Electroconvection in systems with heterogeneous ion-exchange membranes

The results of studying the surface morphology of heterogeneous cation-(MK-40) and anion-exchange (MA-40) membranes and calculating the structure of electroconvective vortices generated by the electric body force are shown. The body force and its distribution are estimated by taking into account real parameters of the membrane surface morphology. The calculations of vortices were carried out by solving the Navier-Stokes equation with the no-slip boundary condition and the preset body force distribution. It is shown that the body force induced by the flowing current can generate pairs of electroconvective vortices (electroosmosis of the second kind), where the size of induced vortices is comparable with the intermembrane gap in electrodialysis cells.     

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.     

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.     

Transport properties of highly ordered heterogeneous ion-exchange membranes

Model "ordered" heterogeneous ion exchange membranes are made with ion exchange particles heaving ion exchange capacity in the range 3 to 2.5 meq/gr (dry basis) and diameters ranging from 37 to 7 microm and 2 component room-temperature vulcanizing silicon rubber as a polymeric matrix, by applying an electric field normal to the membrane surface during preparation. These membranes were shown to have an improved ionic conductivity compared with "nonordered" membranes based on the same ion exchange content (for instance, at 10% resin content "nonordered" membranes show <10(-5) mS/cm while "ordered" membranes have conductivity of 1 mS/cm). The transport properties of ordered membranes were compared with those of nonordered membranes, through the current-voltage characteristics. Limiting currents measured for the ordered membranes were significantly higher than those of the nonordered membranes with the same resin concentration. In addition, higher limiting currents were observed in ordered membranes as the resin particles became smaller. Energy dispersion spectrometry analyses revealed that the concentration of cation exchange groups on the membrane surface was higher for ordered membrane as compared to that of nonordered membranes. This implies that the local current density for the conducting domains at the surface of the nonordered membranes is higher, leading to higher concentration polarization and, eventually, to lower average limiting current densities. The effect of ordering the particles on the membrane conductivity and transport properties was studied, and the advantages of the ordered membranes are discussed.     

Studies with inorganic ion-exchange membranes

The pyridinium molybdoarsenate membrane shows a response to pyridinium ions and can be used to determine the concentration of these ions in the range 10(-3)-1M. The potentials generated across the membrane are reproducible and the response time is less than 1 min. There is no interference from certain inorganic and organic ions. The electrode can be used in the pH range 3-6 as well as in non-aqueous medium. Small additions of cetyltrimethylammonium bromide cause large shifts in the membrane potentials. A membrane, after being treated with this surfactant, shows a wider range of response to pyridinium ions. Precipitation titration of pyridinium nitrate has been monitored by using this membrane electrode.     

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.     

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).     

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.     

Analytical model of laminar flow electrodialysis with ion-exchange membranes

A boundary-value problem for electrodialysis with ion-exchange membranes is posed and its analytical solution obtained. The solution allows one to calculate concentration fields in desalination and brine compartments, the current-density distribution along the flow coordinate and the thickness of diffusion boundary layers. It also makes it possible to estimate the value of local limiting current-density and to obtain the dependence of the process on physico-chemical characteristics of ion-exchange membranes (transport numbers and conductivity). The mathematical model was verified by a local distributive analysis made by means of laser interferometry.     

Ion-selective field effect transistors with heterogeneous membranes

Polyfluorinated polyphosphazene can be used as an elastomer for the preparation of heterogeneous membranes which can be applied to the ISFETs by solvent casting. The performance characteristics of such devices prepared with AgCl, AgCl-Ag₂S and AgI-Ag₂S membranes are very similar to those found for heterogeneous membranes based on silicone rubber, which have been used in the corresponding ion-selective electrodes. The sensor is suitable for direct determinations of chloride in sweat.     

Reformulating the permselectivity-conductivity tradeoff relation in ion-exchange membranes

Polymer membranes used in separation applications exhibit a tradeoff between permeability and selectivity. That is, membranes that are highly permeable tend to have low selectivity and vice versa. For ion-exchange membranes used in applications such as electrodialysis and reverse electrodialysis, this tradeoff is expressed in terms of membrane permselectivity (i.e., ability to selectively permeate counter-ions over co-ions) and ionic conductivity (i.e., ability to transport ions in the presence of an electric field). The use of membrane permselectivity and ionic conductivity to illustrate a tradeoff between counter-ion throughput and counter-ion/co-ion selectivity in ion-exchange membranes complicates the analysis since permselectivity depends on the properties of the external solution and ionic conductivity depends on the transport of all mobile ions within a membrane. Furthermore, the use of these parameters restricts the analysis to ion-exchange membranes used in applications in which counter-ion/co-ion selectivity is required. In this study, the permselectivity-conductivity tradeoff relation for ion-exchange membranes is reformulated in terms of ion concentrations and diffusion coefficients in the membrane. The reformulated framework enables a direct comparison between counter-ion throughput and counter-ion/co-ion selectivity and is general. The generalizability of the reformulated tradeoff relation is demonstrated for cation-exchange membranes used in vanadium redox flow batteries.     

