Ion Permselectivity Research Articles

Labyrinth-inspired nitrogen-sulfur co-doped reduced holey graphene oxide/carbonized cellulose paper: A permselective and multifunctional interlayer for high-performance lithium-sulfur batteries

In this work, we report a rational design and facile synthetic process for the construction of a 3-dimensional N, S co-doped reduced holey graphene oxide/carbonized cellulose paper as interlayer in Li–S batteries. The interlayer has a labyrinth-like structure with plenty of nanosized holes to achieve the permselectivity for Li ions and effective intercept for polysulfides. The coin cell with the interlayer shows a high initial discharge capacity of 1263.5 mAh/g at 0.2 C, a low capacity fading rate of 0.0487% per cycle for 300 cycles at 1 C, and an excellent rate performance of 864.6 mAh/g at 2 C. Moreover, this cell also has excellent anti-self-discharge property and a high areal capacity of 6.1 mA h/cm2 at a sulfur loading of 8.1 mg/cm2 using aluminum foil as the current collector. Thus, this work suggests the permselective and multifunctional interlayer as a practical and effective strategy for the application in the high-performance Li–S battery.

From the Potential of the Mean Force to a Quasiparticle’s Effective Potential in Narrow Ion Channels

We consider the selective permeation of ions through narrow water-filled channels in the presence of strong interaction between the ions. These interactions lead to highly correlated ionic motion, which can conveniently be described via the concept of a quasiparticle. Here, we connect the quasiparticle’s effective potential and the multi-ion potential of the mean force, found through molecular dynamics simulations, and we validate the method on an analytical toy model of the KcsA channel. Possible future applications of the method to the connection between molecular dynamical calculations and the experimentally measured current-voltage and current-concentration characteristics of the channel are discussed.

Theory and Experiments on Multi-Ion Permeation and Selectivity in the NaChBac Ion Channel

The highly selective permeation of ions through biological ion channels is an unsolved problem of noise and fluctuations. In this paper, we motivate and introduce a non-equilibrium and self-consistent multi-species kinetic model, with the express aims of comparing with experimental recordings of current versus voltage and concentration and extracting important permeation parameters. For self-consistency, the behavior of the model at the two-state, i.e., selective limit in linear response, must agree with recent results derived from an equilibrium statistical theory. The kinetic model provides a good fit to data, including the key result of an anomalous mole fraction effect.

Transient membrane potential after concentration step: A new method for advanced characterization of ion-exchange membranes

A better understanding of ion-transport mechanisms requires separate information on the equilibrium (partitioning) and kinetic (diffusivity) properties of the membranes with respect to ions. This work presents a novel non-stationary-diffusion method to determine salt diffusion and partitioning coefficients under (quasi)-linear conditions of relatively small concentration differences.An ion-exchange membrane supported by a relatively thick coarse-porous support (glass frit) is placed in a two-compartment stirred cell. The salt concentration in one compartment is kept stationary during the measurement whereas in the other compartment, the initial solution is rapidly replaced by a solution of different concentration. As a result, there is a time-dependent electrical response due to a progressive redistribution of applied concentration difference between the membrane and the porous support and the different ion perm-selectivities of those media. A mathematical model is developed to interpret the data. The rate of signal relaxation is primarily controlled by the diffusion permeability of the membrane but is also affected by the salt partitioning. In addition, osmotic trans-membrane volume transfer has a significant impact on the relaxation process, so it needs to be taken into account. The osmotic permeability has been determined in separate measurements.Systematic studies have been carried out at various NaCl concentrations with Nafion 120 and type 10 Fujifilm ion-exchange membranes. The results obtained for Nafion 120 are in agreement with the literature data. Well-established techniques have also been used to validate the novel approach. This allows for a relatively simple determination of salt permeability and partitioning coefficient in addition to the ion perm-selectivity, which is the only parameter available from the conventional measurements of stationary membrane potential.

Bionic Thermoelectric Response with Nanochannels.

