Properties and structure of heterogeneous ion-exchange membranes after exposure to chemical agents

This work deals with resistance of ion-exchange membranes in selected chemical solutions. Specimens of heterogeneous membranes were analyzed after exposure to chemically aggressive agents such as nitric acid, sodium hydroxide, etc. The analyses were carried out after period up to 6 months. The chemical resistance was evaluated from the mechanical and electrochemical property changes’ point of view before and after exposure. The tested materials were not only the membranes themselves but also their individual components, i.e., ion-exchange resins from various manufacturers and polyester reinforcing fabrics. The results show that exposure to NaOH solution has the most damaging effect especially on reinforcing fabric and a membrane as whole due mainly to dimension changes. The same stands for electrochemical resistance of the membrane and permselectivity. Ion-exchange capacity remains almost the same after exposure.     

Characterization of ion-exchange membrane materials: properties vs structure

This review focuses on the preparation, structure and applications of ion-exchange membranes formed from various materials and exhibiting various functions (electrodialytic, perfluorinated sulphocation-exchange and novel laboratory-tested membranes). A number of experimental techniques for measuring electrotransport properties as well as the general procedure for membrane testing are also described. The review emphasizes the relationships between membrane structures, physical and chemical properties and mechanisms of electrochemical processes that occur in charged membrane materials. The water content in membranes is considered to be a key factor in the ion and water transfer and in polarization processes in electromembrane systems. We suggest the theoretical approach, which makes it possible to model and characterize the electrochemical properties of heterogeneous membranes using several transport-structural parameters. These parameters are extracted from the experimental dependences of specific electroconductivity and diffusion permeability on concentration. The review covers the most significant experimental and theoretical research on ion-exchange membranes that have been carried out in the Membrane Materials Laboratory of the Kuban State University. These results have been discussed at the conferences "Membrane Electrochemistry", Krasnodar, Russia for many years and were published mainly in Russian scientific sources.     

Physicochemical Properties of Profiled Heterogeneous Ion-Exchange Membranes

Optimum values of parameters of hot pressing of heterogeneous ion-exchange membranes MK-40 and MA-40 (temperature, pressure, exposure duration) with the aim of creating geometrical profiles on their surfaces are determined. A method of optical visualization of membrane profiles and a diffusion technique of diagnostics of through pores in the membranes are developed and substantiated. Most important physicochemical and physicomechanical characteristics of profiled membranes are investigated. It is shown that the major obstacle in the production of profiled cation-exchange heterogeneous membranes is encapsulation of grains of the ion-exchange resin by an inert polyethylene film, which leads to an increase in the surface resistance of the membranes and to a decrease in the fraction of their active surface areas. The profiling of heterogeneous anion-exchange membranes is accompanied by an increase in their microscopic porosity and diffusion penetrability.     

Effect of an ion-exchange filler on the deionized water quality in electrodialysis

Desalination channels, containing an inert separator and a monolayer of ionites AV-17 and KU-2 taken in various volume ratios, are studied while maintaining concentrations of all solution components invariant. It is shown that the composition of the ion-exchange filler of the desalination channel affects the rate of transport of salt ions through relevant membranes, pH, and specific resistance of desalinated solution. The behavior of membrane systems in an overlimiting state is explained by using notions about different mechanisms governing the so-called overlimiting current through anion-exchange and cation-exchange membranes     

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.     

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.     

Perfluorinated ion-exchange membranes

This review concerns the chemical structure and synthesis of perfluorinated sulfocationite membranes, among which Nafion, MF-4SC, Flemion, Aciplex-S, and Dow membranes are the most well-known representatives. Special attention is given to main mechanisms controlling the formation of microstructure and its relationship with transport processes and selectivity levels of membranes. Key approaches to the modification of ion-exchange materials, above all involving the synthesis of hybrid membranes containing nanoparticles of inorganic compounds, are described. It is noted that increases in the conductivity and selectivity levels of hybrid membranes are primarily related to changes in structure of pores and channels and in the distribution of the concentration of carriers in them. Some examples of the practical use of perfluorinated membranes in modern technologies are highlighted.     

Asymmetry of current–voltage characteristics: a bilayer model of a modified ion-exchange membrane

The asymmetry of the current–voltage characteristics of ion-exchange membranes is explained in terms of the model of a bilayer fine porous membrane with constant charge distributions over the thickness of layers. This model has previously been proposed for determining diffusion permeability of membranes. In the case of one uncharged (neutral) layer, a set of two implicit algebraic equations is derived for determining the total current–voltage characteristics (CVC) of a membrane. For the first time, implicit algebraic equations are obtained for calculating the limiting currents at different orientations of an anisotropic membrane in an electrodialysis cell and explicit expressions are derived for determining specific conductivity of the membrane from the slope of the ohmic region of a CVC under the approximation of “excluded coions.” The model may be successfully used for describing the CVCs of perfluorinated MF-4SC sulfonic cation-exchange membranes, the surface layers of which are modified with polyaniline or halloysite.     

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.     

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.     

