Permeation Rates Of Ions Research Articles

Angstrom‐scale channels with versatile ion‐membrane interactions enabling precise ion separation via electrodialysis

AbstractIn nature, efficient and selective ion transport is facilitated by ion‐conductive channels in cell membranes; these channels reveal an architectural design with specialized functionality. Drawing inspiration from this, mechanistic insights into the angstrom‐scale‐channel membrane composed of ionic‐crosslinked polybenzimidazole and sulfonated poly(ether ether ketone), exhibiting functional differentiation and efficient ion‐sieving properties are presented. Nanochannels allow for strong hydrogen‐bonding interactions with hydrated ions of higher polarity, while rendering significant electrostatic charge effects that impede the transition of multivalent ions by compressing effective passageways. Both hydrogen bonding and electrostatic interactions synergistically result in high selectivity for monovalent ions over multivalent ions because the latter requires overcoming higher energy barriers for transport compared with the former, thereby causing varying extents of ion dehydration within the nanochannels. The resulting membrane achieves a high monovalent ion permeation rate of 1.35 mol m−2 h−1 with a high mono/multivalent ion selectivity of 56.5 for K+/Mg2+ and 286 for K+/Al3+.

Electrochemical-repaired porous graphene membranes for precise ion-ion separation

The preparation of atom-thick porous lattice hosting Å-scale pores is attractive to achieve a large ion-ion selectivity in combination with a large ion flux. Graphene film is an ideal selective layer for this if high-precision pores can be incorporated, however, it is challenging to avoid larger non-selective pores at the tail-end of the pore size distribution which reduces ion-ion selectivity. Herein, we develop a strategy to overcome this challenge using an electrochemical repair strategy that successfully masks larger pores in large-area graphene. 10-nm-thick electropolymerized conjugated microporous polymer (CMP) layer is successfully deposited on graphene, thanks to a strong π-π interaction in these two materials. While the CMP layer itself is not selective, it effectively masks graphene pores, leading to a large Li+/Mg2+ selectivity from zero-dimensional pores reaching 300 with a high Li+ ion permeation rate surpassing the performance of reported materials for ion-ion separation. Overall, this scalable repair strategy enables the fabrication of monolayer graphene membranes with customizable pore sizes, limiting the contribution of nonselective pores, and offering graphene membranes a versatile platform for a broad spectrum of challenging separations.

Lithium-ion extraction using electro-driven freestanding graphene oxide composite membranes

Lithium-ion extraction and recovery from unconventional aqueous sources by utilizing membranes as a separation barrier are potential technologies to address the challenge of increased lithium demand. Separating lithium from other competing metal ions is challenging due to their low selectivity when using conventional membranes as selective barriers. Graphene oxide (GO) nanofluidic membranes possess a higher monovalent ion permeation rate than multivalent ions, however, the selectivity of lithium over other monovalent ions is low. In this study, we created freestanding GO-based membranes with enhanced Li+/monovalent ions selectivity to concentrate and recover lithium from aqueous solutions containing competing monovalent ions (e.g., K+ and Na+). Specifically, sulfonate groups are introduced into the membrane matrices by intercalating the spacing agent, polystyrene sulfonate (PSS), into the adjacent GO nanosheets. Additionally, neutral and positively charged spacing agents are also used for comparison, and to further investigate the transport mechanisms. The Li+ permeation rate of GOM-S (PSS-incorporated GO membrane) is higher than the other GO-based membranes in a single salt solution. In a mixed salt solution, the Li+/K+ selectivity of GOM-S depends on concentration and we observed the highest value, 3.43, when using 0.5 M LiCl and 0.5 M KCl as the feed (high) concentrations and 0.001 M LiCl and 0.001 M KCl as permeate (low) concentrations. We observed that as permeate concentration decreased the selectivity increased and the pH values from 3 to 10 did not change the selectivity. DFT calculations show that the Li–SO3H pair has smaller binding energy, facilitating lithium-ion transport along the membrane nanochannel. Also, the solution concentration is crucial to the competitive mixed ionic transport, which results from the preferential surface transport induced by the electrical double layer (EDL) extension within the confined membrane nanochannels.

