Composite sulfonated cation-exchange membranes modified with polyaniline and applied to salt solution concentration by electrodialysis

A method is developed for obtaining anisotropic composites based on the sulfonated cation-exchange MF-4SK and MK-40 membranes and the electroactive polymer polyaniline (PANI). The kinetics of aniline polymerization by successive diffusion in these membranes is investigated, and differences in the transport characteristics of the resulting MF-4SK/PANI and MK-40/PANI composites are identified. It is established from results of electroosmotic and diffusion experiments that the composite MF-4SK/PANI-1 membrane (after 1 h of aniline polymerization) suppresses electrolyte and water flow the most. Diffusion permeability drops by an order of magnitude, and water transport numbers are reduced by 50–70%. In the process of sodium chloride concentration by electrodialysis, the salt content of the concentrate increases by 50–70% with the composite MF-4SK/PANI-1 membrane compared to the base MF-4SK membrane and by 15–20% compared to the electrodialysis MK-40 membrane. Transport characteristics of the membrane pairs under investigation are calculated from the model of limiting concentration by electrodialysis: current efficiency, water transport numbers, osmotic and diffusion permeability. The dominant influence of the electroosmotic mechanism of water transport on the effect of salt solution concentration is established.     

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.     

Potentiometric determination of lysine in aqueous solutions using MF-4SK modified perfluorinated membranes

A selective potentiometric sensor was developed on the basis of MF-4SK modified perfluorinated sulfonic cation-exchange membranes for determining lysine monohydrochloride in mixed aqueous solutions of neutral amino acids. It was shown that the treatment of MF-4SK membranes in ethylene glycol increased the sensitivity of the sensor. The use of MF-4SK perfluorinated sulfonic cation-exchange membranes for determining lysine in aqueous solutions is based on a protolytic reaction; as a result of this reaction, single-charged lysine ions from solution are transferred into doubly charged ions in the membrane phase. The Donnan potential at an individual boundary between the studied solution and the membrane is the sensor response. The response time was 10–15 min. The concentration constants of the sensor selectivity to lysine in the presence of glycine, alanine, and leucine did not exceed 0.019. The relative error of determining lysine monohydrochloride in the studied solutions of neutral amino acids was 2–5%.     

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%.     

A Comparative Study of the Electric Transport of Ions and Water in Sulfonated Cation-Exchange Polymeric Membranes of the New Generation

This work presents the results of the studies concerning the electric transport of ions and water through sulfonated cation-exchange membranes synthesized on the basis of polysulfone (PS) and poly(ether–ether–ketone) (PEEK). The concentration dependences of the water absorption capacity, specific conductance, and diffusion and electroosmotic permeabilities measured in sodium chloride solutions are compared to the analogous characteristics of some commercial membranes (MK-40, MF-4SK, CL-25T) under the same experimental conditions. The model concepts concerning the permeability of ion-conducting membranes as disperse systems are found to be applicable for interpreting the set of the electric transport properties of the membrane samples studied. A cluster–channel type of the membrane structure is identified. The polymeric films based on PS and PEEK are shown to possess characteristics comparable to those of commercial ion-exchange membrane samples and can be recommended for producing polymer compositions with an optimum set of electric transport properties.     

Electrochemical behavior of MF-4SK/polyaniline composites as investigated by membrane voltammetry

Membrane voltammetry was used to investigate composites consisting of perfluorinated MF-4SK membranes and polyaniline (PANI) and synthesized under various conditions. A theoretical analysis of the influence of transport-structure parameters of the ion-exchange membranes on their selectivity and limiting-current value is carried out. For MF-4SK/PANI composites, the increase in the exchange potential at the overlimiting state is found to be more than 2V compared to the original membrane. An analysis of how various factors influence the parameters of a current-voltage curve shows that the presence of anilinium and Fe³⁺ ions in the electromembrane system has no effect on the value of the potential at the onset of the overlimiting state. A phenomenological model is proposed to account for the increased plateau length at the limiting current through a change in the energetic state of water in the perfluorinated-membrane matrix resulting from the template synthesis of polyaniline.     

Influence of modification-regeneration cycles and the charge of the modifying ion on the water permeability of a MF-4SK sulfocationite membrane

The water permeability of the MF-4SK membrane (analogue of the Nafion membrane) modified with singly, doubly, and triply charged metal ions and then regenerated is examined. It is shown that the modification of the membrane with metal ions followed by regeneration substantially increases its water permeability because of a decrease in its thickness. It is found that the introduction of multiply charged ions more significantly affects the characteristics of the membrane.     

Peculiarities of electrotransport characteristics of composite membranes PANi/MF-4SK in sulfuric acid solutions

A methodological approach towards assessing the conducting, diffusion, and selective properties of perfluorinated membranes MF-4SK and also their composites with polyaniline (PANi/MF-4SK) is presented. Studying these properties in NaCl and H₂SO₄ solutions makes it possible to elucidate the role played by the nature of counter-ions and co-ions in the charge and mass transfer. It is shown that both individual and composite membranes in their protonic forms are highly selective and, on average, have a conductivity that is higher by a factor of 3.5 as compared with their sodium forms. In contrast, the diffusion permeability is 4-times lower, in line with variation in the charge of the ions that define the mass transfer (co-ions Cl^? or HSO ₄ ^? ). Within the framework of model assumptions on the structure of ion-exchange membranes, the transport and structure parameters of MF-4SK and PANi/MF-4SK are determined in H₂SO₄ solutions for a PANi-saturation of 25%. It is found that aromatic PANi chains localized in aqueous clusters of the template inhibit the diffusion transfer through the PANi/MF-4SK films, whereas the conductivity and transport numbers of PANi/MF-4SK in both NaCl and H₂SO₄ remain virtually unchanged.     

