Features of the sorption of phenylalanine by profiled ion-exchange membranes

Features of the equilibrium sorption of phenylalanine from neutral media by profiled ion-exchange membranes in a wide range of concentrations is studied under static conditions. The mechanism of phenylalanine sorption by ion-exchange membranes with profiled and smooth surfaces is discussed. It is shown that phenylalanine sorption is accompanied by the formation of spatial associative structures of the aminoacid in an external equilibrium solution, and in a solution of the membrane’s pore spaces or on its surface. The increased sorption capacity of the profiled membranes is explained by features of the microstructure of their surface and volume.     

Effect of an aprotic solvent on the properties and structure of ion-exchange membranes

The effect of the aprotic solvent dimethylacetamide on the equilibrium and transport properties of heterogeneous (MK-40, MA-40, and MA-41) and homogeneous (MF-4SK) ion-exchange membranes is investigated. On the basis of concentration dependences of the conductivity and diffusion permeability of membranes, model calculations of transport-structural parameters that reflect the structural and kinetic characteristics of conducting phases of the swollen polymer are performed. The effect of the aprotic solvent on the flow of current through the structural fragments of the ion-exchange material is estimated. The causes of changes that are induced in the properties of the membranes by the aprotic solvent are ascertained.     

Electric mass transport through homogeneous and surface-modified heterogeneous ion-exchange membranes at a rotating membrane disk

Polarization characteristics of the homogeneous MF-4SK perfluorinated sulfonated cation-exchange membrane and the heterogeneous MK-40 sulfonic acid membrane with its surface modified by a homogeneous film of Nafion are studied at a rotating membrane disk in 0.1 and 0.001 M sodium chloride solutions. Partial current-voltage curves (CVC) are obtained for sodium and hydrogen ions, and limiting current densities in the electromembrane systems (EMS) under study are calculated as a function of the rotation rate of the membrane disk. Contribution from different mechanisms (electrodiffusion, electroconvection, dissociation of water, and the effect of the limiting-current exaltation) to the total ion flow is estimated experimentally and theoretically under conditions that the diffusion layer in the EMS has stabilized in thickness. It is established that surface modification of the heterogeneous MK-40 membrane with a 7 μm layer of a modifying agent almost completely eliminates the dissociation of water molecules, and the properties of the heterogeneous MK-40 membrane approximate those of the homogeneous Nafion membrane. From IR spectra and potentiometric titration curves of the MK-40 and MF-4SK membranes, it is shown that the acidity of the sulfonate groups in these membranes is nearly identical, but a difference in the dissociation rate of water at these membranes is determined by a different character of charge-density distribution and potential near the membrane-solution interphase boundary. By means of the theory of the overlimiting state in EMS, the internal parameters of the systems under investigation are calculated: distribution of space-charge density and electric-field potential in the diffusion layer and in the membrane. Partial CVC are calculated for H⁺ ions for the space-charge region in the phase of the MF-4SK and MK-40/Nafion ion-exchange membranes. Partial CVC with similar characteristics are compared for the heterogeneous monopolar MK-40 and the bipolar MB-2 membranes, which contain sulfonate groups. It is concluded that the membrane surface layer, where the space charge is localized, plays a dominant role in speeding up the dissociation of water in EMS.     

The effect of aprotic solvent on the selectivity of ion-exchange membranes

The effect of N,N-dimethylacetamide on the selectivity of heterogeneous (MK-40, MA-40, and MA-41) and homogeneous (MF-4SK) ion-exchange membranes is studied for the first time. Concentration dependences of electrical conductivity and diffusion permeability of the membranes were measured experimentally over wide lithium chloride concentrations; on their basis, electrodiffusion coefficients of the co- and counterions were calculated. The interrelation between the electrodiffusion coefficients and the specific moisture capacity of the heterogeneous and homogeneous membranes (which affect their selectivity) is revealed. The calculated electrodiffusion coefficients were used in the calculations of the electromigration transport numbers of counterions in the initial membranes and those processed in mixed solvent. It is shown that the heterogeneous membrane selectivity either increased under the action of the aprotic solvent to polymer material (MA-40, MA-41) or remained practically unchanged (MK-40); the selectivity of homogeneous perfluorinated membranes (e.g., MF-4SK) decreased, thus approaching that of the studied heterogeneous anion-exchange membranes.     

