Theoretical study of interdiffusion of aqueous solutions of 1 : 1 electrolytes having the same anion through a cation-exchange membrane

The interdiffusion of aqueous 1: 1 electrolytes having the same anion through a negatively charged (cation-exchange) membrane has been studied without taking into account the diffusive layers. It has been established that the interdiffusion coefficients of the cations depend (in addition to their own diffusion coefficients in the membrane) on the ratio of the diffusion coefficients of both cations to the diffusion coefficient of the anion and the ratio of the density of charges fixed in the membrane to the equal concentration of the electrolytes on both sides of the membrane, as well as the equilibrium distribution coefficients of cationanion ion pairs in the membrane matrix. The conditions have been found under which the membrane plays the role of a “blocking system” (like a diode) that is impenetrable to cations located on both sides of the membranes in spite of the existence of their concentration gradients. The developed approach can be used to describe the interdiffusion of 1: 1 electrolytes through any uniformly charged membrane.     

Effects of Electrolytes on the Transport Phenomena in a Cation-Exchange Membrane

The effect of electrolyte solutions on the characteristics of the current-voltage (I-V) curve in a cation-exchange membrane (CMX membrane, Tokuyama Soda, Inc.) was studied based on the concentration polarization and electroconvection theory. The study includes the limiting current density (LCD), plateau length, and the ratio of resistance of region III to region I of the I-V curve (R(3rd)/R(1st)). Different electrolyte solutions, HCl, LiCl, NaCl, KCl, CaCl(2), MgCl(2), and AlCl(3), were used in this study. The LCD values of the electrolytes were correlated with the diffusion coefficient of the cation (D(+)) and valence of the cation and anion (z(+), z(-)). Except for the HCl solution, the LCD values of the electrolytes increased linearly with D(+)(1-z(+)/z(-)), implying that the current in this region was governed by the concentration polarization phenomena. The deviation of the HCl solution from the linearity is due to a particular transport mechanism of the proton called the Grotthuss-type transport. The differences in the plateau length and the resistance ratio, R(3rd)/R(1st), with the electrolytes were explained by the Péclet number (Pe) representing a transport pattern in the electroconvection theory. The Péclet number is proportional to the Stokes radius of an ion. An electrolyte with a large Stokes radius has a shorter plateau length and a lower ratio of R(3rd)/R(1st) than those of an electrolyte with a small Stokes radius. Water-splitting measurements for the different electrolyte solutions in the CMX membrane revealed that the contribution of water splitting to the overlimting current was insignificant regardless of the electrolytes used in this study. However, when metal hydroxides, such as Al(OH)(3), formed on the surface of the membrane, significant water splitting was observed. Copyright 2001 Academic Press.     

Asymmetry of diffusion permeability of bi-layer membranes

To reveal the reason of asymmetry of the diffusion permeability of bi-layer electrodialysis membranes the following problems have been solved using the model of "homogeneous porous membrane": - diffusion of non-electrolyte solutions across a bi-layer membrane; - diffusion of electrolyte solutions across a non-charged bi-layer membrane; - diffusion of electrolyte solutions across a charged single layer membrane; - diffusion of electrolyte solutions across a charged bi-layer membrane. It is shown that the main factor responsible for the asymmetry is the difference between absolute values of densities of fixed charges (or so called "exchange capacities") of different layers of a membrane under investigation. Only in this case the ratio of the thickness of the membrane layers as well as the ratio of ion diffusivities contributes also to the asymmetry of the diffusion permeability. In the present review we survey and generalize our previous investigations and propose a new theory of asymmetry of diffusion permeability of bi-layer membranes. We have deduced explicit algebraic formulas for the degree of asymmetry of diffusion permeability of bi-layer membranes under consideration.     

