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Interaction-Based Ion Selectivity Exhibited by Self-Assembled, Cross-Linked Zwitterionic SPECIAL FEATURE Copolymer Membranes

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Interaction-Based Ion Selectivity Exhibited by Self-Assembled, Cross-Linked Zwitterionic SPECIAL FEATURE Copolymer Membranes Interaction-based ion selectivity exhibited by self-assembled, cross-linked zwitterionic SPECIAL FEATURE copolymer membranes Samuel J. Loundera and Ayse Asatekina,1 aDepartment of Chemical and Biological Engineering, Tufts University, Medford, MA 02155 Edited by Howard A Stone, Princeton University, Princeton, NJ, and approved May 18, 2021 (received for review January 4, 2021) Water filtration membranes with advanced ion selectivity are assemblies (10), carbon nanotubes (11, 12), and graphene oxide urgently needed for resource recovery and the production of clean laminates (13). These various approaches face severe manufacturing drinking water. This work investigates the separation capabilities challenges (6, 14), however, and the use of polymeric materials is of cross-linked zwitterionic copolymer membranes, a self-assembled preferrable due to low cost and simple scale-up (6). For example, membrane system featuring subnanometer zwitterionic nanochan- block copolymers (BCPs) have been used to fabricate ultrafiltration nels. We demonstrate that selective zwitterion–anion interactions (UF) membranes with controlled pore wall functionality (15), simultaneously control salt partitioning and diffusivity, with the per- leading to tunable selectivity through solute–membrane interactions meabilities of NaClO4, NaI, NaBr, NaCl, NaF, and Na2SO4 spanning (16, 17). However, the relatively large size of BCP membrane pores roughly three orders of magnitude over a wide range of feed con- (>2 to 3 nm) limits its use for ion separations. Membranes formed centrations. We model salt flux using a one-dimensional transport from packed arrays of random copolymer micelles have demon- model based on the Maxwell–Stefan equations and show that diffu- strated selectivity based on solute charge (9) and aromaticity (18), sion is the dominant mode of transport for 1:1 sodium salts. Differ- − − but the pore size of these membranes is also too large to separate ences in zwitterion–Cl and zwitterion–F interactions granted these − − salt ions. It has been shown that the transport properties of ion membranes with the ultrahigh Cl /F permselectivity (PCl-/PF- = 24), exchange membranes are affected by the compatibility between the enabling high fluoride retention and high chloride passage even from ions and fixed charge groups (19, 20), but this approach has not saline mixtures of NaCl and NaF. been utilized to design advanced membrane filters. ENGINEERING Random zwitterionic amphiphilic copolymers (r-ZACs) are membranes | self-assembly | ion separation | synthetic ion channel | random/statistical copolymers that combine zwitterionic and hy- biomimetic drophobic repeat units. Strong interactions between zwitterions (ZIs) drive the formation of a bicontinuous network of zwitter- embranes with advanced ion selectivity could provide a ionic nanochannels surrounded by a hydrophobic nanophase Msustainable technical solution to global resource shortages. (21–24). Thin film composite (TFC) membranes are prepared by For example, dangerously high concentrations of fluoride in avail- coating a thin (∼1 μm) copolymer layer onto porous supports, a able drinking water sources affects many communities, resulting in simple and scalable membrane fabrication process. During aque- widespread, debilitating illnesses such as fluorosis (1, 2). Mem- ous filtration, the zwitterionic domain functions as a network of branes with enhanced chloride/fluoride selectivity could protect these communities from fluorosis without necessitating high- Significance pressure filtration or remineralization of the drinking water (2, 3). Limited geological reserves of lithium and uranium pose a The separation of ions is challenging yet crucial for providing major challenge to sustainable lithium battery production and access to safe water resources as well as recovering valuable nuclear power generation, respectively (4). Membranes capable of ions from water and wastewater. Yet, membranes rarely ex- selective ion retention could enrich aqueous feedstocks with de- hibit selectivity between ions of similar charge and size. We sired ions, enabling efficient capture of precious metals (5). Cur- demonstrate that membranes, prepared by a fully scalable rent synthetic membranes separate solutes by size and charge method that uses self-assembling zwitterionic copolymers, ex- differences (6, 7), limiting their use in advanced ion separations hibit exceptional selectivity between salt anions of similar size such as these. As such, the design of synthetic membrane filters and charge. We show that this unusual capability is derived from with targeted