Novel ion-exchange spacer for improving electrodialysis II. Coated spacer

Ion-conducting spacers were prepared by applying an ion-exchange coating to commercially available polypropylene netting. Homogeneous and heterogeneous types of coating were used. Homogeneous anion-exchange coating consisted of bromomethylated and aminated polysulfone, homogeneous cation-exchange coating of sulfonated polysulfone. All heterogeneous coatings consisted of ground ion-exchange resin, embedded in crosslinked poly(vinyl alcohol). All the coated spacers increased the rate of desalting of sodium chloride solutions, at concentrations of 20 mM or less. The effect increased with the ion-exchange capacity of the spacer per unit area. The spacers suppress polarization, leading to increased current efficiency and decreased cell resistance. As expected, largest decrease of cell resistance is obtained in dilute solutions, <3 mM. The clearest effect on efficiency was observed in ED with heterogeneous ion-exchange membranes, which are by themselves highly polarizing. Most experiments were carried out with anion-exchange spacers, minimizing the water splitting which takes place at the surface of the ion-exchange membrane. Introduction of an anion-exchange spacer near the heterogeneous anion-exchange membrane and a cation-exchange spacer near the heterogeneous cation-exchange membrane led to a dramatic increase in current efficiency.     

Electrochemical properties and structure of ion-exchange membranes upon thermochemical treatment

The effect of temperature-induced morphological changes on the electrochemical and physicochemical properties of the heterogeneous sulfo cation-exchange membrane MK-40 in aqueous, alkaline, and acidic media is subjected to comparative analysis. The deviation between the surface and volume microstructure of swollen membrane samples after their chemical conditioning and thermochemical treatment are visualized by a scanning electron microscope. The porosity and the fraction of the ion-exchange component in membranes subjected to heating in water and aggressive media are observed to increase more noticeably in their surface layer as compared with their volume. The maximum effect thus modified structural characteristics on the transport (conductivity, diffusion permeability, selectivity) and physicochemical (exchange capacity, water content, density, linear sizes) properties is observed for MK-40 membrane samples when heated in a sulfuric acid solution. The effect of thermal destruction of inert polymers (divinylbenzene, caprone) involved in the membrane composition on the transport characteristics is revealed.     

The dissociation rate of water molecules in systems with cation- and anion-exchange membranes

Within the framework of the mathematical model of Nernst-Planck-Poisson, an attempt is undertaken to theoretically describe the electrodiffusion of ions in the system diffusion layer/monopolar ionexchange membrane, which is accompanied by dissociation of water molecules. The formulas for estimating the current density transferred through a monopolar membrane by hydrogen or hydroxyl ions formed in dissociation of water in the space-charge region are derived. The rate constants and other parameters of dissociation of water molecules in the space-charge region of monopolar membranes under conditions of stabilization of the diffusion layer thickness are calculated. Their comparative analysis with the similar characteristics of bipolar membranes is carried out. For the phosphoric-acid heterogeneous membrane MK-41 in which the polarization conditions in the current density range under study are not so severe and the reaction layer is not being depleted as in the bipolar membrane MB-3 (contains the same phosphoric-acid groups), it is shown that only single-charged phosphoric-acid groups are involved in the water dissociation reaction. For MK-41, the calculated constants of the heterolytic reaction of water molecule dissociation are lower than for the heterogeneous membrane MA-40 containing ternary and quaternary amino groups. It is confirmed that the nature of ionogenic groups in membranes is a factor that determines the rate of water dissociation in systems with ion-exchange membranes.     

Steady-state Ion Transport through a Three-Layered Membrane System: A Mathematical Model Allowing for Violation of the Electroneutrality Condition

A relatively simple mathematical model based on the Poisson equation is considered. The model is intended for modeling transport through multilayered ion-exchange membranes operating at overlimiting currents. The boundary-value problem is solved by a numerical method of parallel shooting and by an approximate method based on the assumption that the charge density is distributed quasi-uniformly. Concentration profiles in diffusion layers and membranes, current–voltage curves, and dependences of effective transport numbers on the current density are examined.     

Transport properties of anion-exchange membranes: Effect of the formation of complexes

Electroconductivity and diffusion permeability of heterogeneous anion-exchange membranes MA-40 and MA-41 are studied in solutions containing copper, nickel, and zinc cations. The composition of species present in the contacting solution is calculated on the basis of conditions of equilibrium and material balance with allowance made for the tendency of these cations to form complexes. It is shown that the reason for the electroconductivity of MA-40 increasing in dilute solutions of transition metal chlorides is the complexing between these cations and functional groups and the resultant increase in the overall positive charge of the polymer matrix. The complexing with ions of copper and zinc in the electromembrane phase reduces the mobility of chloride ions, thus lowering the membranes’ electroconductivity with increasing concentration of equilibrium solutions. The membranes are characterized using a method of determination of the transport properties and structure of the membranes. The method is applied for the first time to the complexing co-ions, with allowance made for variations in their charge and composition.     

