Diffusion boundary layers during electrodialysis

A laser interferometry method for direct measurements of thicknesses of diffusion boundary layers is described. The method is employed to explore electromembrane processes. Thicknesses of diffusion boundary layers at below and above limiting diffusion current densities are measured and compared with theoretical values. It is shown that the diffusion boundary layers in narrow channels of electrodialyzer sections overlap one another at high current densities. This radically alters traditional notions concerning the diffusion boundary layer. An interpretation for variations in the thickness of a diffusion boundary layer at current densities in excess of the limiting diffusion current density is given.     

<Emphasis Type="Italic">Ab initio</Emphasis> calculation of the structure of acetate cellulose membranes for reverse osmosis

The elementary unit of the monoacetate cellulose membrane in lithium, sodium, and potassium chloride solutions was simulated with the Gaussian-98 program package. The results are used for qualitative explanation of the selective transport of water in comparison with electrolyte in the case of reverse osmosis.     

Interpretation of infrared spectra for ion-exchange systems

Analytical expressions for the frequencies of stretching OH vibrations of hydrated ions are obtained. The expressions are applied to the interpretation of the experimental IR spectra of cation exchanger KU-2, heterogeneous cation-exchange membrane MK-40, and to the Nafion membrane. The comparison of the experimental and theoretical spectra gives evidence of the existence of water molecules located between fixed and mobile ions, i.e., of the dissociation of ion pairs.     

Walter Nernst and analytical chemistry

The history of the introduction of the notion of the solubility product of slightly soluble electrolytes and the elaboration of the theory of the electromotive forces that describes equilibrium electric potentials of metal ion electrodes, redox reactions, and Donnan membrane potentials is presented. The contribution of W. Nernst to the formation of these notions is considered.     

Standardless structural-group analysis of supramolecular systems

The basics of the standardless structural-group analysis of supramolecular systems are proposed. The steps of analysis approbated on the example of studying the structure of a sulfo and a carboxyl cation exchanger are described.     

Short communications

The transport of carbonic acid anions through an anion-exchange membrane (AEM) during electrodialysis is studied. At current densities above limiting diffusion values, fluxes of hydrocarbonates and carbonates are lower than those of strong-electrolyte anions. The reason for the decreased fluxes is the recombination of carbonic acid ions with hydrogen ions that form during irreversible dissociation of water molecules at the solution/AEM interface     

Mutual Effect of Concentration Fields in Solutions of Deionization and Concentration Compartments during Electrodialysis with Ion-Exchange Membranes

The conjugated mass transfer processes in the neighboring compartments of an electrodialyzer are studied theoretically and experimentally. Dependences of the concentration fields, thicknesses of diffusion boundary layers, and maximum solution concentration in the flow on the geometric size, initial concentrations, and the electrolyte feed rate are obtained. A laser interferometry method is employed for experimental modeling and theory verification.     

Transfer of glycols through an MA-41 ion-exchange membrane from aqueous and salt-aqueous solutions

Experiments on the molecular diffusion of glycols through an MA-41 ionite membrane in the SO ₄ ^2? form demonstrated that the flux of the glycols decreases with increasing sodium sulfate concentration in the feed solution and with increasing molecular mass of the glycol. For ethylene glycol, as an example, it was demonstrated that its flux through the membrane nonlinearly depends on its concentration in the feed solution. Based on the observation that the fluxes of the nonelectrolyte and electrolyte differ significantly, we proposed a method for separating these components by means of dialysis through an ionite membrane. Computer simulations of the structure and IR spectra of the ethylene glycol-nine water molecules and ethylene glycol-nine water molecules-M⁺ (M⁺ = Na⁺, K⁺) systems were performed. It was revealed that, for the latter system, the activation energy of self-diffusion of ethylene glycol is lower than that for the former.     

Structural analysis of a sulfo cation exchanger in the form of glycine

A quantum chemical and molecular dynamics simulation of a fragment of the sulfo cation exchanger in the form of glycine is performed. It is found that in the system studied an ionic pair dissociates.