Cation Transport Selectivity Research Articles

Current Density Dependence of Transport Selectivity of Metal Ions in the Electrodriven Process across the Cation Exchange Membrane.

Understanding the mechanisms leading to the selective transport of cations in an electrodriven process across a cation exchange membrane is important to design and control the potential gradient-based separation process. In this study, a comprehensive description of the current density (I, over a broad current regime) dependence of transport selectivity (Si) between cations of the same/different valence is presented. The role of conventional transport mechanisms such as diffusion, electromigration, and electroconvection in controlling the Si was identified theoretically as well as by multiple experimental approaches. These parameters were found to be dependent on the limiting current density (Ilim). In general, irrespective of the cations involved, Si (over Na+) decreased gradually with increasing I and then increased slowly (and saturated) after Ilim. This extent of variation of Si was heavily dependent on the charge and hydration state of the cations. At I < Ilim, both diffusion and electromigration processes contributed and, notably, the sorption selectivity outweighed the migration selectivity. At I → Ilim, diffusion was the solitary mechanism responsible for cation transport and migration selectivity was the major contributor in Si. At I > Ilim, as also validated by the Peclet numbers, the overall transport was dictated by electroconvection.

Proton- versus Cation-Selective Transport of Saccharide Rim-Appended Pillar[5]arene Artificial Water Channels.

Transport of water across cell membranes is a fundamental process for important biological functions. Herein, we focused our research on a new type of symmetrical saccharide rim-functionalized pillar[5]arene (PA-S) artificial water channels with variable pore structures. To point out the versatility of PA-S channels, we systematically varied the nature of anchoring/gate keepers d-mannoside, d-mannuronic acid, or sialic acid H-bonding groups on lateral pillar[5]arene (PA) arms, known as good membrane adhesives, to best describe the influence of the chemical structure on their transport activity. The control of hydrophobic membrane binding-hydrophilic water binding balance is an important feature influencing the channels' structuration and efficiency for a proper insertion into bilayer membranes. The glycosylated PA channels' transport performances were assessed in lipid bilayer membranes, and the channels were able to transport water at high rates (∼106-107 waters/s/channel within 1 order of magnitude as for aquaporins), serving as selective proton railways with total Na+ and K+ rejection. Molecular simulation substantiates the idea that the PAs can generate supramolecular pores, featuring hydrophilic carbohydrate gate-keepers that serve as water-sponge relays at the channel entrance, effectively absorbing and redirecting water within the channel. The present channels may be regarded as a rare biomimetic example of artificial channels presenting proton vs cation transport selectivity performances.

Selectivity of cation transport across lipid membranes by the antibiotic salinomycin

The ionophoric antibiotic salinomycin is in the phase of preclinical tests against several types of malignant tumors including breast cancer. Notwithstanding, the data on its ion selectivity, although being critical for its therapeutic activity, are rather scarce. In the present work, we studied the ability of salinomycin to exert cation/H+-exchange across artificial bilayer lipid membranes (BLM) by measuring electrical potential on planar BLM in the presence of a protonophore and fluorescence responses of the pH-sensitive dye pyranine entrapped in liposomes. The following order of ion selectivity was obtained by these two methods: K+ > Na+ > Rb+ > Cs+ > Li+. Measurements of the monovalent cation-induced quenching of fluorescence of thallium ions in methanol showed that salinomycin effectively binds potassium and calcium but poorly binds sodium and lithium ions. At high concentrations, salinomycin transports Ca2+ through membranes of liposomes and mitochondria, as measured by using the calcium-sensitive dye Fluo-5 N. The data obtained can be used in the mechanistic studies of the anti-tumor activity of salinomycin and its selective cytotoxicity towards cancer stem cells.

Combining Manning's theory and the ionic conductivity experimental approach to characterize selectivity of cation exchange membranes

