Advances in Colloid and Interface Science (v.139, #1-2)

Contents (vii).

Foreword by Victor Starov; Anatoly Filippov; Ninel Berezina (1-2).

Characterization of ion-exchange membrane materials: Properties vs structure by N.P. Berezina; N.A. Kononenko; O.A. Dyomina; N.P. Gnusin (3-28).
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.
Keywords: Ion-exchange membrane; Water content; Electroconductivity; Diffusion permeability; Electroosmotic transport;

Asymmetry of diffusion permeability of bi-layer membranes by Anatoly N. Filippov; Victor M. Starov; Natalia A. Kononenko; Ninel P. Berezina (29-44).
It is shown that the main factor responsible for the asymmetry is the difference between absolute values of densities of fixed charges (or so called “exchange capacities”) of different layers of a membrane under investigation. Only in this case the ratio of the thickness of the membrane layers as well as the ratio of ion diffusivities contributes also to the asymmetry of the diffusion permeability. In the present review we survey and generalize our previous investigations and propose a new theory of asymmetry of diffusion permeability of bi-layer membranes. We have deduced explicit algebraic formulas for the degree of asymmetry of diffusion permeability of bi-layer membranes under consideration.
Keywords: Bilayer membranes; Reverse osmosis membranes; Ion-exchange membranes; Asymmetry of diffusion permeability;

A brief review of the evolution of the diffusion boundary layer (DBL) conception inspired by the works of Nernst, Levich and Amatore is presented. Experimental methods for studying the DBL in electrode and membrane systems are considered.The electrochemical behaviour of a CM2 cation-exchange membrane in NaCl and KCl solutions is studied by chronopotentiometry at constant under-limiting current. Chronopotentiometric curves are described theoretically by applying the Kedem–Katchalsky equations in differential form to a three-layer system including the membrane and two adjoining DBLs. The conductance coefficients entering the equations are found by treating the results of membrane characterisation: the electrical conductivity, transport numbers of ions and water, electrolyte uptake, as functions of the equilibrium electrolyte solution. The two-phase microheterogeneous model is used for this treatment resulting in presentation of the conductance coefficients as functions of (virtual) electrolyte solution concentration in the membrane.The steady-state DBL thickness (δ) is found by fitting experimental potential drop at sufficiently high times. It is found that δ is proportional to (Δc)− 0.2, where Δc is the difference between the electrolyte concentration in the solution bulk and at the interface. This result differs from the Levich equation, which gives the power equal to − 0.25 for Δc. This deviation is explained by the fact that the theory of Levich does not take into account microscopic chaotic convection motion recently described by Amatore et al.It is shown that the treatment of experimental chronopotentiometric curves with the model developed allows one to observe the role of streaming potential in the membrane. Different mechanisms of streaming potential and their effect on the shape of chronopotentiograms are discussed.A simple analytical solution of Navier–Stokes equations applied to natural convection near an infinite vertical ion-exchange membrane is found. It is shown that the formation of DBL induced by electric current is quasi-stationary. This fact allows the empirical expression found earlier and linking δ with Δc under steady-state conditions to be used in transient regimes. The numerical solution of the non-stationary Kedem–Katchalsky equations together with this empirical expression results in quantitative description of the potential difference (pd) and δ as functions of time in chronopotentiometric experiments. The comparison of theoretical and experimental chronopotentiometric curves shows an excellent agreement, especially for the part after switching off the current. The reasons of a small deviation observed just before the curves attain steady state under a constant current applied are discussed.
Keywords: Ion-exchange membranes; Transport properties; Mathematical modelling; Concentration polarisation; Diffusion boundary layer;

The paper presents a kinetic model developed for ozone dissolution in water and taking into account convective and diffusion processes occurring in the vicinity of floating bubbles that contain an ozone-air mixture. It was shown that the gradient of ozone concentration in a convective-diffusion layer and consequently the rate of ozone transfer from bubbles to the solution depended on the rate of ozone decomposition both in its reaction with organic admixtures and in the conditions of exposure to ultraviolet radiation.The obtained kinetic curves of destruction of organic compounds and changes of ozone concentration in water and ozone-air mixture are compared with experimental data for humic acids. The paper also analyzes additional factors affecting the kinetics of ozone dissolution and the rate of resultant reactions.