ion selectivity represents a crucial challenge for selective zwitterion–ion interactions occurring within the nano- addressing global resource shortages. channels, similarly to biological ion channels. We further dem- Biological ion channels (BICs) exhibit exquisite ion selectivity onstrate these membranes exhibit Cl−/F− permselectivity more that can inspire novel membranes capable of precise ion sepa- than twice the values reported in previous studies, with appli- rations. Potassium channels permeate potassium >10,000 times cations in treating groundwater streams to prevent fluorosis and faster than sodium, despite the sub-Å size difference between in wastewater treatment. these equally charged ions (8). While synthetic membranes separate ions primarily by size and charge differences (6, 7), Author contributions: S.J.L. and A.A. designed research; S.J.L. performed research; S.J.L. and A.A. analyzed data; and S.J.L. and A.A. wrote the paper. BICs rely on interactive functional groups that line the walls of Competing interest statement: A.A. owns a minor equity in and serves as the Senior nanopores (6, 9). The favorability and strength of these inter- Scientific Advisor of ZwitterCo. Inc., which was the lead institution in the NSF grant IIP- actions control ion partitioning and diffusion rate, providing a 1843847 that partially funded this work. ZwitterCo also holds a license from Tufts Uni- powerful mechanism for separating similarly sized ions (6, 9). versity to commercialize the technology described in this manuscript. Crucially, BIC pore diameters are comparable to or slightly This article is a PNAS Direct Submission. smaller than the hydrated diameter of the target ion (6, 9). This Published under the PNAS license. nanoconfinement forces ions to interact with the functional 1To whom correspondence may be addressed. Email: [email protected]. groups lining the pores, greatly amplifying selectivity (6, 9). This article contains supporting information online at https://www.pnas.org/lookup/suppl/ Numerous approaches have attempted to translate this separation doi:10.1073/pnas.2022198118/-/DCSupplemental. mechanism to synthetic systems, including the use of supramolecular Published September 7, 2021. PNAS 2021 Vol. 118 No. 37 e2022198118 https://doi.org/10.1073/pnas.2022198118 | 1of7 Downloaded by guest on September 24, 2021 hydrophilic nanochannels for the transport of water and small composition was selected as it was expected to microphase sepa- solutes. For a broad range of copolymer compositions (21, 22, 25), rate into a bicontinuous morphology leading to highly permeable r-ZAC membranes possess pores that are too large to achieve ion yet stable selective layers (21, 22). The copolymer composition selectivity. closely matched the monomer ratios in the reaction mixture de- Recently, we developed r-ZAC membranes with subnanometer spite the reaction being stopped at ∼10% conversion, implying a channels and exceptional mono/divalent ion selectivity (26). These roughly random sequence of monomers. Using gel permeation TFC membranes are prepared using an r-ZAC that contains cross- chromatography (GPC), we determined that the number-averaged linkable allyl groups (Fig. 1A). After membrane fabrication, we molecular weight was 1.8 × 105 Da with a dispersity of 5.4. The extensively cross-link the hydrophobic domain by photopolymerization. large dispersity was due to the presence of a high molecular weight This prevents the zwitterionic nanochannels from swelling water, fraction, which likely formed due to slight intermolecular chain resulting in a smaller effective pore size during aqueous filtra- cross-linking (SI Appendix,Fig.S2) (27). This copolymer was rod tion (26). Highly cross-linked r-ZAC membranes (ZAC-X) have coated onto a commercial UF membrane support (PS35, Solecta) an effective pore diameter of only ∼0.9 nm, based on the 99.7% and dried using a heat gun, resulting in TFC membranes with an retention of sucrose (SI Appendix,Fig.S1)(26),andachievedone ∼450-nm thick selective layer (Fig. 1A). TFC membranes were − 2− of the highest Cl /SO4 selectivities reported (26). The small pore then soaked in a solution of photoinitiator in isopropyl alcohol, size of ZAC-X is essential for the observed selectivity because which swells the hydrophobic nanodomain but not the zwitterionic uncross-linked r-ZAC membranes with larger pores exhibited rel- nanochannels. Exposure to ultraviolet (UV) light for 20 min led to atively limited ion selectivity. the polymerization of the allyl groups of AMA, reducing the ZAC-X presents a unique self-assembled membrane system nanochannel diameter to <1.0 nm (26). The resultant membrane, − − − featuring subnanometer nanochannels lined with abundant ion- ZAC-X, exhibited a water permeance of 0.37 L · m 2 · hr 1 · bar 1, interactive ZI groups (Fig. 1B). In this work, we investigate the in agreement with previous studies (26). ion selectivity
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