High Selectivities among Monovalent Cations in Dialysis through Cation-Exchange Membranes Coated with Polyelectrolyte Multilayers

Abstract

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 HNO 3 to a source phase containing 0.01 M KNO 3 and 0.01 M LiNO 3 increases the K + /Li + selectivity from 8 to ∼60 through (PAH/PSS) 5 PAH-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 cationexchange 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 KNO 3 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.

Sustainable Membrane Production through Polyelectrolyte Complexation Induced Aqueous Phase Separation

“…In our approach, pH is used to trigger complexation in a simple one‐step process that allows membranes with tunable pore sizes to be prepared. Poly(sodium 4‐styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) are chosen as the strong and weak PEs, respectively, and because of their high charge density, they form smooth, dense, and stable complexes when mixed together . At a pH above its p K a , i.e., ≈9, PAH is uncharged and the chains exist in their collapsed coiled state .…”

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities

“… 188 Compared to the mono/divalent cation selectivity of bare Nafion (<2), they reported Li + /Co 2+ and K + /La 3+ selectivities exceeding 1000. Yang et al used the same approach in Donnan dialysis with (PAH/PSS) 5.5 -coated Nafion 115 membrane to differentiate within the monovalent cations and reached K + /Li + selectivity values between 8 and 60, 78 although later the authors reported lower selectivities due to variations in different batches of the Nafion membranes. 189 The selectivity was attributed to the larger hydrated radius of Li + that resulted in a lower diffusion coefficient through the dense PEM layer.…”

Bio-inspired incorporation of phenylalanine enhances ionic selectivity in layer-by-layer deposited polyelectrolyte films

“…[15][16][17][18][19][20] The bottom-up approach of LbL assembly has been leveraged using different materials to create many active and passive functional materials, including polymer/clay anti-corrosion coatings, [21][22][23][24][25][26][27] fireretardant coatings, 28,29 nanoparticle electrocatalysts 30,31 and ion selective membranes. [32][33][34][35][36] LbL deposition of PEs offers an attractive method for modification of nanoporous membranes, which can be used to tune the ion transport properties of nanoporous membranes for various applications, ranging from water purification to energy storage and chemical separations/synthesis.…”