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.     

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.     

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.     

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%.     

Barrier effects of polyaniline layer in surface modified MF-4SK/Polyaniline membranes

An express method of synthesis of surface-modified membrane composites is suggested that allows obtaining heterogeneous materials featuring effects of the blocking of the transport of solvent ions and molecules. Studies using the methods of atomic force microscopy (AFM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and IR spectroscopy show that polyaniline is in the form of emeraldine and water at the interface of polyaniline aromatic chains and initial membrane sulfogroups is in a structured state due to formation of interpolymer complexes. This determines higher thermal stability of composites and specific morphology of the polyaniline layer. Measurement of electroosmotic and diffusion permeability and also membrane conductivity in HCl solutions (with the concentration of 0.01 to 2 M) shows the blocking effect of the polyaniline layer causing a significant decrease in transport characteristics in a wide range of concentrations of equilibrium solutions. A model of structural heterogeneity of composites is offered that takes into account the presence of an additional structural element, polyaniline, in the basic polymer matrix. Mechanisms of the observed barrier effects are discussed. Comparison of the transport-structural parameters of the MF-4SK membrane and MF-4SK/PAn composites allows elucidating the role of polyaniline in formation of transport routes for ion and solvent fluxes.     

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.     

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.     

Polymerization of aniline in perfluorinated membranes under conditions of electrodiffusion of monomer and oxidizer

Journal of Solid State Electrochemistry - A series of composites based on the perfluorinated MF-4SK membrane and polyaniline was obtained under electrodiffusion of monomer and oxidizer. Aniline was...     

Effect of modification of ion-exchange membrane MF-4SK on its polarization characteristics

Modifications of perfluorinated membrane MF-4SK are studied by a membrane voltammetry method. Various techniques used for physicochemical modification of the membrane (variation of conditions of its chemical conditioning, insertion of tetrabutylammonium cations) exert predicted action on its structural and electrotransport properties. Correlation is established between variations in the slope of the ohmic segment of a voltammetric curve, the magnitude of the limiting electrodiffusion current, and the potential of the system’s conversion into an overlimiting state and the electrotransport properties of membranes and their structural characteristics that are obtained by independent methods of impedance conductimetry and standard porosimetry. The increase in the limiting current for membranes subjected to a thermal oxidizing treatment is explained by a change in the content and state of water in the membrane bulk. Effects of acceleration of the occurrence of an overlimiting state of the membrane saturated with tetrabutylammonium cations is discovered and possible reasons for this phenomenon are discussed. For membranes saturated with tetrabutylammonium cations, a correlation is established between the limiting current and electroosmotic phenomena.     

Ion transport in MF-4SK membranes modified with hydrous zirconium hydrogen phosphate

Composites based on perfluorinated cation-exchange membranes MF-4SK with embedded zirconium hydrogen phosphate were synthesized using various precursors. Physicochemical interfacial interactions were discovered. The composites have higher proton conductivity and another diffusion permeability than unmodified MF-4SK membranes.     

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.     

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.     

Effect of polyaniline on the current passing through structural fragments of ion-exchange sulfonic-cationite resins and membranes

Concentration dependences of the specific electrical conductivity of ion-exchange resin KU-2 and ion-exchange heterogeneous membrane MK-40 made thereof are obtained and compared prior to and after the membrane modification with polyaniline. It is shown that the introducing of the polyaniline to polymer matrices of ionites results in the change of the specific electrical conductivity of both the resin KU-2 and the membrane MK-40, which correlates with the earlier data obtained for the MF-4SK/polyaniline composites. The quantitative evaluation of the polyaniline effect on the mechanism of current passing through structural fragments of initial and modified ionites is carried out. To this purpose, model parameters were used, which reflect the current paths and the conducting phases’ volume fractions and spatial orientation.     

