N. P. Berezina

Effect of conditioning techniques of perfluorinated sulphocationic membranes on their hydrophylic and electrotransport properties

The effect of perfluorinated sulphocation-exchange membrane conditioning on their hydrophilic and electrotransport properties was studied. Exchange capacity and water content of MF-4SC membranes having been exposed to conditioning were determined by different chemical techniques. The estimated values were compared to the same characteristics after thermal pre-treatment by sequent boiling in hydrogen peroxide, water, acid and water. Dependences of electrical conductivity, diffusion and electro-osmotic permeability upon sodium chloride concentration were determined for MF-4SC, Nafion-115 and -117 provided that the conditioning techniques varied. Consistent increase of the electrotransport membrane characteristics due to the water content increase and water energy state alteration after the membranes were thermally treated was established. It was shown that MF-4SC, Nafion-115 and -117 conditioning by this method leads to formation of ion-conducting polymeric membranes identical in the structure and transport properties.

Water electrotransport in membrane systems. Experiment and model description

Abstract The dependencies of the electroosmotic permeability on concentration and the water content of ion-exchange membranes have been investigated together with the dependencies of the conductivity and the ion transport numbers over a wide range of sodium chloride concentrations. The materials tested were samples of heterogeneous membranes based on ion-exchange resins and samples of homogeneous perfluorinated membranes. Theoretical description of the results has been performed with the help of a model approach regarding a membrane as a system consisting of two conventional conductive phases. The final model equation for the water transport number shows their correlation with the physicochemical characteristics of the polymer material and enables the evaluation of the hydration parameters of the ion-dipole complexes in the cluster phase of the membrane.

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.

Electrochemical template synthesis of an polyaniline composite with polymeric perfluorinated sulfo cationite

Combining a tracer technique with electrochemical measurements offers a unique opportunity to study the mechanism of anodic synthesis of a composite based on an electron-conducting polymer (polyaniline) and a polymeric perfluorinated sulfo cationite (Nafion, MF-4SK). The composite formation involves two steps. First, the monomer (aniline) is intercalated into the solid-cationite matrix probably with the formation of a template phase, after which postintercalation polymerization of the template occurs.

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.

Theoretical and experimental study of asymmetry of diffusion permeability of composite membranes

Theoretical approaches to the description of asymmetry effects of transport characteristics of two-layer membranes are discussed. Based on a uniform finely porous model membrane, a new method is proposed for calculating mass transfer through asymmetric membranes with allowance for the physicochemical properties of their layers. The difference between the effective densities of charges fixed in membrane layers is shown to be the key factor that governs the degree of asymmetry of diffusion permeability. The adequacy of the proposed approach is supported by the comparison between theoretical calculations and experimental data on the diffusion permeability of modified ion-exchange membranes formed on polystyrene and perfluorinated matrices. It is shown that this approach is applicable to the description of the asymmetry effects of diffusion characteristics of MF-4SK perfluorinated membranes, the surface layer of which is modified with polyaniline.

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.

Synthesis and diffusion permeability of MF-4SK/polyaniline composite membranes with controlled thickness of the modified layer

A new method is proposed for the synthesis of anisotropic membranes based on F-4SF films, which ensures their modification with polyaniline in a layer with a fixed thickness. The concentration dependences of conductivity and diffusion permeability of the composite membranes are studied. These parameters decrease with increasing thickness of the polyaniline-modified layer. The quantitative correlation is revealed between the regularities of variations in both transport characteristics and the thickness of the modified layer. The absence of the asymmetry of the diffusion permeability of the examined composite membranes is explained in terms of the model of a charged bilayer membrane.

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.

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 HSO4−). 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.