Anionic structure-dependent photoelectrochemical responses of dye-sensitized solar cells based on a binary ionic liquid electrolyte

Room temperature ionic liquids (RTILs) have been used as electrolytes to investigate the anionic structure dependence of the photoelectrochemical responses of dye-sensitized solar cells (DSCs). A series of RTILs with a fixed cation structure coupling with various anion structures are employed, in which 1-methyl-3-propylimidazolium iodide (PMII) and I(2) are dissolved as redox couples. It is found that both the diffusivity of the electrolyte and the photovoltaic performance of the device show a strong dependence on the fluidity of the ionic liquids, which is primarily altered by the anion structure. Further insights into the structure-dependent physical properties of the employed RTILs are discussed in terms of the reported van der Waals radius, the atomic charge distribution over the anion backbones, the interaction energy of the anion and cation, together with the existence of ion-pairs and ion aggregates. Particularly, both the short-circuit photocurrent and open-circuit voltage exhibit obvious fluidity dependence. Electrochemical impedance and intensity-modulated photovoltage/photocurrent spectroscopy analysis further reveal that increasing the fluidity of the ionic liquid electrolytes could significantly decrease the diffusion resistance of I(3)(-) in the electrolyte, and retard the charge recombination between the injected electrons with triiodide in the high-viscous electrolyte, thus improving the electron diffusion length in the device, as well as the photovoltaic response. However, the variation of the electron diffusion coefficients is trivial primarily due to the effective charge screening of the high cation concentration.     

Partition Equilibrium on the Interface Between a Charged Membrane and a Mixed Electrolyte Aqueous Solution

Ionic partition equilibrium on a charged membrane immersed in a mixed electrolyte solution was systematically investigated and several models were established for the determination of partition coefficients.On the basis of theoretical models,the effects of the concentration ratio λof the fixed group(charged density) to reference electrolyte,the concentration ratio η between the two electrolytes existing in the solution and the valence of the electrolyte ions on the partition equilibrium in a positively charged membrane were analyzed and simulated within the chosen parameters in detail.The obtainable results can also be applicable to a sytem of mixed electrolytes contacting with a negatively charged membrane.The theoretical calculations were confirmed with the experimental data of model mixed electrolytes,NaCl HCl and CaCl2 NaCl partitioned in the system of self-made negatively charged membrane-sulphonated poly (phenylene oxide)(SPPO) with different charge densities.     

Contribution of convection,diffusion and migration to electrolyte transport through nanofiltration membranes

Transport mechanisms through nanofiltration membranes are investigated in terms of contribution of convection, diffusion and migration to electrolyte transport. A Donnan steric pore model, based on the application of the extended Nernst-Planck equation and the assumption of a Donnan equilibrium at both membrane-solution interfaces, is used. The study is focused on the transport of symmetrical electrolytes (with symmetric or asymmetric diffusion coefficients). The influence of effective membrane charge density, permeate volume flux, pore radius and effective membrane thickness to porosity ratio on the contribution of the different transport mechanisms is investigated. Convection appears to be the dominant mechanism involved in electrolyte transport at low membrane charge and/or high permeate volume flux and effective membrane thickness to porosity ratio. Transport is mainly governed by diffusion when the membrane is strongly charged, particularly at low permeate volume flux and effective membrane thickness to porosity ratio. Electromigration is likely to be the dominant mechanism involved in electrolyte transport only if the diffusion coefficient of coions is greater than that of counterions.     

Separation of aqueous electrolyte solutions with asymmetric membranes containing one charged layer

The Nernst-Planck equation and fine-pore membrane model are applied to describe the ultra- and nanofiltration of electrolyte solutions through a inhomogeneous membrane containing one charged layer. Concentration and electric potential distributions, as well as dependences of electrolyte rejection coefficient (selectivity) and streaming potential on system parameters are determined. Asymmetry effect is revealed with respect to the rejection coefficient and streaming potential at different orientations of the selective charged layer relative to the direction of the filtration flow. The cases of 1: 1 and 1: 2 electrolytes are investigated in detail. Theoretical calculations demonstrate that the rejection coefficient of a bi-layer membrane rises in the following series of binary electrolytes: 1: 2 < 1: 1 < 2: 1, when the first layer is positively charged, and in the opposite series of these electrolytes, when the first layer is negatively charged.     

静电位阻模型在纳滤膜跨膜电位解析中的应用

采用静电位阻模型对纳滤膜的跨膜电位进行了理论解析, 考察了溶液体积通量密度、原料液浓度、阴阳离子扩散系数比、膜孔半径和膜体积电荷密度对KCl(1-1型电解质)和MgCl2(2-1型电解质)中的纳滤膜跨膜电位的影响. 研究结果表明, 随着通量密度的增大, KCl和MgCl₂的跨膜电位线性程度增强; 两种电解质的跨膜电位均随着原料液浓度和膜孔半径的增大而下降; 在不同的考察范围内, 阴阳离子扩散系数比对1-1型和2-1型电解质的跨膜电位的影响差别较大; KCl的跨膜电位随着膜体积电荷密度的变化关于零点呈现出对称性, 而MgCl₂的跨膜电位零点则出现在膜体积电荷密度为负的条件下.     