Synthesis and transport properties of proton-conducting membranes based on polyvinylidene fluoride films with introduced and sulfonated polystyrene

Thermal polymerization of styrene sorbed into a polyvinylidene fluoride (PVdF) film from a toluene solution followed by sulfonation of the resulting material was performed. The kinetics of polystyrene (PS) accumulation in the PVdF film during thermal polymerization was studied. Samples with 6–30 wt % PS and ∼100% PS sulfonation were obtained. Proton-exchange membranes wsith an ion-exchange capacity of up to 2 mg-eq/g and proton conductivity of up to 0.008 S/cm at 75% relative humidity were prepared. The permeability coefficients of water, methanol, and hydrogen and their dependences on the amount of introduced PS, ion-exchange capacity, and water uptake of membranes were measured. The synthesized materials proved similar to MF-4SK membranes in their basic transport characteristics and can be used as proton-exchange membranes in hydrogen-air and alcohol fuel cells.     

Transport properties of MF-4SK membranes modified with inorganic dopants

Proton transport in MF-4SK perfluorinated sulfo cationic membranes modified by silica and zirconium hydrogen phosphate were studied using pulsed-field gradient NMR and impedance spectroscopy. At high water contents, water molecules are involved in proton transfer. At low water contents, the proton conductivity of modified membranes considerably exceeds the conductivity of an unmodified membrane. The most likely reason for this effect is the generation of an extra H-bond network involving the dopant and sulfo groups of the membrane.     

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.     

Preparation of MF-4SC composite membranes with the anisotropic distribution of polyaniline and ion-transport asymmetry

A method of preparing MF-4SK membranes with the anisotropic distribution of aniline over the thickness is developed. The processes of aniline polymerization in the matrix of the MF-4SC sulfocationite membrane are investigated via electronic-absorption and IR spectroscopy. The processes of ion transport in the obtained composite membranes are studied via impedance spectroscopy, voltammetry, and potentiometry.     

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.     

Composite Polyaniline/MF-4SK Membranes: A Chemical Template Synthesis and the Sorption and Conduction Properties

The sorption and hydrophilic properties of composite membranes PAN/MF-4SK, obtained by a chemical template synthesis, are studied. The synthesis is performed in static and dynamic conditions for perfluorinated sulfo-cationite membranes MF-4SK in 0.01 M aniline solutions in 0.5 M H₂SO₄ and 0.01 M FeCl₃ in 0.5 M H₂SO₄. The aniline sorption and polymerization kinetics is studied by conductimetry, radioisotope, and optical methods. The polymerization rate is found to depend on the oxidant concentration and the sequence of the membrane saturation with aniline. An increase in the intensity of the color of a composite film, which depends on the balance between the quinoimine, amine, and radical-cation fragments in the perfluorinated matrix, does not lead to any substantial changes in the moisture content or the ac electroconductivity. Implanting polyaniline chains into the perfluorinated matrix presumably leads to a molecular reorganization of water at the expense of a change in its association at the point where redox fragments of polyaniline converge with side segments of the template matrix.     

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–Structural Parameters of Perfluorinated Membranes Nafion-117 and MF-4SK

The concentration dependences of the electroconductivity of membranes Nafion-117 and MF-4SK are studied in salt solutions under identical experimental conditions. The results are compared with nanostructural characteristics of Nafion-117 measured independently by the AFM and quasi-elastic neutron scattering methods. The role played by internal interfaces between hydrophobic and water-cluster microphases of Nafion during the membrane transition from the Na⁺ form to the form of tetraalkylammonium ions is determined. It is established that the specific adsorption of organic ions makes the water clusters disintegrate and the elasticity of side segments diminish; as a result, the polymer film's conductivity switches off. This morphological effect is used for interpreting the fact that the conducting and hydrophilic properties of perfluorinated membranes substantially depend on their conditioning prior to measurements. It is shown that expanding the membranes reduces the differences between the electroconductivity and structural organization of specimens of different brands with close values of the exchange capacity.     

Studies of asymmetry of diffusion permeability of nanocomposite ion-exchange membranes: Model of charge density of fixed groups linear by membrane thickness

A new model of a fine-porous membrane with a linear distribution of charges of its fixed groups along its depth is suggested in the work to explain the effect of asymmetry of diffusion permeability of perfluorinated MF-4SK membranes surface-modified by polyaniline. The model allowed better describing the experimental data for the synthesized nanocomposite MF-4SK/PANI membranes than the earlier developed model of a bilayer membrane with constant fixed charge densities in layers, which points to its feasilibity.     

Chronopotentiometric Method of Studying Electroosmosis in Systems with Ion-Exchange Membranes and Lysine Solutions

A new method of chronopotentiometric study of electroosmotic water transfer in the electromembrane system containing amino acid solution is elaborated. An electrodiffusion problem, which allows for a convective solvent transfer in the system, is formulated and solved. By comparing calculated and experimental transition times for MK-40 and MF-4SK cationite membranes in lysine monohydrochloride solutions, the electroosmotic permeability of the membranes and transport numbers of water through them are calculated. A considerably higher electroosmotic water transport through MK-40 as compared with MF-4SK is attributed to differences in their nature and structure.     

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.