Thermosensation, the ability to detect environmental temperature change, is one of most ancient and crucial processes for the survival of all living organisms. Mammals use temperature-sensitive transient receptor potential (thermoTRP) cation channels as thermometers to convert the temperature change into electrical signals that are finally received as action potentials by nerve endings. In this work, we report the bionic thermosensation by solid-state hybrid nanochannels based on the principle of thermally sensitive permselective ion transport. The hybrid nanochannels possess an asymmetric structure, consisting of ultrasmall silica nanochannels (∼2.3 nm in diameter, ∼100 nm in length) supported by large-sized track-etched poly(ethylene terephthalate) conical nanochannels. When the hybrid nanochannels are engineered to separate two electrolyte solutions, the temperature change in one solution can be directly converted into trans-nanochannel diffusion potential, akin to the natural thermosensation process. Two bionic modes, namely, in the absence and presence of a concentration gradient, were studied to imitate the natural thermosensation of thermoTRP ion channels and shark, respectively. In both cases, real-time thermoelectric response was captured with a fast relative response speed (electrical response time versus temperature change time) of higher than 98%, as well as excellent stability and reversibility. Moreover, the nanochannels are highly sensitive to thermal stimulus, showing a sensitivity of 0.71 mV/K comparable to the natural thermosensation. The experimental results coincide well with the theoretical relationship between electrical response and temperature change derived in terms of a quasi-steady-state ion transport model. Finite element simulations based on coupled Poisson-Nernst-Planck (PNP) and Einstein-Stokes equations were also performed, confirming that the sensitive thermoelectric response originates from the highly cationic selectivity of nanochannels.

Ion transport and selectivity in thin film composite membranes in pressure-driven and electrochemical processes

This study investigates ionic transport and selectivity in a thin-film composite nanofiltration membrane (NF270) operated in pressure-driven and electrodialysis processes. The perm-selectivity and permeabilities of anions (SO42− and NO3−, SO42− and Cl−, NO3− and Cl−) and cations (Mg2+ and Na+) were studied in an electrodialysis cell operated with NF membrane (ED-NF) at varied trans-membrane potentials (0.5–2 V) and in conventional nanofiltration process. The permeabilities of all ions in the electrochemical process were one to four orders of magnitude lower than permeabilities reported for the pressure-driven NF operated with the same membrane. In the pressure-driven process, a high selectivity of mono-to di-valent ions was obtained, with anions selectivity higher than cations selectivity (highest Na+ to Mg2+ selectivity value of 4.0 vs. lowest Cl− to SO42− selectivity value of 11.6). In contrast, in ED a low perm-selectivity of mono-to di-valent ions was obtained (1.1–1.2 for Na+/Mg2+, and 2.4–3.4 for Cl−/SO42−).Our findings show that: (1) ionic transport in thin film composite membranes operated in electrochemical processes cannot be described by conventional ion exclusion models applied to pressure-driven processes, such as Born or Donnan exclusion mechanisms; and (2) a different, currently unknown, mechanism dominates the ion separation.

Ultrathin Polyamide Nanofiltration Membrane Fabricated on Brush-Painted Single-Walled Carbon Nanotube Network Support for Ion Sieving.

Recently, ultrathin polyamide nanofiltration membranes fabricated on nanomaterial-based supports have overcome the limitations of conventional supports and show greatly improved separation performance. However, the feasibility of the nanomaterial-based supports for large-scale fabrication of the ultrathin polyamide membrane is still unclear. Herein, we report a controllable and saleable fabrication technique for a single-walled carbon nanotube (SWCNT) network support via brush painting. The mechanical and chemical stability of the SWCNT network support were carefully examined, and an ultrathin polyamide membrane with thickness of ∼15 nm was successfully fabricated based on such a support. The obtained thin-film composite (TFC) polyamide nanofiltration membranes exhibited extremely high water permeability of ∼40 L m-2 h -1 bar-1, a high Na2SO 4 rejection of 96.5%, and high monovalent/divalent ion permeation selectivity and maintained highly efficient ion sieving throughout 48 h of testing. This work demonstrates a practical route toward the controllable large-scale fabrication of the TFC membrane with an SWCNT network support for ion and molecule sieving. This work is also expected to boost the mass production and practical applications of state-of-the-art membranes composed of one-dimensional and two-dimensional nanomaterials as well as the nanomaterial-supported TFC membranes.