Diffusion studies of cobalt ions and their salts in agar gel medium: Obstruction effect and gel hydration

The obstruction effect in the electrolyte diffusion of cobalt halides and in tracer diffusion of Co²⁺ ions in the presence of different supporting electrolytes at various concentrations has been studied at 25 °C using the zone diffusion technique. It has been observed that obstruction effect expressed in terms of increases with concentration and is higher for electrolyte diffusion than in tracer diffusion. Further, for a given concentration it is found to decrease with increasing charge density of an anion. These observations are explained on the basis of competitive hydration between ions and agar molecules.     

Preparation and characterization of N-methylene phosphonic and quaternized chitosan composite membranes for electrolyte separations

Chitosan was functionalized either by introducing a phosphonic acid group or by quaternization of existing primary ammonium groups in order to make it a water-soluble material. Functionalized chitosans and poly(vinyl alcohol) (PVA)-based nanoporous charged membranes were prepared in aqueous media and gelated in methanol at 10 degrees C to tailor their pore structure. These membranes were extensively characterized for their physicochemical, electrochemical, and permeation characteristics using FTIR, TGA, DSC, water content, ion-exchange capacity, ionic transport properties, and membrane permeability studies. N-Methylene phosphonic chitosan (NMPC)/PVA-based membranes exhibited mild cation selectivity and quaternized chitosan (QC)/PVA composite membranes had mild anion selectivity, while a blend of NMPC-QC/PVA membranes exhibited weak cation selectivity because of formation of zwitterionic structure. Viscosity measurements and interaction studies for individual and mixed solutions of NMPC and QC were carried out for the prediction of charge interactions between -PO3H2 and -N+(CH3)3 groups and effect on molecular weight due to functionalization. Elaborate electrochemical and permeation experiments were conducted in order to predict suitability of these membranes for the separation of mono- and bivalent electrolytes based on their hydrated ionic radius, and it was found that among all the synthesized membranes, PC/QC-30 had the highest relative permeability, which may extend its suitability for electrolyte separations. Observations were correlated with equivalent pore radius of the different membranes as estimated by membrane permeability measurements.     

Theoretical estimation of differential coefficients of ion-exchange membrane diffusion permeability

It has been shown that the differential coefficients of the diffusion permeability of MK-40 and Nafion 425 sulfonic cation-exchange membranes to solutions of diverse electrolytes can be calculated within the framework of the theory of generalized conductivity of structurally inhomogeneous membranes with the use of model transport-structural parameters. The calculation has been performed on the basis of experimentally measured concentration dependences of the specific electrical conductivities and diffusion fluxes of electrolytes through membranes into water. The results of the model calculation are in satisfactory agreement with the data obtained by an independent method without resorting to notions of the structural organization of a membrane.     

A comparison of electrochemical and electrokinetic parameters determined for cellophane membranes in contact with NaCl and NaNO3 solutions

Electrochemical and electrokinetic characterizations of cellophane membrane samples have been carried out by measuring membrane potential, salt diffusion, and tangential streaming potential, which allow the determination of different characteristic membrane parameters. Experiments were made with the membrane samples in contact with NaCl and NaNO(3) solutions at different concentrations and under different external conditions (concentration gradients), in order to obtain differences in transport and membrane characteristic parameters, depending on the electrolyte considered. Salt permeability across the membrane, which was obtained from diffusion measurements, is about twice as high for NaCl solutions as for NaNO(3) solutions, which is attributed to the different sizes of the electrolytes. Membrane potential measurements keeping the concentration ratio constant (C(1)/C(2)=2) were used to determine both the effective fixed charge concentration in the membrane, X(f), and the average value of transport numbers, t(i); taking into account these values, concentration dependence of membrane potential under a different external condition (C(1)=cte=0.01 M, 5 x 10(-3)< or =C(M)< or =5 x 10(-2)) was predicted. Results show that cellophane membrane behaves as a weak cation-exchange membrane and its permselectivity to cations is practically independent of the electrolyte considered. From electrokinetic results, assuming a Langmuir-type adsorption of anions on the cellophane surface, the number of accessible sites per surface unit was obtained, which is higher for Cl(-) than for NO(3)(-), in agreement with the small radii of chlorine ions; however, no significant differences in the specific adsorption free energy were found (DeltaG(Nacl)=-22.0 x 10(3) J/mol) and (DeltaG(NaNO(3))=-23.2 x 10(3) J/mol).     

