Interfacial electroneutrality controls transport of asymmetric salts through charge-patterned mosaic membranes

Feng Gao; John R. Hoffman; Jialing Xu; Anton V. Ievlev; Jonathan K. Whitmer; William A. Phillip

doi:10.1073/pnas.2504069122

Interfacial electroneutrality controls transport of asymmetric salts through charge-patterned mosaic membranes

Feng Gao
, John R. Hoffman
, Jialing Xu
, Anton V. Ievlev
, Jonathan K. Whitmer
, William A. Phillip

Functional Atomic Force Microscopy Group

Research output: Contribution to journal › Article › peer-review

Abstract

Membranes that selectively enhance target solute permeation while rejecting competing species are essential for precision separations. This study introduces charge-patterned mosaic membranes (CMMs) that selectively transport divalent asymmetric salts by leveraging a net-neutral membrane–solution interface. This mechanism, dictated by the charge ratio of positive and negative domains on the membrane surface and the balance of cations and anions in the salt, is supported by analytical, numerical, and experimental results. Analytical solutions identified cationic domain coverages (f⁺) of 33%, 50%, and 66% as optimal for the selective transport of +2:−1 salts, +1:−1 salts, and +1:−2 salts, respectively, under conditions where the pattern size (L) is significantly larger than the Debye length. Numerical simulations and experiments using CMMs with alternating charged-stripes inkjet-printed onto nanostructure copolymer substrates confirmed these findings. By varying stripe widths to control f⁺, pressure-driven filtration experiments demonstrated selective enrichment of MgCl₂ and K₂SO₄ at the predicted f⁺ values, with deviations from these values leading to salt rejection. These results highlight the pivotal role of a net-neutral interface in enabling asymmetric salt enrichment. This study positions CMMs as a versatile platform for tuning ion selectivity, addressing challenges in resource recovery, water treatment, and precision separations.

Original language	English
Article number	e2504069122
Journal	Proceedings of the National Academy of Sciences of the United States of America
Volume	122
Issue number	35
DOIs	https://doi.org/10.1073/pnas.2504069122
State	Published - Sep 2 2025

Funding

ACKNOWLEDGMENTS. This work was made possible with support from the NSF through the Advanced Manufacturing Program (Award Number: 1932206), and we appreciatively acknowledge this support. We would like to thank the Center for Environmental Science and Technology at the University of Notre Dame (CEST) and the Notre Dame Integrated Imaging Facility; portions of this research were performed with instrument at these facilities. Time-of-flight secondary ion mass spectrometry characterization was conducted at the Center for Nanophase Materials Sciences, which is a Department of Energy Office of Science User Facility, and using instrumentation within Oak Ridge National Laboratory's Materials Characterization Core provided by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. We would also like to thank Prof. Jeremiah Zartman for use of his fluorescent microscope. F.G. gratefully acknowledges support for this project from the Patrick and Jana Eilers Graduate Student Fellowship for Energy Related Research, administered by the Center for Sustainable Energy at Notre Dame. F.G. and J.R.H. gratefully acknowledges support from the CEST/Bayer Predoctoral Fellowship at the University of Notre Dame.

Keywords

additive manufacturing
charge density
charge-patterned mosaic membranes
ion selectivity
polymer membranes

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10.1073/pnas.2504069122

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title = "Interfacial electroneutrality controls transport of asymmetric salts through charge-patterned mosaic membranes",

abstract = "Membranes that selectively enhance target solute permeation while rejecting competing species are essential for precision separations. This study introduces charge-patterned mosaic membranes (CMMs) that selectively transport divalent asymmetric salts by leveraging a net-neutral membrane–solution interface. This mechanism, dictated by the charge ratio of positive and negative domains on the membrane surface and the balance of cations and anions in the salt, is supported by analytical, numerical, and experimental results. Analytical solutions identified cationic domain coverages (f+) of 33\%, 50\%, and 66\% as optimal for the selective transport of +2:−1 salts, +1:−1 salts, and +1:−2 salts, respectively, under conditions where the pattern size (L) is significantly larger than the Debye length. Numerical simulations and experiments using CMMs with alternating charged-stripes inkjet-printed onto nanostructure copolymer substrates confirmed these findings. By varying stripe widths to control f+, pressure-driven filtration experiments demonstrated selective enrichment of MgCl2 and K2SO4 at the predicted f+ values, with deviations from these values leading to salt rejection. These results highlight the pivotal role of a net-neutral interface in enabling asymmetric salt enrichment. This study positions CMMs as a versatile platform for tuning ion selectivity, addressing challenges in resource recovery, water treatment, and precision separations.",

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AU - Whitmer, Jonathan K.

AU - Phillip, William A.

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N2 - Membranes that selectively enhance target solute permeation while rejecting competing species are essential for precision separations. This study introduces charge-patterned mosaic membranes (CMMs) that selectively transport divalent asymmetric salts by leveraging a net-neutral membrane–solution interface. This mechanism, dictated by the charge ratio of positive and negative domains on the membrane surface and the balance of cations and anions in the salt, is supported by analytical, numerical, and experimental results. Analytical solutions identified cationic domain coverages (f+) of 33%, 50%, and 66% as optimal for the selective transport of +2:−1 salts, +1:−1 salts, and +1:−2 salts, respectively, under conditions where the pattern size (L) is significantly larger than the Debye length. Numerical simulations and experiments using CMMs with alternating charged-stripes inkjet-printed onto nanostructure copolymer substrates confirmed these findings. By varying stripe widths to control f+, pressure-driven filtration experiments demonstrated selective enrichment of MgCl2 and K2SO4 at the predicted f+ values, with deviations from these values leading to salt rejection. These results highlight the pivotal role of a net-neutral interface in enabling asymmetric salt enrichment. This study positions CMMs as a versatile platform for tuning ion selectivity, addressing challenges in resource recovery, water treatment, and precision separations.

AB - Membranes that selectively enhance target solute permeation while rejecting competing species are essential for precision separations. This study introduces charge-patterned mosaic membranes (CMMs) that selectively transport divalent asymmetric salts by leveraging a net-neutral membrane–solution interface. This mechanism, dictated by the charge ratio of positive and negative domains on the membrane surface and the balance of cations and anions in the salt, is supported by analytical, numerical, and experimental results. Analytical solutions identified cationic domain coverages (f+) of 33%, 50%, and 66% as optimal for the selective transport of +2:−1 salts, +1:−1 salts, and +1:−2 salts, respectively, under conditions where the pattern size (L) is significantly larger than the Debye length. Numerical simulations and experiments using CMMs with alternating charged-stripes inkjet-printed onto nanostructure copolymer substrates confirmed these findings. By varying stripe widths to control f+, pressure-driven filtration experiments demonstrated selective enrichment of MgCl2 and K2SO4 at the predicted f+ values, with deviations from these values leading to salt rejection. These results highlight the pivotal role of a net-neutral interface in enabling asymmetric salt enrichment. This study positions CMMs as a versatile platform for tuning ion selectivity, addressing challenges in resource recovery, water treatment, and precision separations.

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Interfacial electroneutrality controls transport of asymmetric salts through charge-patterned mosaic membranes

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