Synthesis and characterization of organic-inorganic cross-linked membrane for the separation of mono-charged and double charged ions using UV irradiation

Currently, it is commonly observed that ion flux decreases as the separation performance of cation exchange membranes between mono- and double-charged ions improves. Therefore, cation exchange membranes with simultaneously excellent separation performance and high ionic flux must be developed. Herein, a series of pure and organic-inorganic cross-linked membranes were synthesized using UV irradiation. The synthetic membranes showed high ion-exchange capacity (IEC) and large ion flux. The organic-inorganic membranes achieved excellent mechanical properties, thermal stability, and separation properties owing to the addition of SiO2. When the silica content was 4 wt%, the three ionic perm-selectivity showed their highest separation performances of 1.70 ± 0.03(PKNa), 5.84 ± 0.04(PKMg), and 25.67 ± 0.04 (PKCa), representing an increase of 70% (PKNa), 279.22% (PKMg), and 1236.98% (PKCa) compared than those of pure membrane. In addition, the limiting current density of the prepared organic-inorganic membranes was all >8.74 mA/cm2, higher than that of commercial membrane. According to these excellent performances, the prepared membranes showed good prospects for separation performance of cation exchange membranes. The addition of a proper amount of inorganic particles can improve the ion perm-selectivity between mono- and double-charged ions.

Surface-Charge Effects on Voltammetry in Carbon Nanocavities.

Ion transport controlled by electrostatic interactions is an important phenomenon in biological and artificial membranes, channels, and nanopores. Here, we employ carbon-coated nanopipets (CNPs) for studying permselective electrochemistry in a conductive nanopore. A significant accumulation (up to 2000-fold) of cationic redox species and anion depletion inside a CNP by diffuse-layer and surface-charge effects in a solution of low ionic strength were observed as well as the shift of the voltammetric midpeak potential. Finite-element simulations of electrostatic effects on CNP voltammograms show permselective ion transport in a single conducting nanopore and semiquantitatively explain our experimental data. The reported results are potentially useful for improving sensitivity and selectivity of CNP sensors for ionic analytes.

Cross-linked anion exchange membranes with hydrophobic side-chains for anion separation

In this work, high performance cross-linked anion exchange membranes with hydrophobic side-chains were prepared via quaternization reaction and a simple heat treatment. The cross-linked membrane structure, the hydrophobic side-chain and the micro-phase separated morphology of membranes have synergistic effect on ion selective separation. Accordingly, the obtained membranes possess excellent ion perm-selectivity and considerable OH− flux. Especially, QPPO-C6-120 membrane exhibits the superior perm-selectivity of 76 for OH−/WO42− system and high OH− flux of 2.72 × 10−7 mol cm−2 s−1 at a current density of 30 mA cm−2. XPS and 1H NMR confirmed the introduction of long side-chain and the thermo-crosslinking reaction. Meanwhile, the microstructure of membranes was investigated by XRD and SAXS. More importantly, a new perspective of mesoscales by combining the molecule structure with the microstructure of membranes is innovatively used to explore the ion separation mechanism.

Systematic study on FRET-pair functionalization of mesoporous thin films for correlation of pH-sensing and ionic mesopore accessibility