Effects of the desalination chamber design on the mass-transfer characteristics of electrodialysis apparatuses at overlimiting current densities

The overlimiting current modes are being increasingly used in electrodialysis (ED) of dilute aqueous solutions. Of great importance is establishing a relationship between the design of ED apparatuses and the character of phenomena observed at overlimiting current densities, primarily, electroconvection and H⁺ and OH^? ion generation during water dissociation at the membrane-solution interface. In this work, we analyze the factors governing the efficiency of dilute solutions using modern theoretical concepts and experimental data obtained in laboratory cells and large-scale electrodialysis apparatuses. We also analyze the relationship between the mechanisms of the overlimiting transfer and the design of the desalinating channel. ED apparatuses of different types are considered, namely, apparatuses with profiled membranes, inert spacers, monolayer of ionite granules, and dipolar fillers of unwoven ionite fibers. The optimum concentration ranges of the desalinated solutions were found, and the operating conditions of membrane stacks, providing maximum overlimiting ion transfer, were determined.     

Effect of thermochemical treatment on the surface morphology and hydrophobicity of heterogeneous ion-exchange membranes

A comparative analysis is performed on the effect thermochemical treatment in aqueous, alkali, and acid media has on the surface morphology and hydrophobicity of swelling heterogeneous ion-exchanged membranes. A correlation between changes in surface morphology and hydrophobicity is established. It is shown that under prolonged (50 h) membrane thermal treatment above room temperature, hydrophobicity is reduced due to substantial enlargement of cavities and cracks resulting from the partial destruction of inert binder (polyethylene) and reinforcing poly-?-caproamide fabric (capron).     

New Composite Proton-Conducting Membranes Based on Nafion and Cross-Linked Sulfonated Polystyrene

New composite membranes based on commercial perfluorinated Nafion-115 membrane and cross-linked sulfonated polystyrene were synthesized and investigated. The membranes were prepared by radical polymerization of styrene in the presence of a cross-linking agent divinylbenzene in Nafion polymer matrix and subsequent sulfonation of formed polystyrene. The membranes containing approximately 5 and 10 wt % of cross-linked polystyrene with ion-exchange capacity of 1.1 to 1.3 mg-eq/g were obtained. Modification with sulfonated polystyrene leads to an increase in the moisture content and proton conductivity of membranes in the humidity range of 15 to 100 RH.     

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.     

Electroanalytical sensors for nonconducting media based on electrodes supported on perfluorinated ion-exchange membranes

Electroanalytical sensors, suitable for the analysis and monitoring of electroactive analytes present in gaseous phase or low-conductive liquid media, and based on electrodes in close contact with perfluorinated ion-exchange polymers are reviewed. The basic operative mechanism of these sensors, in which ion-exchange polymers act as solid polymer electrolytes (SPE's), is thoroughly discussed, while stressing the fundamental reasons why their behavior differs from that of conventional membrane electrodes. The procedures for preparing composite working electrodes by coating one side of ion-exchange membranes with stable porous films of conductive materials are described, along with the most common strategies followed to assemble this type of sensors. Useful examples of measurements in electrolyte-free media of inorganic and organic electroactive species of interest mainly for environmental analysis are given. Future prospects for the development of these sensors are also discussed.     

Effect of anion-exchange membrane surface properties on mechanisms of overlimiting mass transfer

Four effects providing overlimiting current transfer in ion-exchange membrane systems are examined. Two of them are related to water splitting: the appearance of additional current carriers (H+ and OH- ions) and exaltation effect. Two others are due to coupled convection partially destroying the diffusion boundary layer: gravitational convection and electroconvection. Three anion-exchange membranes, which differ in surface morphology and the nature of ion-exchange sites within a surface layer, are examined. The ion transfer across these membranes in NaCl solutions is studied by voltammetry, chronopotentiometry, and pH-metry. By excluding the effects of water splitting and gravitational convection, it is shown that the main mechanism of overlimiting mass transfer in narrow membrane cells at low salt concentrations is electroconvection. The reasons explaining why water splitting suppresses electroconvection are discussed. The scenario of development of potential oscillations with growing current and time is compared with that described theoretically by Rubinstein and Zaltzman.     

Generalization and prognostication of mass exchange characteristics of electrodialyzers operating in overlimiting current regimes with use made of similarity theory and compartmentation method

A semiempirical approach, which allows one to perform generalization, prognostication, and scaling of the mass exchange characteristics of electrodialyzers intended for the desalination of dilute solutions in the overlimiting current regimes, is proposed. The approach is based on modern theoretical notions concerning the dependence of mass transfer on conjugated effects of concentration polarization, such as electroconvection and the exaltation effect and makes use of the similarity theory principles. As a result, there are obtained sufficiently simple equations that describe to within a satisfactory accuracy characteristics of the process of electrodialysis (mass transfer coefficient, Sherwood number, solution desalination degree, and so on) as functions of four parameters, namely, the input concentration, the solution flow rate, the potential drop across the paired chamber of the membrane stack of an electrodialyzer, and the length of the desalination channel. For scaling the obtained dependences with respect to a channel length a compartmentation method is used. Results of experimental investigations and calculations of electrodialyzers with desalination channels without a filler, or containing an inert separator, a monolayer of an ion-exchange resin, a profiled membrane, are presented.     

Ion transport in MF-4SK membranes modified with hydrous zirconia

Composites based on perfluorinated cation-exchange membranes MF-4SK with embedded hydrous zirconia were synthesized. The composites have higher proton conductivity, lower diffusion permeability, and higher ion-transport selectivity than unmodified MF-4SK membranes. Diffusion permeability anisotropy was found in samples with heterogeneous dopant distribution across the membrane.     

Anion-exchange properties of composite ceramic membranes containing hydrated zirconium dioxide

Composite membranes consisting of an inert ceramic matrix based on aluminum and zirconium oxides and of an ion-exchange component, hydrated zirconium dioxide, were prepared. The selectivity of these membranes to Cl^? ions was studied.