Dynamics of ion exchange between self-assembled redox polyelectrolyte multilayer modified electrode and liquid electrolyte

A probe beam deflection (PBD) study of ion exchange between an electroactive polymer poly(allylamine)-bipyridyl-pyridine osmium complex film and liquid electrolyte is reported. The PBD measurements were made simultaneously to chronoamperometric oxidation-reduction cycles, to be able to detect kinetic effects in the ion exchange. Layer-by-layer (LbL) self-assembled redox polyelectrolyte films with osmium bipyridyl complex covalently attached to poly(allylamine) (PAH-Os) and poly(styrene sulfonate) (PSS) have been built by alternate electrostatic adsorption from soluble polyelectrolytes. The ionic exchange during initial conditioning of the film ("break-in") undergoing oxidation-reduction cycles and recovery after equilibration in the reduced state have shown an exchange of anions and cations with time lag between them. The effect of the nature of cation on the ionic exchange has been investigated with dilute HCl, LiCl, NaCl, and CsCl electrolytes. The ratio of anion to cation exchanged at the film-electrolyte interface has a strong dependence on the nature of charge in the topmost layer, that is, when negatively charged PSS is the capping layer, a larger proportion of cation exchange is observed. This demonstrates that the electrical potential distribution at the redox polyelectrolyte multilayer (PEM)/electrolyte interface determines the ionic flux in response to charge injection in the film.     

Transport in polymer-gel composites: response to a bulk concentration gradient

This paper examines the response of electrolyte-saturated polymer gels, embedded with charged spherical inclusions, to a weak gradient of electrolyte concentration. An electrokinetic model was presented in an earlier publication, and the response of homogeneous composites to a weak electric field was calculated. In this work, the influence of the inclusions on bulk ion fluxes and the strength of an electric field (or membrane diffusion potential) induced by the bulk electrolyte concentration gradient are computed. Effective ion diffusion coefficients are significantly altered by the inclusions, so-depending on the inclusion surface charge or zeta potential-asymmetric electrolytes can behave as symmetrical electrolytes and vice versa. The theory also quantifies the strength of flow driven by concentration-gradient-induced perturbations to the equilibrium diffuse double layers. Similarly to diffusiophoresis, the flow may be either up or down the applied concentration gradient.     

Transport and separation of Ag(+) and Zn(2+) by donnan dialysis through a monovalent cation selective membrane

Donnan Dialysis of Ag⁺ and Zn²⁺ was investigated through a cation exchange membrane (CMS Neosepta) when a proton concentration difference was maintained between the two sides of the membrane. Developed for the production of brine from sea water, CMS Neosepta showed a higher permeability to monovalent than to bivalent cations. Several physico-chemical parameters have been determined (electrical resistance, membrane potential, sorption of electrolytes, Zn²⁺ and Ag⁺ diffusion coefficients). The flux of Ag⁺ and the diffusion potential in the membrane increase with HNO₃ concentrations. Ag⁺ and Zn²⁺ can be separated because of the preferential membrane transfer for Ag⁺.     

Diffusion coefficients of lithium chloride and potassium chloride in hydrogel membranes derived from acrylamide

Diffusion of non-associated electrolytes (potassium chloride and lithium chloride) in concentrated aqueous solutions (0.1-1.0 mol dm⁻³) has been studied in hydrogels derived from acrylamide and methyl methacrylate to study the mechanism of electrolyte transport. The preparation of two gels with different monomer ratio compositions resulted in obtaining membranes of substantially different hydrophilic character with polymer fractions of 0.3 and 0.5.Cukier hydrodynamic model was applied to explain the dependence of the diffusion coefficients of KCl and LiCl on the electrolyte concentration in hydrogel obtained experimentally. It was shown that the increase of the diffusion coefficients is accompanied with a decrease of the mean distance of approach of the ions. This can be explained by the formation of ion-pairs, resulting in a further contribution to diffusion once there is a decrease in the hydrodynamic resistance of the medium to the diffusing particles. Parameters, which characterise such a behaviour quantitatively, are different for different electrolytes and depend on water content in the gel.     