Influence of inert components on the formation of conducting channels in ion-exchange membranes

In this work, the results of investigation of the influence of inert binder and reinforcing fabric on structural organization and mechanism of current transfer in homogeneous and heterogeneous ion-exchange membranes are presented by theoretical analysis of parameters of the extended three-wire conductivity model. It was established that analogy in reorganization of the current paths takes place in the course of inclusion of the reinforcing fabric in perfluorinated membranes and addition of polyethylene and nylon 6 to ion-exchange resins during preparation of heterogeneous membranes. In comparison with perfluorinated membranes, the essential difference in conducting properties of heterogeneous membranes is the opportunity for the current transfer via the channel filled with equilibrium solution. The size of this channel decreases with increase in the volume fraction of the inert component inside the membrane.     

Electroconvection in systems with heterogeneous ion-exchange membranes after thermal modification

It is found that the variations in the structure (morphology and microrelief) and chemical composition of surface of heterogeneous ion-exchange membranes as a result of thermal modification have different effects on the current—voltage characteristics and conditions for the generation of electroconvective instability at the membrane/solution interface under intense current modes. After thermal treatment of strongly acidic sulfocation-exchange membrane, which is characterized by a low catalytic activity in the reaction of water dissociation and a high thermal stability of fixed groups, a fraction of conducting surface area increases and the membrane microrelief develops. As a result, the diffusion limiting current density increases and the length of plateau of the current—voltage curve decreases. Therewith, the thickness of the region of electroconvective instability of solution in the near-membrane region increases and the polarization of electromembrane system, at which the mode of unstable electroconvection is reached, decreases. The thermodestruction of strongly basic anion-exchange membranes, conversely, leads to suppression of electroconvection and an increase in the length of plateau of the current—voltage curve due to the formation of fixed weakly basic amino groups, which are catalytically active in the reaction of water dissociation. A linear correlation is found between the dimensions of the region of electroconvective instability and a fraction of weakly basic functional amino groups in the composition of strongly basic membranes.     

Electrical conductivity of cation-and anion-exchange membranes in ampholyte solutions

Several laws governing ampholyte transport through ion-exchange membranes are established by a comparative analysis of the concentration dependence of electrical conductivity for homogeneous (CMX, AMX) and heterogeneous (MK-40, MA-41) membranes in NaCl, LysHCl, and NaH₂PO₄ solutions. The increase in the electrical conductivity of membranes in ampholyte solutions as the solutions become more dilute is explained by the increased fraction of divalent ions of the amino acid (cation-exchange membrane) or from phosphoric acid (anion-exchange membrane) in the membrane as a result of Donnan exclusion of hydrolysis products (hydroxide ions or protons, respectively).     

Electrochemical properties of polyethylene membrane modified with carboxylic acid group

Two cation-exchange membranes modified with the carboxylic acid group for battery separator were prepared by radiation-induced grafting of acrylic acid (AA) and methacrylic acid (MA) onto a polyethylene (PE) film. The surface area, thickness, volume, water uptake, ion-exchange capacity, specific electric resistance, and electrolyte flux were evaluated after PE film was grafted with AA and MA. It was found that KOH diffusion flux of AA-grafted PE membrane and MA-grafted PE membrane increased with an increase in the degree of grafting. AA-grafted PE membrane had a higher diffusion flux than MA-grafted PE membrane. Electrical resistance of two cation-exchange membranes modified with AA and MA decreased rapidly with an increase in the degree of grafting.     

Profiling Functional and Biochemical Phenotypes of Circulating Tumor Cells Using a Two‐Dimensional Sorting Device

During cancer progression, tumors shed circulating tumor cells (CTCs) into the bloodstream. CTCs that originate from the same primary tumor can have heterogeneous phenotypes and, while some CTCs possess benign properties, others have high metastatic potential. Deconstructing the heterogeneity of CTCs is challenging and new methods are needed that can sort small numbers of cancer cells according to their phenotypic properties. Here we describe a new microfluidic approach that profiles, along two independent phenotypic axes, the behavior of heterogeneous cell subpopulations. Cancer cells are first profiled according to expression of a surface marker using a nanoparticle‐enabled approach. Along the second dimension, these subsets are further separated into subpopulations corresponding to migration profiles generated in response to a chemotactic agent. We deploy this new technique and find a strong correlation between the surface expression and migration potential of CTCs present in blood from mice with xenografted tumors. This system provides an important new means to characterize functional diversity in circulating tumor cells.     