A monovalent selective anion exchange membrane made by poly(2,6-dimethyl-1,4-phenyl oxide) for bromide recovery

Bromide recovery from waste streams is crucial for both environmental and economic reasons. Membrane technology has shown great potential in achieving efficient ion separation, credited to the use of separation membranes with well-defined pore structure down to molecular level and surface charge properties. Herein, a class of novel anion exchange membranes (AEMs) was fabricated by brominated poly (2,6-dimethyl-1, 4-phenyl oxide) (BPPO) and different content of 4-hydroxy-1-methylpiperidine (HMP) via the chemical substitution reaction for Br- recovery. The resulting membrane (HMP@BPPO-0.1) had an ion exchange capacity of 1.69 mmol g−1, a water content of 14.4 %, a swelling rate of 4.7 %, a surface electrical resistance of 3.68 Ω cm2 and a limiting current density of 28.4 mA cm−2. In addition, under a constant voltage of 15.0 V for 180 min in electrodialysis, the removal ratio of NaBr solution was 98.15 %, which was higher than that of the commercial AEM (ASE) (97.1 %) under the same testing conditions. To evaluate the Br- selectivity, the membranes were tested for an initial concentration of 50 mM of NaCl, NaBr and Na2SO4 in a mixed solution with a constant current density of 2.5 mA cm−2, 5.0 mA cm−2 and 10.0 mA cm−2. The results showed that the permselectivity of the HMP@BPPO-0.1 for 100 min was as high as 12.6 (Br-/SO42-) and 1.20 (Br-/Cl-) under the 5.0 mA cm−2 electric field, and the ion permeation rate of Cl-, Br- and SO42- were 0.78, 1.20 and 0.20 mol−1 m-2 h−1, respectively. Compared to the ASE membrane and the resulting membrane with negatively charged modification layer, this work suggests novel ideas in fabricating a membrane with Br- selectivity in view of resource recovery.

Hydrophobic hydrogels as internal curing agent for concrete: The double benefit of super high water content and excellent anti-ion permeability

Owing to their high water content, conventional superabsorbent polymer (SAP) hydrogels can serve as internal curing agents for concrete fabrication. However, the hydrophilic network in such hydrogels permits ion diffusion, which induces severe steel corrosion inside concrete in the marine environment. Herein, we introduce a brand-new hydrogel, termed “hydrophobic hydrogels”, for concrete curing. On one hand, these hydrogels possess super high water content (up to 99.6 wt%), which is even higher than that in normal SAP hydrogels; on the other hand, the anhydrous inhomogeneous hydrophobic structures can significantly retard or stop the ion permeation, which is significantly superior to SAP hydrogels. With ion diffusion measurement, the hydrophobic hydrogels achieve much better ion resistance than SAP hydrogels, with 2–5 orders' decrease in ion permeation rate. Among diverse hydrophobic hydrogels, the polymer's hydrophobicity and glass transition directly determine the permeation performance. Hydrogels with a more hydrophobic network and higher polymer Tg possess better anti-ion diffusion properties. At the optimal state, the poly(phenyl acrylate) hydrophobic hydrogels can completely stop the permeation process. Owing to the double benefit, the hydrophobic hydrogel cured concretes demonstrate a notable decrease in ion permeation compared to SAP hydrogel cured concrete, with only 1/20 of ion permeation to that in SAP groups with an electric flux test upon coated on the surface layers. Owing to the double benefit from super high water content and excellent anti-ion permeability, these hydrophobic hydrogels may serve as ideal candidates as concrete curing agents in the future.

Alternating electric field-based ionic control and layer-by-layer assembly of anion exchange membranes for enhancing target anion selectivity

Separation and recovery of target anions from wastewater is a sustainable method for chemical production industry. In this study, the novel anion exchange membranes with high antifouling and target ion selectivity performance were designed by the layer-by-layer (LbL) assembly of dopamine, positive and negative polyelectrolytes under the alternating current electric field and ionic control (AC[ion]) process. The resulting membranes were fabricated with different LbL assembled multilayers to evaluate the target ion selectivity. Besides, the different LbL assembled methods, including the electrostatic deposition self-assembly (EDS), AC[Cl−] assembly and AC[NO3−] assembly methods were carried out to evaluate the ion effect in membrane assembly process on final target ion permeation rate, respectively. With the initial concentration of 50 mM of NaCl, NaNO3 and Na2SO4 in mixed solution and the constant current density of 5.00 mA cm−2 of electrodialysis, the permselectivity of resulting membrane could be enhanced by the increased multilayers. Besides, the ion control in the alternating current electric field assembly process could enhance the relevant target ion permeation rate. The increased values of membrane surface electrical resistance fouled by organic pollutants were used to evaluate the antifouling property of membranes in electrodialysis, and the resulting membranes showed a high antifouling performance. This work provided a promising membrane separation technology in target anion recovery from wastewater for sustainable development.

