Dehydration-enhanced ion-pore interactions dominate anion transport and selectivity in nanochannels

Abstract

State-of-the-art ion-selective membranes with ultrahigh precision are of significance for water desalination and energy conservation, but their development is limited by the lack of understanding of the mechanisms of ion transport at the subnanometer scale. Herein, we investigate transport of three typical anions (F^-, Cl^-, and Br^-) under confinement using in situ liquid time-of-flight secondary ion mass spectrometry in combination with transition-state theory. The operando analysis reveals that dehydration and related ion-pore interactions govern anion-selective transport. For strongly hydrated ions [(H₂O)_nF^- and (H₂O)_nCl^-], dehydration enhances ion effective charge and thus the electrostatic interactions with membrane, observed as an increase in decomposed energy from electrostatics, leading to more hindered transport. Contrarily, weakly hydrated ions [(H₂O)_nBr^-] have greater permeability as they allow an intact hydration structure during transport due to their smaller size and the most right-skewed hydration distribution. Our work demonstrates that precisely regulating ion dehydration to maximize the difference in ion-pore interactions could enable the development of ideal ion-selective membranes.

Fig. 1.. In situ characterization of h_X⁻ distribution during transmembrane ion transport.

(A) Schematic diagram showing in situ liquid ToF-SIMS analysis combined with the microfluidic nanofiltration platform. The detailed procedure of this method is given in Materials and Methods. (B to D) Variation of h_X⁻ distributions before and after filtration of 10 mM NaF, NaCl, and NaBr, respectively. The left panel of each figure represents the h_X⁻ distributions in the bulk solution (i.e., before filtration), the middle panel represents the h_X⁻ distributions after filtration by nonsterically limited membrane (NF 800), and the right panel represents the h_X⁻ distributions after filtration by sterically limited membrane (NF 200). h_avg denotes the weighted average strongly bound water number of the corresponding h_X⁻ distributions only for mathematical comparison. The dotted lines in all panels are provided to guide the eye. Error bars represent SDs.

Fig. 2.. Elucidation of the critical role of dehydration during ion partition using TST.

(A) The apparent energy for hydrated anions entering the polyamide membranes (i.e., partitioning) and diffusing inside the polyamide membranes (i.e., intrapore diffusion) obtained by the hindered transport theory in combination with TST. (B) Cross-sectional illustration of the simulation platform. The cyan part represents the water-filled reservoirs with NaX solution (X = F, Cl, and Br) connected by a polyamide nanochannel. The simulated bulk polyamide was omitted for improved clarity (more details in the Supplementary Materials, fig. S2). (C) MD simulation of the change in the PMF during ion transport across polyamide nanochannels with different size. (D) Apparent energy for NaX salts partitioning into the active layer of membranes quantified by QCM. (E) Measured overall apparent transmembrane energy barriers (E_a) for anion transport through polyamide membranes. (F) Correlation between measured energy of enthalpic component (ΔH^‡) and entropic component (−TΔS^‡) for anions at 298.15 K, showing an EEC in anion transport. The data were collected from the ΔH^‡ and −TΔS^‡ of the three anions (i.e., F⁻, Cl⁻, and Br⁻) during transport in NF 200 and NF 800. The shaded areas represent the SDs of the enthalpic component (ΔH^‡). The blue arrow indicates that stronger dehydration is accompanied by higher entropic compensation.

Fig. 3.. Illustration of dehydration-enhanced electrostatic interactions on ions during transmembrane transport.

(A) Energy contributed to apparent energy barriers by various steps throughout the transmembrane transport. (B) Variation of hydration number and PMF of (H₂O)_nF⁻ in polyamide nanochannels with different sizes. The gray shading indicates that the change in hydration number at the pore entrance is accompanied by an increase in PMF. The blue shading indicates the change in energy following dehydration. (C) DFT calculation of effective charge for hydrated anions with various hydration numbers. (D) Schematic diagram showing (i) ion transport and (ii) energy profile of strongly hydrated anions for elucidating the mechanisms of dehydration-enhanced electrostatic interactions. Hydrates permeate across the membrane as a result of partitioning into the active layer and diffusion through the active layer. For strongly hydrated ions [e.g., (H₂O)_nF⁻] under confinement, these larger hydrates would partially dehydrate to reduce their effective size for permeation, but this simultaneously enhances the electrostatic repulsion between negative carboxyl groups and dehydrated anions, contributing higher energy barriers. Yellow and green balls represent the fluoride (F⁻) and negative carboxyl, respectively.

Fig. 4.. Ion transport and selectivity of nanoporous polymeric nanochannels.

(A) Schematic diagram showing dehydration-enhanced electrostatic interactions on hydrates. Partially dehydrated anions with higher effective charge [e.g., (H₂O)_nF⁻] could interact more closely with the ionized carboxyl groups and are thus repulsed intensely by the membrane. Red arrows represent the intensity of electrostatic repulsion and dominant transport direction of hydrated anions, where a lighter color represents stronger repulsion. (H₂O)_nBr⁻, which maintains an intact hydrated shell during permeation and interacts weakly with the ionized carboxyl groups thus has greater permeability across the membrane (light blue arrow). (B) Intrinsic permeability of monovalent anions in nanofiltration under different pH. (C) Anion selectivity ratio (SR_X⁻/F⁻) of the two polyamide nanofiltration membranes (i.e., sterically limited NF 200 and nonsterically limited NF 800) under different pH. (D) Zeta potential change with respect to pH for NF 200 and NF 800 membranes. (E) Normalized selectivity ratio between Br⁻ and F⁻, i.e., SR_Br⁻/F⁻, under different driving forces in NF 200. All selectivity ratios were normalized on the basis of the SR_Br⁻/F⁻ of NF 200 in nanofiltration at pH 6.0.