Template synthesis and electrotransport behavior of polymer composites based on perfluorinated membranes incorporating polyaniline

A method of chemical template synthesis is described for producing composites based on a perfluorinated matrix with polyaniline chains implanted. It has been shown that the choice of experimental and conditioning techniques is relevant for the composites’ investigation. The conductivity, diffusion permeability, selectivity, and electroosmotic permeability of the composites have been investigated in comparison with the same properties of the initial MF-4SC membrane. A model describing the transport behavior of the composites in the doped state as a fibrous-cluster system is proposed. A set of transport and structural parameters of the composites in a H₂SO₄ solution has been calculated and an analysis of the results observed has been carried out. The set of electrotransport properties is explained by the morphological features of the composites, taking into account the redox heterogeneity of polyaniline. The contribution of electron conductivity to the mixed conductivity of composites with a certain saturation degree by polyaniline has been estimated.     

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%.     

Synthesis and characterization of MF-4SK + SiO<Subscript>2</Subscript> hybrid membranes modified with tungstophosphoric heteropolyacid

Composites based on MF-4K perfluorinated cation-exchange membranes doped with hydrous silica nanoparticles, which were precipitated under various conditions, and with tungstophosphoric heteropolyacid nanoparticles were synthesized. The proton conductivity of the composites was studied as a function of temperature and relative ambient humidity. As a result of modification, the water content and ion conductivity of the membranes in low humidities increase by 2.5 orders of magnitude compared to unmodified MF-4SK membranes and the material is rendered less water-dependent.     

Distribution of luminescence excitation energy between Eu<Superscript>3+</Superscript> and Tb<Superscript>3+</Superscript> ions fixed in a perfluorosulfonic membrane

Ion-exchange modification of MF-4SK perfluorosulfonic membranes with Eu³⁺ and Tb³⁺ cations was realized. The state of cations in the membrane was studied by X-ray photoelectron spectroscopy. Special features of the luminescence of the resulting systems point to preferential excitation energy transfer from europium to terbium.     

Synthesis and characterization of red mud/polyaniline composites: Electrical properties and thermal stability

New types of conducting composites using red mud as an inorganic substrate and polyaniline as the conducting phase were prepared. Red mud/polyaniline (RM/PANI) composites were synthesized in acidic aqueous solution by the chemical oxidative polymerization of aniline using ammonium peroxydisulfate as the oxidant. The composites exhibit conductivities in the 0.42-5.2 S cm⁻¹ range, depending on the amount of polyaniline. They were characterized by infrared and UV-vis spectroscopy, scanning electron microscopy and X-ray diffraction. The IR and X-ray results show that PANI is deposited on the RM surface. The composites have a globular structure and the PANI globules synthesized on the surface of RM are smaller than those prepared under the same conditions without the substrate. Thermogravimetric analysis was used for investigation of the thermal stability of the composites. The thermal stability of the conductivity of RM/PANI composites was studied by ageing at 125 °C, the conductivity being measured in situ during this process.     

Diffusion of solutions in the course of the matrix synthesis of composite membranes MF-4SC—polyaniline and their transport properties

The dependence of the kinetics of diffusion transfer of solutions through MF-4SC and MC-40 ion-exchange membranes during the matrix synthesis of polyaniline on the composition of feed solutions and the nature of the initiating agents and the polymer matrix is studied. The effect of the conditions of the modification of the parent membrane on the resultant physicochemical characteristics is studied. The conditions of synthesis for the preparation of MF-4SC—polyaniline (emeraldine) composites with an optimized combination of conducting and diffusion characteristics are found. The contribution of electron conductivity to the overall proton-electron conductivity of the composite films is estimated.     

Electrokinetic Phenomena in Sulfonated Cation-Exchange Membranes with Tetraalkylammonium Ions

The electric transport of solvent ions and molecules through membrane systems containing cations of tetraalkylammonium bases is studied. The interrelation between electrokinetic characteristics of sulfonated cation-exchange membranes at different degrees of their saturation with tetrabutylammonium ions and the content of water in membranes at equilibrium is established. A new type of conductor-insulator percolation transitions is revealed for homogeneous MF-4SK perfluorinated membranes saturated with organic counterions. This transition is interpreted with allowance for the concept of the governing role of narrow channels in the membrane microstructure. The electroosmotic permeability of MF-4SK membranes in solutions of sodium and tetraethylammonium chloride, as well as in their mixtures, is studied. An experimentally observed abnormally high amount of water transferred by tetraethylammonium ions is discussed with allowance for the dynamic hydration characteristics of ions.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 485–493.Original Russian Text Copyright © 2005 by Kononenko, Berezina, Shkirskaya.