Enhancement mechanism of an improved liquid membrane using selective permeation retardant for heavy metal ions separation

Liquid membrane (LM) separation is a kinetic process that can be enhanced by increasing the difference between the permeation rates of the components. The addition of H3Cit/Na3Cit as a selective permeation retardant can enlarge the difference between the permeation rates of Cu(II) and Cd(II) ions when P507 was used as the extractant. This retardant is analogous to a masking agent employed in liquid–liquid extraction (LLE). However, the retardant mainly affects the LM’s kinetics, while the masking agent impacts the LLE's equilibrium. It was found that the separation coefficient of the H3Cit/Na3Cit LM system could be much greater than that of the H3Cit/Na3Cit LLE system owing to the LM's kinetic effect. Following the addition of H3Cit/Na3Cit, the Cu(II) ion permeation rate was selectively retarded, and the LM selectivity of Cd(II)/Cu(II) was significantly enhanced in comparison with a control LM system. To reveal this enhancement mechanism, a new LM kinetic model was established in which the interfacial chemical reactions and mass transfer resistance in the middle connection zone of a H-type device were considered. The model's predictions agreed well with the experimental data. According to the model' quantitative analysis, the retardant affected the apparent reaction rate constants of Cd(II) and Cu(II) ions differently, and this is key to the enhancement mechanism. Aside from concentration, the LM model also predicted the variations in separation coefficient, purity, and yield with time. An optimal LM extraction time zone was defined to obtain high yield and high purity simultaneously. Finally, the enhanced LM was further verified and applied on a preliminary basis in a supported liquid membrane with strip dispersion, which is a LM technology with potential for industrial application.

チャネルの入口がイオンの透過速度を決める

チャネルの入口がイオンの透過速度を決める

Potential application of graphene oxide membranes for removal of Cs(I) and Sr(II) from high level-liquid waste

Abstract As a new carbon-based membrane material used for ionic and molecular sieving, the graphene oxide (GO) membrane also has potential application in radiochemical separation. In this work, the permeation performances of some typical metallic elements in high level-liquid waste (HLLW) through the ionic sieving 5-μm-thick GO membranes were investigated. It was found that the permeation rates of ions through GO membranes markedly decreased with the increase of hydrated ionic radii. Cs(I) and Sr(II) permeated through GO membranes quickly, whereas lanthanide ions and actinide ions infiltrated much more slowly, thus providing the feasibility of separation according to the difference of hydrated ionic radii. Moreover, the concentrations of initial metal ions and acidity in feed solutions also had influences on the permeation of ions through GO membranes, and higher initial metal ions and feed solution acidity were favorable to the removal of Cs(I) and Sr(II). The barrier separation test indicated that GO membranes had promising ability for the separation of Cs(I) and Sr(II) from lanthanide and actinide ions due to the difference of hydrated ion radius, and the separation factor of Cs(I)/U(VI) and Sr(II)/U(VI) would reach about 10 and 3 in single penetration. The presented results demonstrated that the membrane separation based on GO membrane would be a promising candidate for the removal of Cs(I) and Sr(II) from HLLW.

K+ Occupancy in the Cavity Determines the Ion Permeation Rate through the KV1.2 Channel

K+ Occupancy in the Cavity Determines the Ion Permeation Rate through the KV1.2 Channel

Photocontrol of ion permeation in lipid vesicles with (bola)amphiphilic spirooxazines.

Three (bola)amphiphilic spirooxazines have been synthesized and their photochromism has been characterized. The large biphotochromic structure of 2 significantly affects its conformational flexibility and the rate constants for thermal ring closure are particularly dependent on the lipid phase state. Two comprehensive ion permeation studies were performed to examine the effect of spirooxazine inclusion and isomerization on membrane permeability. In all cases, the open-ring isomers of these spirooxazines are more disruptive in bilayer membranes than their closed-ring isomers. Further, the rate of ion permeation and net release are highly dependent on the lipid bilayer phase state and the relative position of the photochromic moiety in the bilayer membrane. Moreover, the difference in potassium ion permeability under UV and visible irradiation is more pronounced than previously reported photoresponsive membrane disruptors with reversible photocontrols.

The First Extracellular Domain Plays an Important Role in Unitary Channel Conductance of Cx50 Gap Junction Channels.

