Synthesis,characterization and electrochemical properties of cation selective ion exchange composite membranes

In this work polystyrene based strontium phosphate membranes (SPMs) were prepared by applying different pressures. The membrane potential is measured with uni-univalent electrolytes (KCl, NaCl, and LiCl) solutions using saturated calomel electrodes (SCEs). The effective fixed charge density of these membranes is determined by the Torell, Meyer and Sievers method and it showed the dependence of membrane potential on the porosity, the charge on the membrane matrix, charge and size of permeating ions. The membranes are characterized by X-ray diffraction, scanning electron microscopy and IR spectroscopy. The order of surface charge density for electrolytes is KCl > NaCl > LiCl. Other parameters such as transport number, distribution coefficient, charge effectiveness and related parameters are calculated. The membrane was found to be mechanically stable, and can be operated over a wide pH range.     

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.     

Studies with nickel phosphate membranes: evaluation of charge density and test of recently developed theory of membrane potential

The preparation of polystyrene-based nickel phosphate 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 controlling the transport phenomena. Teorell, Meyer and Sievers (TMS) method was used for the estimation of the thermodynamically effective fixed charge density of membranes. The data were then utilized to calculate membrane potential using the extended TMS theory. It was interesting to note that the theoretical predictions were borne out quite satisfactorily with experimental results. scanning electron microscope (SEM) micrographs of the membranes have also been presented.     

1-1型单组分盐溶液中荷电膜膜电位的研究

根据固定电荷模型和非线性最小二乘法, 研究膜体积电荷密度为定值和其大小随电解质主体溶液浓度呈指数变化的两种初始条件下, 五种纳滤膜(NTR 7450, ESNA 1, ESNA 1-LF, LES 90和UTC 60)在不同浓度的氯化钠和氯化钾溶液中的膜电位, 获得膜体积电荷密度与电解质主体溶液浓度的关系. 结果表明, 当体积电荷密度随浓度呈指数变化时, 拟合的膜电位与实验结果更接近, 得到的固定电荷密度更精确. 膜电位的大小与膜两侧电解质溶液浓度的比值相关. 在较高浓度时, 膜电位的值还与扩散系数相关; 其中阴阳离子的扩散系数之比大于1.0是膜电位反号的标志. 在中间浓度时, 膜电位随电解质主体溶液浓度近似呈线性变化.     

Dielectric characterization of a nanofiltration membrane in electrolyte solutions: its double-layer structure and ion permeation

Dielectric spectroscopy (DS) was applied to a nanofiltration (NF) membrane to detect its double-layer structure and ion permeation. Dielectric measurements were carried out on the systems composed of the NF membrane NTR7450 and dilute solutions of eight electrolytes, LiCl, NaCl, KCl, NH(4)Cl, MgCl(2), CaCl(2), BaCl(2), and CuCl(2). Two relaxations were observed in the frequency range from 40 Hz to 4 MHz for each system. On the basis of characteristics of the dielectric spectra and the Maxwell-Wagner interfacial polarization theory, the low-frequency relaxation was attributed to inhomogeneity of the membrane structure itself, whereas the high-frequency relaxation was attributed to interfacial polarization between the membrane and the solution. A multiphase dielectric model previously developed by one of the authors and co-workers was adopted to present systems to analyze the dielectric spectra, and electric parameters, i.e., capacitance and conductance, of the two layers composing the membrane were obtained. The electric properties estimated for the two layers were different and changed with the environment in a different manner. Further analyses suggest that the two layers had a different separation mechanism due to their difference in materials, looseness, and fixed charge content. The fixed charge density of one layer was estimated, and the ion permeation difficulties in both layers was compared. This research revealed that DS was by far an effective method to obtain detailed electric parameters about the inner multilayer structure of the NF membrane and to elucidate separation mechanisms of each layer.     

Surface Charge Density of Silica Suspended in Water-Acetone Mixtures

The net proton surface charge density of silica suspended in water-acetone mixtures was studied by potentiometric titration. LiCl and NaCl were used as background electrolytes at concentrations of 10(-1), 10(-2), and 10(-3) mol L-1. The results showed that acetone lowers the net proton surface charge density of silica and that the greater the decrease, the greater the acetone concentration. The surface charge density of silica also is very sensitive to the nature of the background electrolyte, LiCl producing much lower surface charge densities than NaCl. The concept of free energy of transfer (DeltaG0t) of an electrolyte between two different solvents was applied to explain the results in a qualitative manner. Copyright 1999 Academic Press.     