The interferometric investigations of electromembrane processes by Vladimir A. Shaposhnik; Vera I. Vasil'eva; Olga V. Grigorchuk (74-82).
The multi-frequency laser interferometric method for measuring concentration profiles under electrodialysis with ion-exchange membranes including the case of small intermembrane distances and in the presence of ion-conducting spacers is developed. Experimental results obtained by laser interferometric method are in good agreement with the mathematical models of electrodialysis. The amplitude of the concentration field oscillations is shown to increase at the current density exceeding the limiting current density, and these oscillations transfer into the state of dissipative chaos. This instability is presumably a consequence of irregularly distributed heat sources, namely, the Joule heat and the heat of dissociation and recombination of water molecules.
Keywords: Electrodialysis; Ion-exchange membranes; Laser interferometry; Membrane-solution interface; Limiting current; Concentration fields; Self-oscillating process; Water dissociation;

A review is presented on an application of a cell method for investigations of hydrodynamic permeability of porous/dispersed media and membranes. Based on the cell method, a hydrodynamic permeability is calculated of a porous layer/membrane built up by solid particles with a porous shell and non-porous impermeable interior. Four known boundary conditions on the outer cell boundary are considered and compared: Happel's, Kuvabara's, Kvashnin's and Cunningham's (usually referred to as Mehta–Morse's condition). For description of a flow inside the porous shell Brinkman's equations are used. A flow around an isolated spherical particle with a porous shell is considered and a number of limiting cases are shown. These are compared with the corresponding results obtained earlier.
Keywords: Permeability; Porous shell; Membranes; Porous media;

Metallocene-containing conjugated polymers by Mikhail A. Vorotyntsev; Svetlana V. Vasilyeva (97-149).
The paper gives a review of publications on polymers with conjugated matrices (PPy, PTh, PAni, hydrocarbon or mixed chains…) which incorporate metallocene complexes (Fe, Ru, Co; Ni, Ti, Zr, Ta) with two cyclopentadienyl ligands (Cp) and their derivatives, in particular with methylated cyclopentadienyl rings (Cp), as well as hemi-metallocene complexes (Fe, Ru, Co, Mn), as pendant groups or inside the principal chain (part B). The information on related short-chain systems, monomers and oligomers, is also included. In part A, a brief overview of various conjugated polymer materials is presented, with their classification in accordance with the conductivity mechanism (ionic, electronic or mixed conductors) or with the structural type (linear-chain organic or mixed polymers, derivatization, metallopolymers, multi-dimensional structures, alternating and block copolymers with organic or mixed units, hybrid materials with a mixture of conjugated and inert polymers, polymers inside a solid matrix, conjugated polymers with incorporated nanoelements of transition metals, carbon, semiconductors etc.
Keywords: Conducting polymer; Ferrocene; Metallocene; Hemi-metallocene; Modified electrode; Hybrid material;

Negative rejections of ions in pressure-driven membrane processes can be caused by several distinct mechanisms. In a number of cases, in a final count, the phenomenon is brought about by increased concentration of an ion in the membrane phase. In the case of charged membranes, the increased concentration has to be accompanied by a weakening of electric field of filtration potential, which normally retards counter-ions and prevents the increased concentrations from manifesting themselves in negative rejections. This occurs in charge-mosaic membranes due to the so-called current circulation phenomenon or in electrolyte mixtures due to the presence of more mobile counter-ions.Negative rejections can also occur for ions whose concentration is decreased in the membrane phase. This occurs in electrolyte mixtures due to the acceleration of such ions by the electric field of diffusion potential arising because of strong rejections of other mixture components. This phenomenon is most pronounced for single-charge ions in the presence of predominant amounts of ions of higher charge of the same sign.All those mechanisms are considered within the scope of a common theoretical framework. An attempt is made of a tentative classification of mechanisms of negative rejections. An overview of available literature data is provided and it is shown that in a number of cases the published information is not sufficiently detailed for a reliable identification of the mechanisms. It is concluded that the studies of negative rejections could be a valuable membrane characterization tool but they need to be more systematic and targeted to fulfil this role.
Keywords: Membrane; Negative rejection; Ion; Pressure-driven; Nanofiltration; Charged membrane; Neutral membrane; Charge-mosaic; Donnan equilibrium; Preferential solvation;