Gap junction (GJ) channels provide direct passage for ions and small molecules to be exchanged between neighbouring cells and are crucial for many physiological processes. GJ channels can be gated by transjunctional voltage (known as Vj-gating) and display a wide range of unitary channel conductance (γj), yet the domains responsible for Vj-gating and γj are not fully clear. The first extracellular domain (E1) of several connexins has been shown to line part of their GJ channel pore and play important roles in Vj-gating properties and/or ion permeation selectivity. To test roles of the E1 of Cx50 GJ channels, we generated a chimera, Cx50Cx36E1, where the E1 domain of Cx50 was replaced with that of Cx36, a connexin showing quite distinct Vj-gating and γj from those of Cx50. Detailed characterizations of the chimera and three point mutants in E1 revealed that, although the E1 domain is important in determining γj, the E1 domain of Cx36 is able to effectively function within the context of the Cx50 channel with minor changes in Vj-gating properties, indicating that sequence differences between the E1 domains in Cx36 and Cx50 cannot account for their drastic differences in Vj-gating and γj. Our homology models of the chimera and the E1 mutants revealed that electrostatic properties of the pore-lining residues and their contribution to the electric field in the pore are important factors for the rate of ion permeation of Cx50 and possibly other GJ channels.

Functional roles of the amino terminal domain in determining biophysical properties of Cx50 gap junction channels

Communication through gap junction channels is essential for synchronized and coordinated cellular activities. The gap junction channel pore size, its switch control for opening/closing, and the modulations by chemicals can be different depending on the connexin subtypes that compose the channel. Recent structural and functional studies provide compelling evidence that the amino terminal (NT) domains of several connexins line the pore of gap junction channels and play an important role in single channel conductance (γj) and transjunctional voltage-dependent gating (Vj-gating). This article reviews recent studies conducted on a series of mutations/chimeras in the NT domain of connexin50 (Cx50). Functional examination of the gap junction channels formed by these mutants/chimeras shows the net charge number at the NT domain to be an important factor in γj and in Vj-gating. Furthermore, with an increase in the net negative charge at the NT domain, we observed an increase in the γj as well as changes in the parameters of the Boltzmann fit of the normalized steady-state conductance and Vj relationship. Our data are consistent with a structural model where the NT domain of Cx50 lines the gap junction pore and plays an important role in sensing Vj and in the subsequent conformational changes leading to gating, as well as in limiting the rate of ion permeation.

Crucial Points within the Pore as Determinants of K+ Channel Conductance and Gating

While selective for K+, K+ channels vary significantly among their rate of ion permeation. Here, we probe the effect of steric hindrance and electrostatics within the ion conduction pathway on K+ permeation in the MthK K+ channel using structure-based mutagenesis combined with single-channel electrophysiology and X-ray crystallography. We demonstrate that changes in side-chain size and polarity at Ala88, which forms the constriction point of the open MthK pore, have profound effects on single-channel conductance as well as open probability. We also reveal that the negatively charged Glu92s at the intracellular entrance of the open pore form an electrostatic trap, which stabilizes a hydrated K+ and facilitates ion permeation. This electrostatic attraction is also responsible for intracellular divalent blockage, which renders the channel inward rectified in the presence of Ca2+. In light of the high structural conservation of the selectivity filter, the size and chemical environment differences within the portion of the ion conduction pathway other than the filter are likely the determinants for the conductance variations among K+ channels.

Electrochemical Charaterization of Steel Concrete under Multi-Factors Influence

The corrosion of steel concrete under the multi-factor of hygrothermal marine environment and flexural stress-salt spray accelerated corrosion environment was studied by potentodynamic polarization method and electrochemical impedance spectroscopy. Under the hygrothermal marine environment (Wan-ning), with the prolonging of period, passive film on the steel was depolarized, and when the period was two years, the steel started corrosion. The result show that flexural stress could accelerate chlorine ion permeation rate on the concrete, and make passive film depolarize more early. With the passive film’s depolarization, the interfacial reaction of steel/concrete develops from electrochemical activation to mass transport limitation, and the reaction resistance decrease.

Computational prediction of ion permeation characteristics in the glycine receptor modified by photo-sensitive compounds.

We conduct computational analyses of ion permeation characteristics in a model glycine receptor (GlyR) modified by photo-sensitive compounds. In particular, we consider hypothetical attachment to the channel of charge-neutral chemical groups which can be photo-activated by shining light of an appropriate wavelength on the system. After illumination, the attached molecules become charged via a photodissociation process or excited into a charge-separated state (thus generating a significant electric dipole). We carry out Brownian Dynamics simulations of ion flow through the channel in the presence of the additional charges generated in this fashion. Based on these calculations, we predict that photo-activation of appropriately positioned photo-sensitive compounds near the channel mouth can significantly modify the rate of ion permeation and the current rectification ratio. Possible implications for GlyR-based device designs are briefly discussed.

