膜
选择性
聚电解质
电渗析
二价
无机化学
Nafion公司
离子交换
化学
反离子
质子化
离子
有机化学
电化学
电极
物理化学
聚合物
催化作用
生物化学
作者
Ling Yang,Chao Tang,Muhammad Ahmad,Andriy Yaroshchuk,Merlin L. Bruening
标识
DOI:10.1021/acsami.8b16434
摘要
Cation-exchange membranes allow preferential passage of cations over anions, but they show minimal selectivity among cations, which limits their use in ion separations. Recent studies show that modification of cation-exchange membranes with polyelectrolyte multilayers leads to exceptional monovalent/divalent cation electrodialysis selectivities, but no studies report high selectivity among monovalent ions. This work demonstrates that adsorption of protonated poly(allylamine) (PAH)/poly(4-styrenesulfonate) (PSS) multilayers on Nafion membranes leads to high K+/Li+ selectivities in Donnan dialysis, where K+ and Li+ ions in a source phase pass through the membrane and exchange with Na+ ions in a receiving phase. Addition of 0.01 M HNO3 to a source phase containing 0.01 M KNO3 and 0.01 M LiNO3 increases the K+/Li+ selectivity from 8 to ∼60 through (PAH/PSS)5PAH-coated Nafion membranes, primarily because of a ≥fivefold increase in K+ flux. These selectivities are much larger than the ratio of 1.9 for the aqueous diffusion coefficients of K+ and Li+, and uncoated Nafion membranes give a K+/Li+ selectivity <3. Bi-ionic transmembrane potential measurements at neutral pH confirm that the membrane is more permeable to K+ than Li+, but this selectivity is less than in Donnan dialysis with acidic solutions. In situ ellipsometry data indicate that PAH/PSS multilayers (assembled at pH 2.3, 7.5, or 9.3) swell at pH 2.0, and this swelling may open cation-exchange sites that preferentially bind K+ to enable highly selective transport. The coated membranes also exhibit modest selectivity for K+ over H+, suggesting selective transport based on preferential partitioning of K+ into the coatings. Selectivity declines when increasing the source-phase KNO3 concentration to 0.1 M, perhaps because the discriminating transport pathway saturates. Moreover, selectivities are lower in electrodialysis than in Donnan dialysis, presumably because electrodialysis engages other transport mechanisms, such as electroosmosis and strong electromigration.
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