Небавская Ксения Андреевна

The mechanism of electroconvection at a permselective surface presents a high interest for electrodialysis separation processes as well as for microfluidics and other applications. We have studied a commercial Neosepta AMX-Sb anion-exchange membrane and its three modifications differing in the surface charge and, as a consequence, in the degree of hydrophobicity. The zeta-potential and the contact angle were measured; the membranes were characterized by chronopotentiometry and voltammetry. It is shown that at the current densities slightly lower or equal to the limiting current density, the mass transfer rate is mainly affected by the membrane surface charge. However, at the higher current densities, the main factor is the degree of hydrophobicity: the samples with a weakly charged highly hydrophobic surface show lower voltage under the same current density. This peculiarity is explained by the fact that the mechanism of electroconvection (EC) depends on the current density. At underlimiting currents and low voltages, EC occurs as electroosmosis of the first kind; the surface charge determines the parameters of the (quasi)equilibrium electric double layer (EDL), playing the main role in the phenomenon. At overlimiting currents and high voltages, it is the extended space charge region (much thicker than the EDL), which controls EC occurring apparently as electroosmosis of the second kind (nonequilibium EC). Then the contribution of the EDL is less important, while the impact of hydrophobicity increases. It is shown that the equilibrium EC may be quite strong at the AMX-Sb membrane having a highly developed surface roughness of different scales. In the range of 0.03–0.06 V there is an “anomaly”: with increasing current density the potential drop over the AMX-Sb is decreasing instead of increasing.

Журнал физической химии: научный журнал. - Т. 88. - № 7-8. - с. 1114-1120 Москва Издательство "Наука" Проведен сравнительный анализ влияния термического воздействия в водной, щелочной и кислотной средах на морфологию и степень гидрофобности поверхности набухших гетерогенных ионообменных мембран. Установлена корреляция между изменениями морфологии и степени гидрофобности поверхности. Показано, что при длительном (50 часов) термостатировании мембран при температурах выше комнатной степень гидрофобности снижается вследствие значительного увеличения размеров каверн и трещин, которые образуются в результате частичной деструкции инертного связующего (полиэтилена) и армирующей ткани (капрон).

Surface properties were measured together with electrochemical characteristics of a CMX (Neosepta, Tokuyama Corp.) cation-exchange membrane. Relative hydrophobicity was controlled by the contact angle; XPS and SEM were used for characterizing chemical composition and microrelief of the surface, respectively. Voltammetry, chronopotentiometry, and mass transfer rate measurements were made as well. A "fresh" membrane and samples after 10, 25, 100, and 150 h of operation in an electrodialysis cell at an overlimiting current equal to 3 theoretical limiting currents, in a 0.02 M NaCl solution, were characterized. Some electrochemical properties were also measured for a Neosepta cation-exchange membrane, aged 2 years, in an industrial food process. It was found that the hydrophobicity of the CMX membrane has increased after the first 10 h of operation; more and more cavities of the dimension of the order of 1 μm have appeared with time testifying electrochemical erosion of the surface. The limiting current density (i(lim)) and the overlimiting transfer rate through the CMX membrane increased with time of its operation under overlimiting current. In the case of new CMX, i(lim) was very close to the theoretical value i(lim)(theor) calculated by the Lévêque equation. After 10 h of operation, i(lim) increased by 5%, and after 25, 100, and 150 h, the increase was by 30%, 70%, and 100%, respectively. Similarly, the mass transfer rate was found to increase up to 5 times (when desalting 0.005 M NaCl under 3 V) in comparison with the theoretical value. The ensemble of data was explained by the hypothesis that the passage of intensive current produces erosion of the ion-exchange polymer forming a continuous phase in CMX. This erosion results in exposure at the surface of the other constituent of CMX: small (about 100 nm) particles of relatively hydrophobic polyvinylchloride. Increasing surface hydrophobicity facilitates the slip of electroconvective vortexes along the surface. Besides, the geometry of the cavities gives rise to appearing tangential electric force applied to the extended space charge density at cavity's walls. As the local limiting current density within a cavity is lower than at the flat surface, electroconvective vortices arise at current densities lower than i(lim)(theor). With time, the number and the size of cavities increase (apparently, due to paired electroconvective vortices occurring inside them) that seems the main reason for overlimiting transfer increase.