Adsorption of Polyelectrolyte Multilayers Imparts High Monovalent/Divalent Cation Selectivity to Nafion and Fujifilm Cation-Exchange Membranes

Although commercial ion-exchange membranes show high selectivities between cations and anions, they have modest selectivities among monovalent and divalent cations or monovalent and divalent anions. Such selectivities among cations or among anions are important, however, in applications such as purifying salts. This presentation will describe layer-by-layer adsorption of polyelectrolytes on cation-exchange membranes to impart extreme selectivities among monovalent and divalent cations.  Deposition of protonated poly(allylamine) (PAH)/poly(4-styrenesulfonate)(PSS) films on Nafion membranes leads to K⁺/Mg²⁺ electrodialysis selectivities >1000. Unfortunately, the current efficiency is only 0.5, so unwanted ions (such as protons) carry 50% of the current. Moreover, the high price of Nafion membranes ($800 for 0.5 m² ) limits their practical application. We recently found that (PAH/PSS)₅PAH-modified Fujifilm cation-exchange membranes show a K⁺/Mg²⁺ electrodialysis selectivity of ~800 with a current efficiency of 0.5.  The corresponding unmodified membranes show selectivities <2.  The Fujifilm substrates do not contain expensive fluorinated polymers and are thus much less expensive than Nafion.  To increase current efficiency, we coated Nafion membranes with polydiallyldimethylammonium (PDADMAC)/PSS films.  (PDADMAC/PSS)₅PDADMAC-modified Nafion membranes exhibit K⁺/Mg²⁺ and Li⁺/Co²⁺ selectivities > 1000, but in Li⁺/Co²⁺ separations, the current efficiency is 0.6 for (PDADMAC/PSS)₅PDADMAC-modified Nafion and only 0.45 for (PAH/PSS)₅PAH-modified Nafion. The higher current efficiency may stem from swelling of PDADMAC/PSS films. However, the PDADMAC/PSS films likely delaminate at high ionic strength, so we are working on cross-linking chemistries to stabilize these membranes.