Physicochemical characterization of ion-exchange membranes in water-methanol mixtures

A complete physicochemical characterization of two ion-exchange membranes—CM2 and Nafion^®117—used in electrodialysis and in direct methanol fuel cells (DMFC) has been carried out. For each membrane, in different methanol-water mixtures—0%, 20%, 40%, 60%, 80% and 100%—and at different temperatures (25.0; 40.0 et 55.0 °C), we have measured the variations of the geometrical dimensions, the proton electrical conductivity, the swelling rate and the amount of methanol in the membrane. The FTIR analysis of Nafion^®117 was performed at different methanol contents of the external solution.The results show that the CM2 membrane presents the best geometrical stability, and the lowest conductivity at any methanol content. At high methanol contents, Nafion^®117 is 10 times more conductive than the CM2 membrane. It was found that the methanol is absorbed more by Nafion^®117, and its effect is more noticeable on the microstructure of this membrane, under standard conditions. The high methanol permeability of these membranes, particularly of the Nafion^®117, induces bad cell efficiencies and lifetimes.     

Mathematical model of electrodialysis with ion-exchange membranes and inert spacers

A mathematical model describing the two-dimensional concentration field of an electrodialysis device with inert spacers is proposed. The boundary-value problem includes the Navier-Stokes, continuity, and steady-state convective diffusion equations and well-defined conditions and is solved by the control-volume numerical method. Results are expressed in the form of functional relationships of generalized variables. It is shown that when channels of the electrodialysis device are filled with spacers that do not conduct electric current, mass transport increases by several times in comparison to devices with open channels. The possibility is discussed for replacing the inert spacers with ones that conduct ion, not only in the complete demineralization of natural waters, but also in the desalination of brackish ground waters.     

Electrodialysis of pectin-containing solutions of hydrochloric acid with ion-exchange membranes

The process of electrodialysis of 0.05 M hydrochloric acid solutions containing 0.2% pectins with different molecular mass was studied using ion-exchange membranes. It is found that occurrence of the process at the current density of 5 mA/cm² allows achieving the solution demineralization degree of 87–90% and acid regeneration degree of 86–92% at current efficiency of 63–77%. Higher process indicators are observed in the case of electrodialysis of solutions containing pectin of a lower molecular mass.     

Electroluminescence of ion-exchange polymeric membranes in the swollen state

The first results on the electroluminescence of the MK-40L, MK-40K, MF-4SK, MA-40L, MA-41L, MA-40K, and MA-41K ion-exchange polymeric membranes in the swollen state are reported. It was found that electroluminescence had the character of flashes. Intensity and time characteristics of electroluminescence were determined over the range of NaCl solution concentrations from 0 to 0.1 M. The special features of electroluminescence from the ion-exchange membranes were determined from their photographs. It was found that the membranes in the air-dry state did not exhibit electroluminescence.     

Electrotransport properties and morphology of MF-4SK membranes after surface modification with polyaniline

The method of template synthesis is used for the surface modification of MF-4SK membranes with polyaniline. The influence of the time of polyaniline synthesis in the surface layer of a perfluorinated MF-4SK membrane on its morphology and electrotransport properties is investigated. It is established that under the synthesis conditions, a gradient distribution of polyaniline develops across the thickness of the membrane, and as a result of this, an anisotropic composite structure is formed. It is shown that the specific electrical conductivity and the electroosmotic and diffusion permeability exhibit an extremal character as functions of the time of polyaniline synthesis. When the orientation of these composite membranes is changed with respect to electrolyte flow, an asymmetry effect in their diffusion characteristics is found. With the application of the bilayer fine porous membrane model, the modified-layer thickness is estimated, and the determining influence of the difference in absolute values of effective fixed-charge volume densities on the development of the asymmetry effect is found.     

Transport of glycine in systems with cation-exchange membranes in hydrochloric acid solutions: Effect of a heterogeneous protonation reaction

The electrical mass transfer of cations in electromembrane systems (EMS) with an MK-40 cation-exchange membrane and glycine in aqueous solutions of hydrochloric acid is studied using the method of a rotating membrane disk. Limiting current densities and limiting steps of the transport of cations in such systems are determined. Shown is the possibility of an increase in the electrical mass transfer of glycine as a consequence of the occurrence of a heterogeneous reaction of protonation of its zwitterions. The effect of the membrane surface state on the kinetic regularities of transport of cations in EMS with glycine in solutions of hydrochloric acid is exposed.     

Contact ion-exchange interactions in systems involving zeolites

Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1505–1506, August, 1994.     

Effects of inorganic substances on water splitting in ion-exchange membranes; I. Electrochemical characteristics of ion-exchange membranes coated with iron hydroxide/oxide and silica sol

The effects of inorganic substances on water splitting in ion-exchange membranes (IEMs) were investigated. In this study, iron hydroxide/ oxide and silica sol were immobilized on the surface of the IEMs. The water-splitting capabilities of the metal-embedded cation-exchange membranes were 10(4)-10(5) times greater than those of the virgin membranes at the same current density. Similarly, silica sol (i.e., triple bond Si-OH groups) deposited on the anion-exchange membrane surface also drastically increased the proton transport numbers. It was thought that the bipolar structure consisting of H- and OH-affinity groups immobilized on the IEM surface increased water-splitting due to the enhancement of water polarization with the help of strong electric fields. This study revealed that metal oxides or silica groups (triple bond Si-OH), as well as metal hydroxides, can be used as catalysts for water splitting.