The transport selectivity of different cations through cation exchange membranes (CEMs) could be estimated with the partitioning selectivity factor (Kji) and the cation mobility ratio in the membrane (uim/ujm), which in turn can be related to corresponding membrane conductivity and dimensional swelling ratio data [Journal of Membrane Science, 2020, 597, 117645]. This method has been validated in two hydrocarbon polymer-based CEMs, and the obtained K+/Na+ selectivity equals to the one obtained with conventional electrodialysis (ED) method. However, the K+/Na+ selectivity of perfluorosulfonic acid (PFSA) membranes, and the bi-/monovalent cation (Mg2+/Na+) selectivity of all three types of CEMs estimated with this ionic conductivity experimental approach deviate noticeably from corresponding values obtained with ED. In this work, it is proved that this deviation is partly due to the simplification of cation activity coefficients in the membrane. Here, the cation activity coefficients in three types of CEMs are calculated according to Manning's counter-ion condensation model. In this model, the Manning parameter (ξ) characterizing the dimensionless linear charge density is determined by the average distance between two adjacent fixed sulfonate groups (b) and the permittivity of hydrated membranes (ε). In hydrocarbon polymer-based CEMs, the average distance between fixed sulfonate groups can be estimated by assuming homogeneous distribution of the fixed groups, while in PFSA membranes three representative structure models are employed to estimate this average distance. After accounting for the cation activity coefficients in the membrane, the Mg2+/Na+ cation transport selectivity obtained with the ionic conductivity experimental approach gets closer to, but is still smaller than the selectivity obtained with the ED method. This work shows the importance of cation activity coefficients in the membrane phase in interpreting the membrane transport properties, and complements the proposed conductivity approach to characterize the counter-ion selectivity of ion exchange membranes.

Artificial K+ Channels Formed by Pillararene-Cyclodextrin Hybrid Molecules: Tuning Cation Selectivity and Generating Membrane Potential.

A class of artificial K+ channels formed by pillararene-cyclodextrin hybrid molecules have been designed and synthesized. These channels efficiently inserted into lipid bilayers and displayed high selectivity for K+ over Na+ in fluorescence and electrophysiological experiments. The cation transport selectivity of the artificial channels is tunable by varying the length of the linkers between pillararene and cyclodexrin. The shortest channel showed specific transmembrane transport preference for K+ over all alkali metal ions (selective sequence: K+ > Cs+ > Rb+ > Na+ > Li+ ), and is rarely observed for artificial K+ channels. The high selectivity of this artificial channel for K+ over Na+ ensures specific transmembrane translocation of K+ , and generated stable membrane potential across lipid bilayers.

Artificial K+Channels Formed by Pillararene‐Cyclodextrin Hybrid Molecules: Tuning Cation Selectivity and Generating Membrane Potential

AbstractA class of artificial K+channels formed by pillararene‐cyclodextrin hybrid molecules have been designed and synthesized. These channels efficiently inserted into lipid bilayers and displayed high selectivity for K+over Na+in fluorescence and electrophysiological experiments. The cation transport selectivity of the artificial channels is tunable by varying the length of the linkers between pillararene and cyclodexrin. The shortest channel showed specific transmembrane transport preference for K+over all alkali metal ions (selective sequence: K+&gt; Cs+&gt; Rb+&gt; Na+&gt; Li+), and is rarely observed for artificial K+channels. The high selectivity of this artificial channel for K+over Na+ensures specific transmembrane translocation of K+, and generated stable membrane potential across lipid bilayers.

Short Side Chain Aquivion Perfluorinated Sulfonated Proton-Conductive Membranes: Transport and Mechanical Properties

Data on the water uptake, proton conductivity, diffusion permeability, cation transport selectivity, and mechanical properties of short side chain Aquivion sulfonated perfluoropolymer membranes with an equivalent weight of 870 and 965 have been described. Properties of the membranes have been compared with those of a long side chain Nafion 212 membrane (equivalent weight of 1100). An increase in the equivalent weight leads to an increase in the sorption exchange capacity and water uptake of the membranes and a decrease in their proton conductivity. The conductivity of the Aquivion membrane with an equivalent weight of 870 is 1.4–1.5 times higher than that of the Nafion 212 membrane; it reaches 13.6 mS/cm in contact with water and 1.0 mS/cm at a relative humidity of 32% at 25°C. Diffusion permeability and cation transport selectivity exhibit a nonmonotonic dependence on the equivalent weight of the material. The lowest diffusion permeability, the highest Na+ cation transport selectivity (99.5%), and the best mechanical properties have been found for the Aquivion membrane with an equivalent weight of 965, which is characterized by the highest degree of crystallinity.

MF-4SC hybrid membranes doped with carbon nanotubes functionalized with proton-acceptor groups

Carbon nanotubes functionalized with proton acceptor groups are obtained for their use as dopants for hybrid materials based on an MF-4SC perfluorinated sulfonated membrane. Ion conductivity at high and low relative humidity, diffusion permeability of hydrochloric acid and methanol permeability, interdiffusion of H+/Na+ ions and mechanical properties of the final hybrid materials are studied as well. Introducing a small amount of dopant (0.5 wt %) is shown to favor an increase in ion conductivity at both high and low humidity, along with a decrease in methanol permeability, compared to the analogous initial membrane. In the specimens with 1–3 wt % of dopant, cation transport selectivity is higher than in the initial MF-4SC membrane. Furthermore, the mechanical properties of hybrid materials are found to not deteriorate, at least.