Abstract Ordered mesoporous silica materials (OMS) are relevant in various applications such as separation, catalysis, or drug delivery. Thereby their performance is mainly dependent on mesopore accessibility and molecular transport within mesopores which is strongly influenced by the spatial confinement. This is especially relevant in case of charged mesopore walls and thus electrostatically controlled transport. Therefore, it is highly important to monitor charged states of functionalized mesopores. In this study we adapt pH-probing strategies, known from pH-monitoring e.g. in cells, to mesoporous thin films aiming to monitor mesopore charge and correlate this to mesopores ionic permselectivity. Therefore, grafting of a FRET dye pair, consisting of a pH-sensitive dye (fluorescein isothiocyanate (FITC)) and a pH-insensitive reference dye (rhodamine B isothiocyanate (RITC)), to the mesopore wall is optimized. The grafted FITC-RITC FRET pair allows to monitor mesopore “pH” over a broad range of up to six pH-units after optimization of FRET pair functionalization. Thereby, “pH” monitoring is performed based on absorption and fluorescence readout using confocal microscopy baring the potential of localized detection. The detected pH-dependent absorption and fluorescence emission is in good agreement with the pH-dependent ionic mesopore permselectivity, and thus with the influence of mesopore confinement on ionic permselectivity as detected by cyclic voltammetry. In addition, this study not only shows the possibility of using FRET dye pairs for monitoring charged states in mesopores but as well the influence of these probing dye molecules on the mesopores characteristics, such as ionic permselectivity, indicating the need of optimizing functionalization with probe molecules.

Application of homogeneous membrane electrodialysis in the treatment of high-salt organic wastewater

In this paper, a homogeneous membrane electrodialysis system was used to separate the high COD and high sulfate concentrated water from a non-ferrous metal plant. After continuous pilot test, the results showed that the homogeneous membrane electrodialysis method has the characteristics of low energy consumption, high concentration ratio, strong ion selective permeability and effectively remove organic matter. Therefore, the production cost can be reduced and the recycling of waste water resources can be realized.

Selective Ion Exchange in Supramolecular Channels in the Crystalline State

AbstractArtificial ion channels are of increasing interest because of potential applications in biomimetics, for example, for realizing selective ion permeability through the transport and/or exchange of selected ions. However, selective ion transport and/or exchange in the crystalline state is rare, and to the best of our knowledge, such a process has not been successfully combined with changes in the physical properties of a material. Herein, by soaking single crystals of Li2([18]crown‐6)3[Ni(dmit)2]2(H2O)4 (1) in an aqueous solution containing K+, we succeeded in complete ion exchange of the Li+ ions in 1 with K+ ions in the solution, while maintaining the crystalline state of the material. This ion exchange with K+ was selectively conducted even in mixed solutions containing K+ as well as Na+/Li+. Furthermore, remarkable changes in the physical properties of 1 resulted from the ion exchange. Our finding enables not only the realization of selective ion permeability but also the development of highly sensitive biosensors and futuristic ion exchange agents, for example.

Selective Ion Exchange in Supramolecular Channels in the Crystalline State.

Artificial ion channels are of increasing interest because of potential applications in biomimetics, for example, for realizing selective ion permeability through the transport and/or exchange of selected ions. However, selective ion transport and/or exchange in the crystalline state is rare, and to the best of our knowledge, such a process has not been successfully combined with changes in the physical properties of a material. Herein, by soaking single crystals of Li2 ([18]crown-6)3 [Ni(dmit)2 ]2 (H2 O)4 (1) in an aqueous solution containing K+ , we succeeded in complete ion exchange of the Li+ ions in 1 with K+ ions in the solution, while maintaining the crystalline state of the material. This ion exchange with K+ was selectively conducted even in mixed solutions containing K+ as well as Na+ /Li+ . Furthermore, remarkable changes in the physical properties of 1 resulted from the ion exchange. Our finding enables not only the realization of selective ion permeability but also the development of highly sensitive biosensors and futuristic ion exchange agents, for example.

Preparation of PVA-Based Hollow Fiber Ion-Exchange Membranes and Their Performance for Donnan Dialysis.

