Membrane Current-voltage Characteristics Research Articles

Current-Voltage and Transport Characteristics of Heterogeneous Ion-Exchange Membranes in Electrodialysis of Solutions Containing a Heterocyclic Amino Acid and a Strong Electrolyte.

The alterations in current-voltage and transport characteristics of highly basic and strongly acidic ion-exchange membranes, during the electrodialysis of solutions containing a heterocyclic amino acid and a strong electrolyte, were studied. An increase in the catalytic activity of the water splitting process at the surface of heterogeneous MK-40 and MA-41 membranes upon prolonged contact with proline and tryptophan solutions was found. A significant effect of electroconvection on the components mass transfer through the cation-exchange membrane in the intensive current mode of electrodialysis was revealed for the solution containing a heterocyclic amino acid along with mineral salt (NaCl). This led to a reduction in the length of the "plateau" of the membrane's current-voltage characteristics, in comparison with the characteristics for an individual sodium chloride solution with the same concentration. The changes in the characteristics of the studied ion-exchange membranes caused by contact with solutions containing heterocyclic amino acids during electrodialysis were reversible when applying electrochemical regeneration (cleaning in place) using the overlimiting current mode, corresponding to the region of facilitated transport for these ampholytes.

Current-Voltage Characteristics of Membranes with Different Cation-Exchanger Content in Mineral Salt-Neutral Amino Acid Solutions under Electrodialysis.

The features of the electrochemical behavior of experimental heterogeneous ion-exchange membranes with different mass fractions of sulfonated cation-exchange resin (from 45 to 65 wt%) have been studied by voltammetry during electrodialysis. Electromembrane systems with 0.01 M NaCl solution and with a mixed 0.01 M NaCl + 0.05 M phenylalanine (Phe) solution have been investigated. A significant influence of the ion-exchanger content on the parameters of current-voltage curves (CVCs) was established for the first time. Electrodialysis of the sodium chloride solution revealed a decrease in the length of the limiting current plateau and in the resistances of the second and third sections of the CVCs with an increase in the resin content in the membrane. The fact of the specific shape of the CVCs of all studied cation-exchange membrane samples in mixed solutions of the mineral salt and the amino acid was established. A specific feature of current-voltage curves is the presence of two plateaus of the limiting current and two values of the limiting current, respectively. This phenomenon in electromembrane systems with neutral amino acids has not been found before. The value of the first limiting current is determined by cations of the mineral salt, which are the main current carriers in the system. The presence of the second plateau and the corresponding second limiting current is due to the appearance of additional carriers due to the ability of phenylalanine as an organic ampholyte to participate in protolytic reactions. In the cation-exchange electromembrane system with the phenylalanine containing solution, two mechanisms of H+/OH- ion generation through water splitting and acid dissociation are shown. The possibility of the generation of H+/OH- ions at the enriched solution/cation-exchange membrane interface during electrodialysis of amino acid containing solutions is shown for the first time. The results of this study can be used to improve the process of electromembrane demineralization of neutral amino acid solutions by both targeted selection or the creation of new membranes and the selection of effective current operating modes.

Effect of current density, concentration of ternary electrolyte and type of cations on the monovalent ion selectivity of surface-sulfonated graft anion-exchange membranes: modelling and experiment

The dependence of monovalent-ion selectivity of surface-sulfonated anion exchange membranes (s-AEMs) on the electrodialysis (ED) desalination current density, the concentration of equimolar ternary electrolyte (XCl + X2SO4) in the total concentration range 0.04–0.2 M, and the type of cations (X = H, Cs, Na) has been studied. The membrane current-voltage characteristics and values of the diffusion permeability coefficients in electrolyte solutions and their mixtures were measured. It was shown that Cl/SO4-selectivity coefficients (PCl|SO4) increase along with current density, reaching maximum values at the limiting current density (Ilim). The PCl|SO4 maximum value of 5.5 was achieved for the s-AEM-18 membrane in the ED-desalination of equimolar ternary electrolytes NaCl/Na2SO4, and CsCl/Cs2SO4 (total salt concentration 0.04 M) at 1.4 and 2.4 mA cm−2 current densities, respectively. At the overlimiting currents, the splitting of water and the decrease of Cl/SO4-selectivity were observed. The obtained experimental results are interpreted using simplified mass transfer and numerical simulations in the framework of Nernst-Planck-Poisson equations implemented in the COMSOL® Multiphysics software. The model represents the modified membrane as an asymmetric bipolar membrane with two diffusion boundary layers (DBL) in electrolyte solutions on both sides of the membrane interface. The model includes the water splitting at the bipolar boundary and the chemical equilibrium between sulfate and hydrogen sulfate ions. The numerical simulations qualitatively predict the behaviour of s-AEMs membranes when the abovementioned conditions change.

Multiple conductance states in single channels of variable resistance lipid bilayer membranes

The properties of the cation-permeable excitability-inducing material (EIM) channels in lipid bilayer membranes have been investigated using membranes penetrated by only one to four active channel units. Under these conditions, individual channels may be observed to undergo on-off conductance transitions that may be correlated with the negative resistance characteristics of membranes having a large number of channels.In many lipid membrane systems, EIM channels have several conductance levels for each channel. Such channels may open or close with either a single, large conductance step or in several smaller jumps having a sum equivalent to the large jump. The conductance of the fully closed state in such channels is generally less than 2% of the open-state conductance, too low to be differentiated from membrane conductance in the absence of an EIM channel. This ratio is reflected in the current-voltage characteristics of membranes containing large numbers of channels, where ratios of maximum conductance to minimum conductance may range from 50 to 200.EIM channels in oxidized cholesterol bilayers are exceptional in showing only two conductance levels normally. The closed state in this system corresponds to a low intermediate state in the general channel.The conductance of each state apears to be linearly related to the activity of the electrolyte.

Apparatus for measurement of the dynamic current-voltage characteristics of membranes

A variable limit linear sweep generator and an electrometer voltmeter with differential input are employed for measurement of the dynamic current-voltage characteristics of membranes.