Hybrid materials based on MF-4SK membranes and hydrated silica and zirconia with sulfonic acid-functionalized surface: transport properties and characteristics of DP-sensors in amino acid solutions with varying pH

Hybrid ion-exchange membranes of MF-4SK brand containing hydrated silica and zirconia nanoparticles with sulfonic acid-functionalized surface have been synthesized. The effect of modification on the properties of materials in the proton and potassium forms and the characteristics of potentiometric sensors based on these materials have been studied. It has been found that membrane modification leads to an increase in the ionic conductivity compared with that of the unmodified MF-4SK sample and, in some cases, to a decrease in diffusion permeability. Variations in the cation transport selectivity provided by the modification of MF-4SK membrane have made it possible to decrease the sensitivity of sensors to interfering hydronium cations in an acid medium and increase the sensitivity to amino acid anions and zwitterions in an alkaline medium. Optimum membrane compositions for the determination of histidine cations at pH 7 have been selected.

Hybrid materials based on the Nafion membrane and acid salts of heteropoly acids M x H3 – x PW12O40 and M x H4 – x SiW12O40 (M = Rb and Cs)

The effect of the modification of Nafion membranes with nanoparticles of acid salts of heteropoly acids has been described. Ion conductivity at high and low relative humidity (RH), diffusion permeability, and mechanical properties of hybrid materials have been studied. It has been shown that the membrane modification with the acid salts of heteropoly acids makes it possible to increase the ionic conductivity both at high and low humidity, with the conductivity being determined by the amount of charge carriers at the surface of the dopant particles. The most significant effect of dopant incorporation is observed at low humidity. The conductivity of the sample Nafion + 5 wt % of Cs x H4 – x SiW12O40 at room temperature is σ = 4.1 mS/cm and it is more than two times as high as the conductivity of the initial Nafion membrane at RH = 30%. By modifying with the acid salts of heteropoly acids, the diffusion permeability of the hybrid membranes is reduced as compared to the initial membrane, thereby suggesting an increase in cation transport selectivity along with an increase in proton conductivity. Introduction of small amounts of the dopant do not cause deterioration of the mechanical properties of the membrane.

Sensitivity of potentiometric sensors based on Nafion®-type membranes and effect of the membranes mechanical, thermal, and hydrothermal treatments on the on their properties

The effect of heat treatment and mechanical deformation at different relative humidity on the transport properties of ion exchange Nafion® type membranes and characteristics of the potentiometric sensors based on them were studied. Methods for treatment of ion-exchange Nafion® type membranes were proposed for obtaining materials with given properties, including materials with improved selectivity of cation transport. An explanation for the changes in the pore and channel system during membrane treatment was proposed. A correlation between water uptake, transport properties of Nafion® type membranes and sensitivity of potentiometric sensors based on Donnan potential (DP) to amino acid and hydroxonium ions was revealed. Thermal treatment of membranes at RH<100% results in higher determination accuracy of amino acid ions due to decreasing off of the sensor sensitivity to interfering hydroxonium ions. The best value of DP-sensor sensitivity to amino acids ions was 51±2mV/pC in a concentration range 1.0·10−4–1.0·10−1M. The detection limit reached 7·10−6M. The relative error for amino acid determination in the pH range from 3 to 5 was 0.9–10%, the response time and drift of sensor did not exceed 1min and 7mV/h respectively. Presumably, targeted changes in the membrane properties due to their treatment will improve the parameters of processes and devices based on ion-exchange membranes.

Synthesis and study of hybrid materials based on a nafion membrane and hydrated titania

The properties of hybrid membranes based on a perfluorosulfonated Nafion polymer and hydrated titania are described. The membranes have been prepared by casting from a solution containing particles of a dopant preformed by precipitation at different pH values or a precursor for further synthesis of titania. The dependence of the conductivity of these samples on relative humidity and temperature has been studied. Modification leads to an increase in the proton conductivity of the membranes; higher values are obtained for the samples prepared using preformed TiO2 particles. It has been shown that the background of the dopant has an effect on the conductivity value: at low relative humidity, higher values are obtained using titania with a higher degree of hydration. The introduction of small amounts of the dopant (1.5–3 wt %) leads to an increase in the cation transport selectivity compared to that of the unmodified membrane.