Ion transfer across ion-exchange membranes with homogeneous and heterogeneous surfaces

A homogeneous (AMX) and two heterogeneous (MA-40, MA-41) anion-exchange membranes, as well as a heterogeneous cation-exchange membrane (MK-40), are studied by electronic scanning microscopy, voltammetry, and chronopotentiometry. The presence of conducting and nonconducting regions on the surfaces of heterogeneous membranes is established by means of element analysis. The fraction of conducting regions is found by an image treatment. The surface of the AMX membrane was partially coated with microspots of a paint to make it heterogeneous (AMXheter). Voltammetric and chronopotentiometric measurements for AMX, AMXheter, and MA-41 membranes in NaCl solutions are carried out and the pH changes in the solution layers adjoining to these membranes are recorded. Analysis of obtained results shows that the concentration polarization of studied membranes characterized by the potential drop and the rate of water dissociation at the interface is mainly governed by the properties of their surfaces. It is found that the local limiting current density through conducting regions of a heterogeneous membrane is several times higher than the average limiting current through a homogeneous membrane.     

Comparative studies on electrochemical characterization of homogeneous and heterogeneous type of ion-exchange membranes

Interpolymer films of poly-ethylene and styrene-divinyl benzene copolymer were subjected to chlorosulfonation or chloromethylation then amination for the preparation of homogeneous type of cation- or anion-exchange membranes, respectively. Heterogeneous types of ion-exchange membranes were prepared from polyvinyl chloride (PVC) as binder and ion-exchange resin powder in tetrahydrofuran solvent. Membrane potential and conductance measurements have been carried out in NaCl(aq), CuCl₂(aq) and AlCl₃(aq) solutions at different concentrations to investigate the relationship between concentration of fixed charges and electrochemical properties of these membranes. On the basis of the micro-heterogeneous model, describing the micro-structure of the membrane material, the counter-ion diffusion coefficients were estimated. Membrane conductance data, along with values of concentration of fixed ionic site in the membrane, were used for the estimation of the tortuosity factor and salt permeability employing non-equilibrium thermodynamic principles. It was concluded that electrochemical transport properties of homogeneous type of ion-exchange membranes are superior to those for heterogeneous type of ion-exchange membranes. However, both types of membranes are suitable for electrodriven separation of mono-, bi- and tri-valent electrolytes.     

Sodium chloride concentration by electrodialysis with hybrid organic-inorganic ion-exchange membranes: An investigation of the process

This study examines how conditions for modifying homogeneous MF-4SK and heterogeneous MK-40 membranes with tetraethoxysilane affect membrane properties. The microstructure of the bulk membrane and its surface, both before and after exposure to the modifying agent, is examined by scanning electron microscopy, spark spectrophotometry, and standard contact porosimetry. The process of sodium chloride concentration by electrodialysis with hybrid organic-inorganic membranes in cells with noncirculating concentration compartments is investigated, and a mathematical model of the concentration process by electrodialysis is used to determine transport properties: current efficiency, diffusion and osmotic permeabilities, and the salt hydration number. For highly hydrophilic membranes, it is shown that water transport occurs both in ion hydration shells and also as free water. It is established that after modified membranes undergo additional heat treatment, the transport of free water ceases, and the water transport number decreases. This is in accord with an increase in the salt content of the concentrate during concentration by electrodialysis.     

Assessing the selectivity of composite ion-exchange membranes within the framework of the extended three-wire model of conduction

The possibility of assessing the selectivity of composite perfluorated membranes MF-4SK modified with polyaniline (PANI), based on the concentration dependence of their specific conductivity in terms of the extended three-wire model of conduction of ion-exchange materials is investigated. To check the reliability of results obtained, the transport numbers of ions are calculated based on the electrodiffusion coefficients of counter ions and coions determined from concentration dependences of conductivity and diffusion permeability of ion-exchange membranes. The transport numbers of hydrogen ions found by these two methods are shown to coincide with high accuracy throughout the whole range of concentrations of HCl solutions for the original MF-4SK membrane and membranes with the fixed thickness of the modified polyaniline layer on their one side. For the MF-4SK sample the with gradient distribution of polyaniline, which exhibits asymmetrical transport characteristics, the deviation between the transport numbers of ions calculated with the use of electrodiffusion coefficients of counter ions and coions and those determined in terms of the extended three-wire model of conduction is shown to increase with the increase in solution concentration but not exceed 4%.     

Effect of amino acid sorption on formation of water clusters in ion-exchange membranes

The average size and number of water clusters inside ion-exchange membranes are calculated from experimental isotherms of water vapor sorption as a result of considering the sorption in terms of the clusterization theory. It is established that, in MK-40 heterogeneous cation-exchange membrane, water clusters are not formed, while, inside MF-4SK perfluorinated homogeneous membrane, intense cluster formation takes place. The effect of amino acid sorption on cluster water is considered. An increase in the membrane hydrophobicity as a result of the incorporation of amino acid ions leads to prevailing interaction of water molecules with one another rather than with the polymer phase, which is evident from an enlargement of water clusters.