Theory of pressure-driven transport of neutral solutes and ions in porous ceramic nanofiltration membranes

Hindered transport theory and homogeneous electro-transport theory are used to calculate the limiting, high volume flux, rejection of, respectively, neutral solutes and binary electrolytes by granular porous nanofiltration membranes. For ceramic membranes prepared from metal oxides it is proposed that the membrane structural and charge parameters entering into the theory, namely the effective pore size and membrane charge density, can be estimated from independent measurements: the pore radius from the measured hydraulic radius using a model of sintered granular membranes and the effective membrane charge density from the hydraulic radius and the electrophoretic mobility measurements on the ceramic powder used to prepare the membrane. The electro-transport theory adopted here is valid when the membrane surface charge density is low enough and the pore radius is small enough for there to be strong electrical double layer overlap in the pores. Within this approximation the filtration streaming potential is also derived for binary electrolytes.     

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.     

Effect of different electrolytes on the swelling properties of calyx[4]pyrrole-containing polyacrylamide membranes

Calix[4]pyrrole (1) was synthesized and characterized and this macrocycle was incorporated in polyacrylamide gels. The presence of meso-octamethyl-porphyrinogen inside of gel was checked using infrared spectroscopy, differential scanning calorimetry, and swelling studies. The swelling degree of these hydrogels in equilibrium with different electrolytes (NaCl, LiCl, KCl, CaCl₂ and AlCl₃) was measured in a concentration range 0.1-0.5 mol dm⁻³. Although no significant alterations in the swelling degree can be found for the different 1:1 electrolytes, when the cation charge of unsymmetrical electrolytes increases, the gel swells in a significant way. This swelling process is enhanced by the presence of calyx[4]pyrrole. The effect of alkaline hydrolysis of polaycrylamide-based hydrogels was also studied. The hydrolysed hydrogels collapse in the presence of the electrolytes; this behavior is dependent on the hydrolysis degree, electrolyte charge and calyx[4]pyrrole presence and concentration; the latter leads to polyacrylamide with tailor-made properties.     

Studies on the Electrochemical Characterization of Cellulose Acetate and Dowex-50 Membranes for Uni-Univalent Electrolytes in Aqueous Solutions

Membrane potential and bi-ionic potential studies using cellulose acetate and Dowex-50 membranes and sodium chloride and potassium chloride aqueous solutions have been carried out. The results have been used to estimate solute permeability, ionic transport numbers, fixed charge density, and surface charge density of both the membranes. Both membrane potential and bi-ionic potential are affected by adsorption of ions. Adsorbed ions affect the surface potential as well as solute retention capacity of the membranes. Solute retention expressed in terms of the “maximal retention” of membranes could thus be estimated. Furthermore, a correlation between permselectivity of the membrane and effective concentration with the dielectric constant of the membrane has also been established.     

Electrochemical characterization of an asymmetric nanofiltration membrane with NaCl and KCl solutions: influence of membrane asymmetry on transport parameters

Electrochemical characterization of a nanofiltration asymmetric membrane was carried out by measuring membrane potential, salt diffusion, and electrical parameters (membrane electrical resistance and capacitance) with the membrane in contact with NaCl and KCl solutions at different concentrations (10(-3)< or =c(M)< or =5 x 10(-2)). From these experiments characteristic parameters such as the effective concentration of charge in the membrane, ionic transport numbers, and salt and ionic permeabilities across the membrane were determined. Membrane electrical resistance and capacitance were obtained from impedance spectroscopy (IS) measurements by using equivalent circuits as models. This technique allows the determination of the electrical contribution associated with each sublayer; then, assuming that the dense sublayer behaves as a plane capacitor, its thickness can be estimated from the capacitance value. The influence of membrane asymmetry on transport parameters have been studied by carrying out measurements for the two opposite external conditions. Results show that membrane asymmetry strongly affects membrane potential, which is attributed to the Donnan exclusion when the solutions in contact with the dense layer have concentrations lower than the membrane fixed charge (X(ef) approximately -0.004 M), but for the reversal experimental condition (high concentration in contact with the membrane dense sublayer) the membrane potential is practically similar to the solution diffusion potential. The comparison of results obtained for both electrolytes agrees with the higher conductivity of KCl solutions. On the other hand, the influence of diffusion layers at the membrane/solution interfaces in salt permeation was also studied by measuring salt diffusion at a given NaCl concentration gradient but at five different solutions stirring rates.     