Determinants of cation transport selectivity: Equilibrium binding and transport kinetics.

The crystal structures of channels and transporters reveal the chemical nature of ion-binding sites and, thereby, constrain mechanistic models for their transport processes. However, these structures, in and of themselves, do not reveal equilibrium selectivity or transport preferences, which can be discerned only from various functional assays. In this Review, I explore the relationship between cation transport protein structures, equilibrium binding measurements, and ion transport selectivity. The primary focus is on K+-selective channels and nonselective cation channels because they have been extensively studied both functionally and structurally, but the principles discussed are relevant to other transport proteins and molecules.

Hybrid perfluorosulfonated membranes with zirconia(IV) nanoparticles as an electrode-active material for potentiometric sensors

The effect that modifying MF-4SC and Nafion membranes with ZrO2 nanoparticles has on the sensitivity of PD-sensors (devices with an analytical signal in the form of a Donnan potential) to bulky organic ions in acid solutions of novocaine and lidocaine hydrochlorides and in alkaline solutions of glycine and cysteine is studied. The hybrid membranes are in situ synthesized by introducing dopant particles into the matrix of a finished membrane. An examination of the diffusion permeability of the membranes reveals that modification with ZrO2 nanoparticles leads to a deceleration of the anion transport rate and an improvement in the cation transport selectivity when compared to the original membranes. Potentiometric measurements show that the sensitivity of PD-sensors based on hybrid membranes to cations in acid solutions of NovHCl and LidHCl significantly increases compared to sensors based on unmodified membranes. An increase in the ZrO2 concentration in the membrane matrix results in a considerable increase in the sensitivity of PD-sensors to anions in alkaline solutions of amino acids, while the sensitivity to cations exhibits a tendency to decrease. The results show that hybrid membranes can be used in potentiometric sensors as electrode-active materials that are highly sensitive to various ions.

Transport properties of hybrid materials based on MF-4SC perfluorinated ion-exchange membranes and nanosized ceria

Hybrid materials based on MF-4SC perfluorinated cation-exchange membranes containing different amounts of ceria are prepared. It is shown that the modification of the membranes leads to an increase in their conductivity of up to 2.5 orders of magnitude, and the highest increase is observed at a humidity of 32%. The cation transport selectivity of the modified membranes increases in comparison with the initial MF-4SC membrane.

The hZIP4 transporter has two binding affinities and transports multiple transition metal ions.

Cells possess numerous membrane proteins to transport essential metal ions across cellular organelles. hZIP4 (human ZIP4) is one member of the ZIP (Zrt‐ and Irt‐like proteins) family of proteins that imports zinc to the cytoplasm. Autosomal mutations in hZIP4 have been associated with the zinc deficient genetic disorder Acrodermatitis Enteropathica. hZIP4 is expressed in small intestine, stomach, colon, cecum, kidney and pancreas. Recently, hZIP4 overexpression has also been associated with the pancreatic cancer. hZIP4 has 8 transmembrane domains with extracellular N‐ and C‐termini. Furthermore, the histidine rich non‐conserved N‐terminus and intracellular domain between transmembrane‐III and IV are believed to be important for Zn2+ coordination.We have established a radio‐isotope uptake experiment in Xenopus leavis oocytes to study the selectivity and kinetics of hZIP4 cation transport. An analysis of our experimental results demonstrate that hZIP4 transports Ni2+ and Cu2+ in addition to Zn2+. Our data also shows that hZIP4 has two binding affinities where one is in the μM range and the other one is in nM concentration. Subsequent experiments have also identified possible zinc coordination amino acid residues.

Separation of cobalt-60, strontium-90, and cesium-137 radioisotopes by competitive transport across polymer inclusion membranes with organophosphorous acids

The removal and separation of Co-60, Sr-90, and Cs-137 radioisotopes from nitrate aqueous solutions were studied by competitive transport across cellulose triacetate plasticized membranes using organophosphorous acids compounds (D2EHPA, Cyanex 272, Cyanex 301 and Cyanex 302). Competitive transport of trace radionuclides ions from 0.10 M NaNO 3 aqueous solutions across polymer inclusion membranes containing cellulose triacetate as the support, D2EHPA as the ion carrier and o-nitrophenyl pentyl ether as the plasticizer provide the selectivity order: Co(II) > Cs(I) > Sr(II). The affinities of the radioisotopes investigated for Cyanex 272 and Cyanex 302 follow the order Cs(I) > Sr(II) > Co(II). The influence of ion carrier acidity on cation transport selectivity and effectiveness was explored. The results show a linear decrease of permeability coefficient values for Co(II) with an increase of pK a values of ion carriers. A long-term transport experiment was carried out to demonstrate the durability of polymer inclusion membranes.

