Layer-by-layer modification of membranes for ion separations and catalysis

Layer-by-layer (LBL) adsorption of polycations and polyanions enables the formation of functional thin films containing charged materials ranging from polymer electrolytes to metal nanoparticles and viruses. This adsorption method is applicable to a variety of substrates including flat surfaces, colloids, and membrane pores. This dissertation examines the use of LBL adsorption to modify membranes for specific applications in ion separations and catalysis.;To simultaneously achieve high permeability and selectivity in ion separations, composite membranes contain a thin, selective layer on a thicker porous support that provides mechanical stability. Polyelectrolyte multilayer films adsorbed on porous ultrafiltration membranes present an ultrathin, selective skin that can be tailored for selective removal of multivalent ions in the presence of monovalent ions. Deposition conditions such as the type of terminating layer, the number of bilayers deposited, and the pH and ionic strength of the polyelectrolyte deposition solutions allow for optimization of film properties for selected ion separations. Specifically, five bilayers of poly(styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH) on porous alumina supports allow a solution flux of 0.85 m3/(m2 day) at 4.8 bar and exhibit 95% rejection of MgCl2 along with a Na+/Mg2+ selectivity of 22. Films with 4.5 bilayers of PSS/poly(diallyldimethylammonium chloride) (PDADMAC) deposited on alumina supports show a 98% rejection of phosphate, a chloride/phosphate selectivity of 48, and a solution flux of 2.4 m3/(m2 day). Both membranes exhibit higher fluxes and selectivities than commercially available nanofiltration membranes.;Metal nanoparticles are attractive catalysts due to their high surface area to volume ratio and unique electronic properties. However, their high surface energy often leads to aggregation, which greatly reduces catalytic activity. Alternating adsorption of polyelectrolytes and metal nanoparticles is a simple and effective method to load flat sheet and hollow fiber membranes with metal nanoparticles without nanoparticle aggregation Catalytic, immobilized nanoparticles such as Au and Pd allow the use of such membranes as catalytic reactors. Hollow fiber membrane reactors coated with films of Au nanoparticle/polyelectrolyte show high catalytic activity in the reduction of 4-nitrophenol with NaBH 4, and >99% initial conversion of 4-nitrophenol by HCOONa also takes place in flat sheet membranes containing Pd nanoparticles. However, a slight conversion decline over time takes place in both cases. This conversion decline probably stems from catalyst fouling by byproducts of 4-aminophenol oxidation.;Overall, LBL adsorption provides a simple, versatile method for modifying a variety of porous substrates to create functional membranes. Deposition of polyelectrolyte films on the surface of porous support yields composite membranes for ion separations, whereas adsorption of metal nanoparticle/polyelectrolyte films in porous supports gives catalytic membrane reactors. Further work is needed, however, to decrease the time and processing required in the LBL method and to increase the stability of membrane reactors prepared through LBL deposition.