Stability of Properties of Layer-by-Layer Coated Membranes under Passage of Electric Current

Abstract

Electrodialysis with layer-by-layer coated membranes is a promising method for the separation of monovalent and polyvalent ions. Since the separation selectivity is significantly reduced in the presence of defects in the multilayer system, the stability of the modifiers becomes an important issue. This article reports the i-V curves of layer-by-layer coated membranes based on the heterogeneous MK-40 membrane before and after 50 h long electrodialysis of a solution containing sodium and calcium ions at an underlimiting current density, and the values of concentrations of cations in the desalination chamber during electrodialysis. It is shown that the transport of bivalent ions through the modified membranes is reduced throughout the electrodialysis by about 50%, but the operation results in decreased resistance of the membrane modified with polyethylenimine, which may suggest damage to the modifying layer. Even after electrodialysis, the modified membrane demonstrated experimental limiting current densities higher than that of the substrate, and in case of the membrane modified with polyallylamine, the limiting current density 10% higher than that of the substrate membrane.

Short-Term Stability of Electrochemical Properties of Layer-by-Layer Coated Heterogeneous Ion Exchange Membranes

“…We suppose that the large uncertainty margins, exceeding sometimes the average values, are at least partially the consequence of the use of differential methods of calculation of layer thicknesses, since the absolute errors add up in subtraction, and of the compressibility of swollen polymeric layers by faces of the micrometer. Additional reasons might be found through comparison with the results obtained by scanning electrical microscopy of a similar membrane with a larger number of adsorbed layers (differing by addition of layer of polystyrene sulfonate, PSS and polyamine) [ 21 ]. Scanning electron microscopy showed that the thickness of the coating differs by the coordinate at the membrane, presumably due to filling of the geometry of heterogeneous membrane by the modifying layers, and gives the thickness of assembly of LF-4SC + PAH + PSS + PAH of 5.12 ± 2.96 μm and the thickness of assembly of LF-4SC + PEI + PSS + PEI of 8.89 ± 7.25 μm.…”

“…It was expected that possible damage to the modifying layer would have a stronger effect on the electrochemical characteristics of the membrane if this layer is the only one present, than in the case of the damage of the last layer among the multiple layers, and therefore such changes would be easier to detect. Stability of the MK-40 membrane modified with a higher number of layers of polymers in desalination of a mixed solution in under-limiting modes is discussed in our other article [ 21 ].…”

“…In this study, the number of layers bearing polar groups charged oppositely to the polar groups of the membrane is limited to one; membranes with a higher number of formed layers are discussed in our previous work [ 21 ]. Membranes with a single oppositely charged layer also possess monovalent selectivity and are used in fractionation of solutions [ 17 ], as well as the membranes with multiple layers.…”

“…As an example of spectroscopic detection of changes occurring during membrane operation, we provide attenuated total reflection Fourier transform infrared spectra of MK-40 membrane and two series of membranes produced by the similar technique that operated in electrodialysis of mixed solution (15 mM NaCl and 7.5 mM CaCl 2 ) for 50 h in under-limiting current mode (approximately 0.5 theoretical limiting values calculated by the approach described in [ 60 ]). Membrane preparation and operation are described in [ 21 ]; in brief, the membranes were also based on MK-40 and underwent surface homogenization with LF-4SC layer and layer-by layer absorption of three layers total: two of polyamine (polyallylamine orpolyethylenimine, depending on series) and one middle layer of sodium polystyrene sulfonate (purchased from Merck KGaA, Darmstadt, Germany). The ATR-FTIR spectra were registered by the “Diagnostics of structure and properties of nanomaterials” Centre for Collective Use of Kuban State University using a Bruker Optics Vertex 70 spectrometer (Bruker, Billerica, MA, USA) for newly prepared membranes and for membranes after operation under current; the measurement were made for dry membranes.…”

“…If the changes occur early during the operation of the membranes, it will cause shift of measured properties during the membrane characterization, making the values time dependent and, consequently, the average values would depend on the number of repeats, hindering the membrane characterization. Short-term operation measurements detect such changes and additionally help in determining a timeframe in which the difference in membrane properties begin to appear, and it was shown that the 50 h long operation of the layer-by-layer coated heterogeneous membrane, modifying layers one of which was polyethylenimine, in under-limiting current mode electrodialysis creates differences in the voltammetric curve of the membrane [ 21 ]. The changes in structure can be attributed to multiple causes including partial drying and swelling of the membrane leading to cracks and bends.…”