Stability of the Shab K + Channel Conductance in 0 K + Solutions: The Role of the Membrane Potential

Shab channels are fairly stable with K + present on only one side of the membrane. However, on exposure to 0 K + solutions on both sides of the membrane, the Shab K + conductance ( G K) irreversibly drops while the channels are maintained undisturbed at the holding potential. Herein it is reported that the drop of G K follows first-order kinetics, with a voltage-dependent decay rate r. Hyperpolarized potentials drastically inhibit the drop of G K. The G K drop at negative potentials cannot be explained by a shift in the voltage dependence of activation. At depolarized potentials, where the channels undergo a slow inactivation process, G K drops in 0 K + with rates slower than those predicted based on the behavior of r at negative potentials, endowing the r- V m relationship with a maximum. Regardless of voltage, r is very small compared with the rate of ion permeation. Observations support the hypothesized presence of a stabilizing K + site (or sites) located either within the pore itself or in its external vestibule, at an inactivation-sensitive location. It is argued that part of the G K stabilization achieved at hyperpolarized potentials could be the result of a conformational change in the pore itself.

Spermine block of the strong inward rectifier potassium channel Kir2.1: dual roles of surface charge screening and pore block.

Inward rectification in strong inward rectifiers such as Kir2.1 is attributed to voltage-dependent block by intracellular polyamines and Mg2+. Block by the polyamine spermine has a complex voltage dependence with shallow and steep components and complex concentration dependence. To understand the mechanism, we measured macroscopic Kir2.1 currents in excised inside-out giant patches from Xenopus oocytes expressing Kir2.1, and single channel currents in the inside-out patches from COS7 cells transfected with Kir2.1. We found that as spermine concentration or voltage increased, the shallow voltage-dependent component of spermine block at more negative voltages was caused by progressive reduction in the single channel current amplitude, without a decrease in open probability. We attributed this effect to spermine screening negative surface charges involving E224 and E299 near the inner vestibule of the channel, thereby reducing K ion permeation rate. This idea was further supported by experiments in which increasing ionic strength also decreased Kir2.1 single channel amplitude, and by mutagenesis experiments showing that this component of spermine block decreased when E224 and E299, but not D172, were neutralized. The steep voltage-dependent component of block at more depolarized voltages was attributed to spermine migrating deeper into the pore and causing fast open channel block. A quantitative model incorporating both features showed excellent agreement with the steady-state and kinetic data. In addition, this model accounts for previously described substate behavior induced by a variety of Kir2.1 channel blockers.

Selective Ion Transport across Self-Assembled Alternating Multilayers of Cationic and Anionic Polyelectrolytes

Ultrathin membranes consisting of an alternating sequence of cationic and anionic polyelectrolytes were prepared by means of electrostatic layer-by-layer adsorption and investigated on their permeability for NaCl, Na2SO4, and MgCl2 in aqueous solution. It is demonstrated that the multi-bipolar structure of the polyelectrolyte membranes favors the separation of mono- and divalent ions by Donnan exclusion of the divalent ions. Various effects on the rate of ion permeation and the selectivity were investigated. Addition of salt to the polyelectrolyte solutions used for membrane preparation led to improved ion separation, while an increase of the pH had the opposite effect. Use of polyelectrolytes with high charge density also improved the ion separation. Especially good results were obtained if membranes containing polyallylamine (PAH) as the cationic polyelectrolyte were used. For 60 layer pairs of PAH/polystyrenesulfonate, for example, a separation factor α for Na+/Mg2+ up to 112.5 and for Cl-/SO42- up to 45.0 was found. The origins of the various effects are discussed in terms of different charge density and concentration of excess charges in the polyelectrolyte membrane.

Irreversible Electrochemical Processes of Membranes

Theoretical equations of membrane potentials and of the permeation rates of ions through membranes are derived from the entropy production in the membranes. The entropy production is calculated by means of the irreversible thermodynamics of continuous systems based on hydrodynamics. The calculation was carried out under the assumptions that the membrane has a porous mosaic structure and that the ionic concentration in the membrane capillaries ought to be determined by means of the Poisson-Boltzmann equation, and also that there is no pressure difference between two sides of the membrane. The equation of the membrane potential here obtained agrees with that derived from the Eyring theory of rate processes, but the former is more rigorous than the latter. The experimental data were found to be in good agreement with the theory. Furthermore, a new equation for the permeability of ions was derived by the same procedure.