DIFFUSION BOUNDARY LAYERS IN THE SOLUTIONS AT INTERPHASE WITH THE SULFOCATION-EXCHANGE MEMBRANE AFTER TEMPERATURE MODIFICATION

Abstract

The use of the laser interferometry method allows measuring local concentrations and visualizing the process of formation and development of diffusion layers in solutions near the surface of the ion-exchange membrane. The prolonged exposure of the current and elevated temperatures to heterogeneous ion-exchange membranes leads to irreversible changes in the structure, which are interrelated with transport properties. Therefore, the purpose of this work is to measure the concentration profiles and parameters of the diffusion boundary layers in the solution at the interphase with heterogeneous sulfocation-exchange membranes after temperature effect by laser interferometry.

A commercial heterogeneous MK-40 membrane was chosen as the study object, it consisted of the KU-2 sulfocation-exchanger, polyethylene, and caprone. After chemical conditioning, the membrane samples were thermostated at 100 °C in water for 50 h. The experiments were performed in a seven-compartment electrodialysis cell with alternating cation-exchange and anion-exchange membranes at stable concentration-temperature stratification of the electrodialyzer in a gravitational field. To visualize the transport processes at the membrane-solution boundary, a Mach-Zehnder setup was used.

The concentration profiles and the dimensions of the diffusion boundary layers in the solution at the interphase with the MK-40 sulfocation-exchange membrane after chemical conditioning and temperature influence were determined using laser interferometry. It is shown that at current densities, exceeding the limiting diffusion values, the concentration profiles are transformed from stationary to nonstationary due to electroconvective instability arising in the solution at the interphase. It is established that at the overlimiting the current densities, changes in the surface morphology and microstructure of membranes after heating determine the intensity of the electroconvective mixing of the solution at the interphase and the parameters of the diffusion layers. After the temperature modification, it was established that there was more intense electroconvective instability and a decrease in the thickness and then the destruction of the Nernst diffusion layer, with smaller degree of membrane polarization than for the conditioned membrane.

ACKNOWLEDGMENTS

This work was financially supported by RFBR grant (project No 16-38-00572 mol_a).