Spectral properties of concentration field fluctuation in electromembrane systems with sulfocation-exchange membranes with different ion-exchanger dispersity
Abstract
One of the formation mechanisms of the concentration field oscillations in electromembrane systems under overlimiting current modes is electroconvective instability. The intensity of electroconvective mixing of the solution at the interface depends not only on current regimes and hydrodynamic conditions, but also is determined by heterogeneity of membrane surface. The aim of this work is to study the effect of surface properties of experimental samples of heterogeneous sulfocation-exchange membranes on the spectral composition of the concentration field fluctuations in solution under intensive current regimes.
The Fourier analysis method was used to determine the spectral composition of concentration field fluctuations in stratified systems with experimental heterogeneous sulfocation-exchange membranes Ralex CM Pes («MEGA» a.s., Czech Republic) containing an ion-exchanger of various dispersity. The degree of dispersity of the sulfocation-exchanger was varied by using a milling time from 5 to 80 minutes. The experiments were performed in a seven-compartment electrodialyzer at its stable concentration-temperature stratification in a gravitational field. To visualize the concentration fields at the membrane – solution boundary, a set up based on a Mach-Zehnder interferometer was used. The spectral density of the optical noise was calculated based on the Fast Fourier Transformation of time series of interference fringe fluctuations.
Using scanning electron microscopy, it was established a decrease in the spacing of electrical heterogeneity (the total size of the conductive and non-conductive regions) of the membrane surface with an increase in the milling time of ion-exchange particles from 5 to 80 minutes. Visualization by laser interferometry showed that reducing the spacing of the electrical surface heterogeneity leads to an increase in the intensity of the electroconvective solution instability at the interface. For experimental samples of CM Pes membranes, it was found increasing amplitude and frequency of oscillations of the interference band with increasing current density. The maximum values of the amplitude and average frequency of oscillations of the interference band, as well as the highest degree of electroconvective mixing of the solution, were observed at the interface with the membrane with a more homogeneous surface, i.e. containing the ionexchanger after 80 minutes of milling.
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