membranes. While selectivity depends on membrane surface charge, the flux through these membranes is affected by film composition. PAH/PAA membranes show similar selectivities as PAH/PSS membranes but with only 70% of the ion flux. To enhance selectivity while maintaining flux, composite membranes were constructed that utilize the high flux of PAH/PSS membranes along with the blocking effects of PAH/PAA membranes. Membranes composed of 5 PSS/PAH bilayers capped with 2.5-bilayers of PAA/PAH show Cl−/SO ₄2− selectivity as high as 350. Utilization of charge, membrane composition, and cross-linking provide for specific tailoring of membrane properties.; Utilization of polyelectrolyte membranes for water purification will require a pressure gradient to achieve sufficient water flux. When used as nanofiltration elements, PSS/PAH membranes show Ca2+ rejections of 90+% along with very high water flux. Cross-linking and fluorination of polyelectrolyte membranes further increases Na+ and SO ₄2− rejections, with respect to PSS/PAH membranes, while maintaining an appreciable flux. Na+ and SO₄ 2− rejection values for composite membranes are >60 and >90%, respectively, for both 50 and 1000 ppm solutions. Thus, polyelectrolyte membranes not only offer easy means for membrane modification, but may provide solutions to current problems in membrane separations.