Transport of Zn(II), Cd(II), and Pb(II) across CTA plasticized membranes containing organophosphorous acids as an ion carriers

In this work a competitive transport of an equimolar mixtures of Zn(II), Cd(II), and Pb(II) across polymer inclusion membranes (PIMs) is presented. In PIMs di-2-ethylhexylphosphoric acid (D2EHPA) as well as commercial extractants, i.e. Cyanex ® 272, 301, and 302 were used as ion carriers. The influence of nature extractants on cation transport selectivity and efficiency has been explored. Cyanex ® 301 and 302 used as ionic carriers for competitive transport of Zn(II), Cd(II) and Pb(II) give preferential selectivity order: Pb(II) > Cd(II) > Zn(II). The selectivity coefficients for Pb(II)/Cd(II) and Pb(II)/Zn(II) for transport with Cyanex ® 302 have been found, they were equal to 2 and 4.4, respectively. To understand the influence of plasticizers nature on Zn(II), Cd(II), Pb(II) ions transport through PIM, membranes with different compounds, i.e. dioctyl phthalate (DOP), o-nitrophenyl octyl ether (ONPPE), bis(2-ethylhexyl) adipate (DOA), and tris(2-etylhexyl) phosphate (TOF) were prepared. The initial fluxes of metal ions increase in the following order of plasticizers: DOA < TOF < ONPOE, i.e. with their viscosity decrease, whereas DOP proved to be equally effective as ONPOE. The addition of 3,7-dinonyl-naphthalene-1-sulfonic acid into PIM with Cyanex ® 302 plays a role of a synergetic factor in the membrane process. A long-term experiment was carried out to demonstrate the durability of PIMs. The application of PIM transport process for 92% and 96% removal of lead(II) and cadmium(II), respectively, from technological sludge eluates is also shown.

Separation of Co(II) and Ni(II) ions by supported and hybrid liquid membranes

Abstract In this work a competitive transport of an equimolar mixture of cobalt(II) and nickel(II) across supported and hybrid liquid membranes is presented. In both types of membranes di-2-ethylhexylphosphoric acid (D2EHPA) as well as commercial extractants, i.e. Cyanex® 272, 301, and 302 were used as ion carriers. In experiments with the supported liquid membranes, the support applied for the liquid membrane was microporous, hydrophobic polypropylene Celgard® 2500. This film was soaked in 0.1 M organic solution of an ionic carrier in kerosene. The hybrid liquid membranes were made by composition of the cation-exchange membranes (CEM) with bulk liquid membrane in the system: CEM–organic phase–CEM. The source aqueous phase was composed of an equimolar mixture of Co(II) and Ni(II) ions in sulfuric acid aqueous solutions. As receiving phase, sulfuric acid solutions of higher concentration than the source phase were utilized. The influence of several factors on cation transport selectivity and effectiveness has been explored. These factors include the effect of (a) acidity of the source and receiving phases, (b) initial concentration of metal ions in the source phase and (c) concentration of ion carrier in membranes. It is shown that supported and hybrid liquid membranes, containing phosphoorganic acids allow to separate a Co(II)/Ni(II) equimolar mixture. The separation of Co(II)/Ni(II) was found to be governed by the ionic carrier used as well as the acidity of the aqueous source phase. In the hybrid liquid membrane processes lower metal ion fluxes than in supported liquid membranes processes were observed. On the other hand, higher separation coefficients for Co(II)/Ni(II) were found for hybrid than for supported liquid membrane processes.

PH-switchable, ion-permselective gold nanotubule membrane based on chemisorbed cysteine.

An electroless gold plating method was used to deposit Au nanotubules within the pores of a polycarbonate template membrane. pH-switchable ion-transport selectivity was introduced by chemisorbing L-cysteine to the inside tubule walls. At low pH, where both the amino and carboxyl groups of the cysteine are protonated, these membranes preferentially reject cations and transport anions. At high pH, where both the amino and carboxyl groups are deprotonated, these membranes preferentially reject anions and transport cations. At pH = 6.0, near the isoelectric point of the chemisorbed cysteine, these membranes show neither cation nor anion transport selectivity. In addition to this electrostatically based selectivity, because of the small inside diameter of the Au nanotubules (as small as 0.9 nm), these membranes show molecular-size-based selectivity.