machine learning reveals key ion selectivity mechanisms in polymeric membranes with subnanometer pores
C.L. Ritt, M. Liu, T.A. Pham, R. Epsztein, H.J. Kulik, M. Elimelech, Science Advances, 8, eabl5771 (2022)
abstract
Designing single-species selective membranes for high-precision separations requires a fundamental understanding of the molecular interactions governing solute transport. Here, we comprehensively assess molecular-level features that influence the separation of 18 different anions by nanoporous cellulose acetate membranes. Our analysis identifies the limitations of bulk solvation characteristics to explain ion transport, highlighted by the poor correlation between hydration energy and the measured permselectivity (R^2 = 0.37). Entropy-enthalpy compensation, spanning 40 kilojoules per mole, leads to a free-energy barrier (∆G‡) variation of only ~8 kilojoules per mole across all anions. We apply machine learning to elucidate descriptors for energetic barriers from a set of 126 collected features. Notably, electrostatic features account for 75% of the overall features used to describe ∆G‡, despite the relatively uncharged state of cellulose acetate. Our work presents an approach for studying ion transport across nanoporous membranes that could enable the design of ion-selective membranes.
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ionization behavior of nanoporous polyamide membranes
C.L. Ritt, J.R. Werber, M. Wang, Z. Yang, Y. Zhao, H.J. Kulik, M. Elimelech, Proceedings of the National Academy of Science, 117, 30191–30200 (2020)
Abstract
Escalating global water scarcity necessitates high-performance desalination membranes, for which fundamental understanding of structure–property–performance relationships is required. In this study, we comprehensively assess the ionization behavior of nanoporous polyamide selective layers in state-of-the-art nanofiltration (NF) membranes. In these films, residual carboxylic acids and amines influence permeability and selectivity by imparting hydrophilicity and ionizable moieties that can exclude coions. We utilize layered interfacial polymerization to prepare physically and chemically similar selective layers of controlled thickness. We then demonstrate location-dependent ionization of carboxyl groups in NF polyamide films. Specifically, only surface carboxyl groups ionize under neutral pH, whereas interior carboxyl ionization requires pH >9. Conversely, amine ionization behaves invariably across the film. First-principles simulations reveal that the low permittivity of nanoconfined water drives the anomalous carboxyl ionization behavior. Furthermore, we report that interior carboxyl ionization could improve the water–salt permselectivity of NF membranes over fourfold, suggesting that interior charge density could be an important tool to enhance the selectivity of polyamide membranes. Our findings highlight the influence of nanoconfinement on membrane transport properties and provide enhanced fundamental understanding of ionization that could enable novel membrane design.
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a molecularly impermeable polymer from two-dimensional polyaramids
C.L. Ritt, M. Quien, Z. Wei, H. Gress, M.T. Dronadula, K. Altmisdort, Y.-M. Tu, M. Gadaloff, N.R. Aluru, K.L. Ekinci, J.S. Bunch, M.S. Strano, (Submitted to Science — https://doi.org/10.26434/chemrxiv-2024-c8b17-v2)
Abstract
All polymers exhibit gas permeability through the free volume of entangled polymer chains. However, two-dimensional (2D) materials including graphene stack densely and can exhibit molecular impermeability. Solution-synthesized 2D polymers that exhibit the latter by poly-condensation have been a longstanding goal. Herein, we demonstrate self-supporting, spin-coated 2D polyaramid nanofilms that exhibit N2 permeability below 3.1E-9 Barrer, 6500-fold lower than existing polymers, and similar for other gases. Optical interference during the pressurization of nanofilm-coated microwells allows measurement of mechanosensitive rim opening and sealing, creating gas-filled bulges stable exceeding 3 years. This discovery enables 2D polymer resonators with high resonance frequencies (~8 MHz) and Q factors up to 537, similar to graphene. A 60-nm coating of air-sensitive perovskites reduces the lattice degradation rate 14-fold with an O2 permeability of 3.3E-8 Barrer. Molecularly impermeable polymers promise the next generation of barriers that maximize chemical rejection with minimal material, ultimately advancing sustainability goals.