Transport Studies with Composite Membrane by Sol-Gel Method

The preparation of polystyrene-based composite membrane at different pressures with varying amounts of material has been described. In order to understand the mechanism of transport of ions, membrane potential measurements were carried out using different concentrations of 1:1 electrolyte (KCl, NaCl, and LiCl) solutions and also to evaluate various membrane parameters such as mobility, distribution coefficient, and charge effectiveness controlling the transport phenomena. The membrane potential offered by the electrolytes is in the order of LiCl > NaCl > KCl, and the membrane is found to be cation selective. The large deviation in the value of K _± at the lower concentration of electrolytes was attributed to the high mobility of comparatively free charges of the strong electrolyte. Teorell, Meyer, and Sievers (TMS) method was used for the estimation of the thermodynamically fixed charge density of membranes. The data were then utilized to calculate membrane potential using the TMS theory. It was interesting to note that the theoretical predictions were borne out quite satisfactorily with experimental results.     

Electrolyte Anion Affinity and Its Effect on Oxyanion Adsorption on Goethite

The influence of various types of background electrolytes (NaCl, NaNO(3), and NaClO(4)) on the proton adsorption and on the adsorption of sulfate and phosphate on goethite have been studied. Below the PZC the proton adsorption on goethite decreases in the order Cl>NO(3)>ClO(4). The decreasing proton adsorption affects the adsorption of oxyanions on goethite. Anion adsorption of strongly binding polyvalent anions is lower in the studied electrolytes in the order Cl相似文献   

Permselectivity of Gramicidin A Channels Based on Single‐channel Recordings

The expression mechanism of permselectivity through a gramicidin A (gA) channel between two aqueous phases (W1 and W2) was investigated. When the concentration of CsCl or CsBr in W1 was equivalent to that in W2, the single‐channel current was proportional to the absolute value of the applied membrane potential. Although the single‐channel current linearly increased with increasing electrolyte concentration in W1 and W2 until about 0.1 M (mol dm^?3), it began to level off around 0.1 M, indicating that ion permeation across the channel pore is the rate‐determining step and that the saturation of the transporting ion within the channel pore provokes the leveling off. In the case of asymmetric composition of the electrolyte in W1 and W2, the monovalent cation and the counter anion were transported in the opposite direction through the gA channel pore or the bilayer lipid membrane around the gA channel. Finally, the experimental data was fitted using the Goldman‐Hodgkin‐Katz equation based on the relationship between the membrane potential and the single‐channel current to define the ratio of the diffusion coefficients of Cs⁺, Cl^?, and Br^? as 5.7 : 1.0 : 0.26.     

丝素膜上药物渗透量对溶液PH值的响应

蚕丝丝素蛋白膜是一种具有弱碱性和弱酸性的两性荷电膜。因此在丝素膜－水溶液体系中，水溶液ｐＨ值变化会影响丝素膜上溶质的渗透速度和渗透量。丝素膜的等电点ｐＨ≈４．５。丝素膜上药物渗透实验的结果表明：在ｐＨ＝３．０～９．０的范围内，当溶液的ｐＨ＜４．５时，丝素膜带正电，正离子苄三甲氯化铵的渗透系数下降；当溶液的ｐＨ＞４．５时，丝素膜带负电，负离子酚磺酸钠的渗透系数明显下降。中性分子间苯二酚在丝素膜上的透过不受外部溶液ｐＨ值变化的影响。当溶液的ｐＨ＞８或ｐＨ＞４．５时，离子化的药物５－氟尿嘧啶或维生素Ｃ的渗透系数明显变小，这是因为两者分别在ｐＨ＝８（ｐＫａ＝８．０）和ｐＨ＝４．５（ｐＫａ＝４．２５）以上变成带电离子。这表明离子化药物在蚕丝丝素膜上的渗透速度和渗透量对溶液ｐＨ值变化有较好的响应特性。