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Understanding the mechanisms of molecular transport in polyamide membranes is imperative to improve their solute-specific selectivity. We explored the partitioning behaviors of water and salts in polyamide membranes to elucidate the role of ion-membrane interactions in the transport. Quartz crystal microbalance (QCM) was employed to quantify the mass uptake at different temperatures and determine partition energies (Ek) for water and salts under two different pH values. Zeta potential and permeability tests were conducted to support the ionmembrane affinity trends observed with QCM and link these trends to ion-ion selectivity. Our results demonstrate a high affinity of water to the polyamide membrane (Ek < 0), with a significant swelling effect attributed to dipole interactions and hydrogen bonding. Ion partitioning revealed distinct differences between monovalent and divalent cations, as well as between kosmotropic and chaotropic anions. Specifically, divalent cations (Ca2+ and Mg2+) exhibited considerably lower partition energies (-0.99 and 0.29 kcal mol-1, respectively) and more efficient charge neutralization, indicating stronger interactions with the membrane compared to monovalent cations (~2.2 kcal mol-1). The partition energies of the chaotropic iodide and kosmotropic sulphate anions were substantially different (-5.5 and 4.0 kcal mol-1, respectively), likely due to the different tendency of these anions to shed their hydration shell and stick to the polymer. Last, our permeability tests indicate the potential existence of an intrinsic tradeoff between ion partitioning and intrapore diffusion, presumably due to the opposite effects that ion-membrane interactions have on these transport steps. Overall, our work underscores the role of ionspecific interactions in membrane transport and selectivity.
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Unlocking the potential of polymeric desalination membranes by understanding molecular-level interactions and transport mechanisms
Polyamide reverse osmosis (PA-RO) membranes achieve remarkably high water permeability and salt rejection, making them a key technology for addressing water shortages through processes including seawater desalination and wastewater reuse. However, current state-of-the-art membranes suffer from challenges related to inadequate selectivity, fouling, and a poor ability of existing models to predict performance. In this Perspective, we assert that a molecular understanding of the mechanisms that govern selectivity and transport of PA-RO and other polymer membranes is crucial to both guide future membrane development efforts and improve the predictive capability of transport models. We summarize the current understanding of ion, water, and polymer interactions in PA-RO membranes, drawing insights from nanofiltration and ion exchange membranes. Building on this knowledge, we explore how these interactions impact the transport properties of membranes, highlighting assumptions of transport models that warrant further investigation to improve predictive capabilities and elucidate underlying transport mechanisms. We then underscore recent advances in in situ characterization techniques that allow for direct measurements of previously difficult-to-obtain information on hydrated polymer membrane properties, hydrated ion properties, and ion–water–membrane interactions as well as powerful computational and electrochemical methods that facilitate systematic studies of transport phenomena.
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- Award ID(s):
- 2136835
- PAR ID:
- 10436508
- Date Published:
- Journal Name:
- Chemical Science
- Volume:
- 14
- Issue:
- 4
- ISSN:
- 2041-6520
- Page Range / eLocation ID:
- 751 to 770
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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In many applications of hydrated, dense polymer membranes—including fuel cells, desalination, molecular separations, electrolyzers, and solar fuels devices—the membrane is challenged with aqueous streams that contain multiple solutes. The presence of multiple solutes presents a complex process because each solute can have different interactions with the polymer membrane and with other solutes, which collectively determine the transport behavior and separation performance that is observed. It is critical to understand the theoretical framework behind and experimental considerations for understanding how the presence of multiple solutes impacts diffusion, and thereby, the design of membranes. Here, we review models for multicomponent diffusion in the context of the solution-diffusion framework and the associated experiments for characterizing multicomponent transport using diffusion cells. Notably, multicomponent effects are typically not considered when discussing or investigating transport in dense, hydrated polymer membranes, however recent research has shown that these effects can be large and important for understanding the transport behavior.more » « less
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Understanding the effects of polymer chemistry on membrane ion transport properties is critical for enabling efforts to design advanced highly permselective ion exchange membranes for water purification and energy applications. Here, the effects of fixed charge group type on anion exchange membrane (AEM) apparent permselectivity and ion transport properties were investigated using two crosslinked AEMs. The two AEMs, containing a similar acrylonitrile, styrene and divinyl benzene-based polymer backbone, had either trimethyl ammonium or 1,4-dimethyl imidazolium fixed charge groups. Membrane deswelling, apparent permselectivity and ion transport properties of the two AEMs were characterized using aqueous solutions of lithium chloride, sodium chloride, ammonium chloride, sodium bromide and sodium nitrate. Apparent permselectivity measurements revealed a minor influence of the fixed charge group type on apparent permselectivity. Further analysis of membrane swelling and ion sorption, however, suggests that less hydrophilic fixed charge groups more effectively exclude co-ions compared to more hydrophilic fixed charge groups. Analysis of ion diffusion properties suggest that ion and fixed charge group enthalpy of hydration properties influence ion transport, likely through a counter-ion condensation, ion pairing or binding mechanism. Interactions between fixed charge groups and counter-ions may be stronger if the enthalpy of hydration properties of the ion and fixed charge group are similar, and suppressed counter-ion diffusion was observed in this situation. In general, the hydration properties of the fixed charge group may be important for understanding how fixed charge group chemistry influences ion transport properties in anion exchange membranes.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1039/d2sc04920a