Hollow fiber type cation-exchange (C-HF) membranes and hollow fiber type anion-exchange (A-HF) membranes were prepared from poly (vinyl alcohol) (PVA)-based copolymer with cation-exchange groups and by blending PVA and polycation, respectively, by a gel fiber spinning method. In order to control the water content of the hollow fiber membranes, the membranes were cross-linked physically by annealing, and then cross-linked chemically by using glutaraldehyde (GA) solutions at various GA concentrations. The outer diameter of C-HF and A-HF membranes were ca. 1000 μm and ca. 1500 μm, respectively, and the thickness of the membranes were ca. 170 μm and 290 μm, respectively. Permeation experiments were carried out in two Donnan dialysis systems, which included mixed 0.1 M NaCl and 0.1 M CaCl2/C-HF /3 × 10−4 M CaCl2 and mixed 0.1 M NaCl and 0.1 M NaNO3/A-HF/3 × 10−4 M NaNO3 to examine ionic perm selectivity of the membranes. In the Donnan dialysis experiments using C-HF membranes, uphill transport of the divalent cations occurred, and, in the case of A-HF membranes, uphill transport of NO3− ions occurred. C-HF and A-HF membranes had about half of the flux in the uphill transported ions and also about half of the selectivity between the uphill transport ions and driven ions in comparison with those of the commercial flat sheet cation-exchange membrane (Neosepta® CMX) and anion-exchange membrane (Neosepta® AMX). Yet, IEC of C-HF and A-HF membranes were about one fifth of CMX and less than half of AMX, respectively. Since hollow fiber membrane module will have higher packing density than a flat membrane stack, the hollow fiber type ion-exchange membranes (IEMs) prepared in this study will have a potential application to a Donnan dialysis process.

Synthesis and characterization of a plat sheet potassium ion sieve membrane and its performances for separation potassium

Most of the ion exchange membranes distinguished monovalent ions from the multivalent ions according to the charge difference, but scarcely had selectivity to the specific ions. In order to effectively separate potassium ions from seawater containing more than 30 times sodium ions, potassium ion-sieve composite membranes with specific selectivity to potassium ions had been synthesized in this paper. The potassium ion sieve was successfully synthesized on the flat α-Al2O3 membrane by secondary hydrothermal method to form potassium ion sieve composite membrane. Then, the results of SEM, XRD and EDS showed that the potassium ion sieve had been successfully bonded with base membrane. The results of electrodialysis experiments showed that the ionic perm-selectivity were α (K/Na) = 7.62, α (K/Mg) = 68.43 and α (K/Ca) = 162.92 respectively, which indicated that the composite membrane had special selectivity for potassium ions. The composite membrane had been evaluated under different voltages and temperatures. As the voltage increased, the ion flux and the perm-selectivity to potassium were all improved. Moreover, with the increase of temperature, the ion flux increased but the selectivity to potassium decreased. The separation performance of the composite membrane was studied by using equimolar solution (containing K+, Na+, Ca2+, Mg2+), seawater and twice concentrated seawater as the test solutions, and the results had exhibited that the composite membrane had good application performance in extracting potassium from seawater.

Low-Resistant Ion-Exchange Membranes for Energy Efficient Membrane Capacitive Deionization

Membrane capacitive deionization (MCDI) has emerged as an effective and energy efficient desalination technology for treating brackish water streams used in numerous industrial processes. Most material research studies on MCDI focus on improving the porous electrodes or using flowing electrode architectures, and little emphasis is given to the rationale design of ion-exchange membranes (IEMs) for MCDI. In this work, the ionic conductivity, permselectivity, and thickness for three different IEM chemistries (polyaliphatic, poly(arylene ether), and perfluorinated) were correlated to MCDI performance attributes: energy expended per ion removed, salt removal efficiency, and Coulombic efficiency. A 5- to 10-fold reduction in area specific resistance, which accounts for thickness and ionic conductivity, with unconventional perfluorinated and poly(arylene ether) IEMs reduced the energy expended per ion removed in MCDI by a factor of 2 when compared to conventional electrodialysis IEMs. In situ electrochemical imp...

Selective ion permeation involves complexation with carboxylates and lysine in a model human sodium channel.

