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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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Assessing the Intrinsic Strengths of Ion–Solvent and Solvent–Solvent Interactions for Hydrated Mg2+ Clusters
Information resulting from a comprehensive investigation into the intrinsic strengths of hydrated divalent magnesium clusters is useful for elucidating the role of aqueous solvents on the Mg2+ ion, which can be related to those in bulk aqueous solution. However, the intrinsic Mg–O and intermolecular hydrogen bond interactions of hydrated magnesium ion clusters have yet to be quantitatively measured. In this work, we investigated a set of 17 hydrated divalent magnesium clusters by means of local vibrational mode force constants calculated at the ωB97X-D/6-311++G(d,p) level of theory, where the nature of the ion–solvent and solvent–solvent interactions were interpreted from topological electron density analysis and natural population analysis. We found the intrinsic strength of inner shell Mg–O interactions for [Mg(H2O)n]2+ (n = 1–6) clusters to relate to the electron density at the bond critical point in Mg–O bonds. From the application of a secondary hydration shell to [Mg(H2O)n]2+ (n = 5–6) clusters, stronger Mg–O interactions were observed to correspond to larger instances of charge transfer between the lp(O) orbitals of the inner hydration shell and the unfilled valence shell of Mg. As the charge transfer between water molecules of the first and second solvent shell increased, so did the strength of their intermolecular hydrogen bonds (HBs). Cumulative local vibrational mode force constants of explicitly solvated Mg2+, having an outer hydration shell, reveal a CN of 5, rather than a CN of 6, to yield slightly more stable configurations in some instances. However, the cumulative local mode stretching force constants of implicitly solvated Mg2+ show the six-coordinated cluster to be the most stable. These results show that such intrinsic bond strength measures for Mg–O and HBs offer an effective way for determining the coordination number of hydrated magnesium ion clusters.
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- Award ID(s):
- 1464906
- PAR ID:
- 10342523
- Date Published:
- Journal Name:
- Inorganics
- Volume:
- 9
- Issue:
- 5
- ISSN:
- 2304-6740
- Page Range / eLocation ID:
- 31
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/inorganics9050031