Bacterial and human voltage-gated sodium channels (Navs) exhibit similar cation selectivity, despite their distinct EEEE and DEKA selectivity filter signature sequences. Recent high-resolution structures for bacterial Navs have allowed us to learn about ion conduction mechanisms in these simpler homo-tetrameric channels, but our understanding of the function of their mammalian counterparts remains limited. To probe these conduction mechanisms, a model of the human Nav1.2 channel has been constructed by grafting residues of its selectivity filter and external vestibular region onto the bacterial NavRh channel with atomic-resolution structure. Multi-μs fully atomistic simulations capture long time-scale ion and protein movements associated with the permeation of Na+ and K+ ions, and their differences. We observe a Na+ ion knock-on conduction mechanism facilitated by low energy multi-carboxylate/multi-Na+ complexes, akin to the bacterial channels. These complexes involve both the DEKA and vestibular EEDD rings, acting to draw multiple Na+ into the selectivity filter and promote permeation. When the DEKA ring lysine is protonated, we observe that its ammonium group is actively participating in Na+ permeation, presuming the role of another ion. It participates in the formation of a stable complex involving carboxylates that collectively bind both Na+ and the Lys ammonium group in a high-field strength site, permitting pass-by translocation of Na+. In contrast, multiple K+ ion complexes with the DEKA and EEDD rings are disfavored by up to 8.3 kcal/mol, with the K+-lysine-carboxylate complex non-existent. As a result, lysine acts as an electrostatic plug that partially blocks the flow of K+ ions, which must instead wait for isomerization of lysine downward to clear the path for K+ passage. These distinct mechanisms give us insight into the nature of ion conduction and selectivity in human Nav channels, while uncovering high field strength carboxylate binding complexes that define the more general phenomenon of Na+-selective ion transport in nature.

Directional coupling of oligodendrocyte connexin-47 and astrocyte connexin-43 gap junctions.

Intercellular communication via gap junction channels between oligodendrocytes and between astrocytes as well as between these cell types is essential to maintain the integrity of myelin in the central nervous system. Oligodendrocyte gap junction connexin-47 (Cx47) is a key element in this crosstalk and indeed, mutations in human Cx47 cause severe myelin disorders. However, the permeation properties of channels of Cx47 alone and in heterotypic combination with astrocyte Cx43 remain unclear. We show here that Cx47 contains three extra residues at 5' amino-terminus that play a critical role in the channel pore structure and account for relative low ionic conductivity, cationic permselectivity and voltage-gating properties of oligodendrocyte-oligodendrocyte Cx47 channels. Regarding oligodendrocyte-astrocyte coupling, heterotypic channels formed by Cx47 with Cx43 exhibit ionic and chemical rectification, which creates a directional diffusion barrier for the movement of ions and larger negatively charged molecules from cells expressing Cx47 to those with Cx43. The restrictive permeability of Cx47 channels and the diffusion barrier of Cx47-Cx43 channels was abolished by a mutation associated with leukodystrophy, the Cx47P90S, suggesting a novel pathogenic mechanism underlying myelin disorders that involves alterations in the panglial permeation.

Determinants of selective ion permeation in the epithelial Na+ channel.

The epithelial Na+ channel (ENaC) is a key transporter mediating and controlling Na+ reabsorption in many tight epithelia. A very high selectivity for Na+ over other cations, including K+, is a hallmark of this channel. This selectivity greatly exceeds that of the closely related acid-sensing channels (ASICs). Here, we assess the roles of two regions of the ENaC transmembrane pore in the determination of cation selectivity. Mutations of conserved amino acids with acidic side chains near the cytoplasmic end of the pore diminish macroscopic currents but do not decrease the selectivity of the channel for Na+ versus K+ In the WT channel, voltage-dependent block of Na+ currents by K+ or guanidinium+, neither of which have detectable conductance, suggests that these ions permeate only ∼20% of the transmembrane electric field. According to markers of the electric field determined by Zn2+ block of cysteine residues, the site of K+ block appears to be nearer to the extracellular end of the pore, close to a putative selectivity filter identified using site-directed mutations. To test whether differences in this part of the channel account for selectivity differences between ENaC and ASIC, we substitute amino acids in the three ENaC subunits with those present in the ASIC homotrimer. In this construct, Li:Na selectivity is altered from that of WT ENaC, but the high Na:K selectivity is maintained. We conclude that a different part of the pore may constitute the selectivity filter in the highly selective ENaC than in the less-selective ASIC channel.