A novel measurement method of Donnan potential at an interface between a charged membrane and mixed salt solution

We developed a novel measurement method of the Donnan potential difference at a charged membrane/salt solution interface. The method can measure the potential under the condition that the membrane charge density is much lower than the KCl concentration of the salt bridge. This method is very useful for obtaining the effective charge density of each layer of a bipolar membrane. The present experiments in a system of a negatively charged poly(vinyl alcohol) membrane and a single salt solution of KCl, NaCl, LiCl, CaCl₂ and LaC₃ revealed that the membrane effective charged density has the same value for all the ions. The experiments in mixed KCl and CaCl₂ solution revealed that the potential in the system is governed mainly by the concentration of the counterion having the highest valence in the system.     

Towards Reversibility of Ion Transfer Across the Interface between Valinomycin Membranes and Aqueous Electrolyte Solutions

Valinomycin-based potassium-selective membranes doped with potassium tetrakis(4-chlorophenyl)borate (KClTPB) or sodium tetrakis(4-fluorophenyl)borate (NaFTPB) are studied in KCl, NaCl, and CaCl₂ solutions by potentiometric and electrochemical impedance methods. Before contact with KCl, membranes doped with NaFTPB provide Nernstian potentiometric response to Na⁺ ions, which is lost after conditioning the membranes in KCl. The membranes doped with KClTPB even before contact with KCl give no Nernstian response to Na⁺ ions. In CaCl₂ solutions, none of the membranes provide a regular potentiometric response. Despite the difference in potentiometric behavior, the impedance spectra of the membranes are very similar in all solutions regardless of prior conditioning of the membranes. No evidence for a hindrance towards charge transfer processes is observed. The results suggest that the membrane/solution interface is reversible for interfering ions as well as for potassium, and the contamination of solutions with the latter is the sole reason for the lack of Nernstian response in the interfering electrolytes.     

Diffusion of Manganese Ions in Agar Gel Containing Alkali Metal Chlorides

The obstruction effect for tracer-diffusion of Mn²⁺ ions in the presence of different supporting electrolytes (LiCl, NaCl, KCl, CsCl) at various concentrations has been studied at 25 °C using the zone diffusion technique. It has been observed that the obstruction effect determined in terms of increases with concentration of the electrolyte. Further, for a given concentration it is found to decrease with increasing charge density of the cation. We also report here on the effect of temperature on obstruction and found that is constant over the temperature range studied (25–45 °C). These observations are explained on the basis of competitive hydration between ions and agar macromolecules.     

Thermoosmosis of various electrolyte solutions through anion-exchange membranes

Thermoosmosis through anion-exchange membranes was measured for 10^-3 to 2 mol/kg of aqueous KCl, LiCl, and NH₄Cl and for 10^-3 to 3 x 10^-1 mol/kg of aqueous KIO₃ and K₂SO₄. For all electrolytes used the direction of thermoosmosis was from the cold side to the hot side over the whole range of concentrations. For KCl and LiCl the experimental results were analyzed with an extension of a previously published theory, using additional data for transport numbers of ions in membranes and for electroosmosis. The agreement between theory and experiment is satisfactory. The absolute value of thermoosmosis for KIO₃ is larger than that for other electrolytes because the pore volume fraction of the membrane for KIO₃ is larger than that for the other electrolytes.     

Surface charge of detonation nanodiamond particles in aqueous solutions of simple 1 : 1 Electrolytes

Adsorption isotherms of potential-determining H⁺ and OH⁻ ions and the pH dependences of the specific surface charge of detonation nanodiamond (DND) particles are obtained in a pH range of 3–10 by the acid-base titration of their hydrosols containing 0.001–1 M LiCl, NaCl, KCl, NaNO₃, KNO₃, and NaClO₄ as background electrolytes. The data obtained attest to the chemical nonuniformity (heterogeneity) of a DND surface and different degrees of binding of background electrolyte cations and anions with ionized groups. It is revealed that the adsorption of OH-anions diminishes in the lyotropic series of cations Na⁺ > K⁺ > Li⁺ and increases with a decrease in the adsorbability of anions in the following series: NO₃⁻ ≊ ClO₄⁻ > Cl⁻. The adsorption of potential-determining H⁺ and OH⁻ ions on a DND surface containing two types of functional groups, i.e., acidic carboxyl and amphoteric hydroxyl groups, is simulated by the Protofit software package. The optimal surface densities and ionization constants that correspond to minimal deviations of model adsorption isotherms from the experimental curves are found for these groups.     

Studies with inorganic precipitative membranes: XI. Membrane potential response,characterization and evaluation of effective fixed charge density

Membrane potentials arising across parchment supported nickel and cobalt phosphate membranes when they separete 1:1 electrolyte solutions of concentration c₁ and c₂ such that c₁=10 c₂, have been measured. The membranes in contactwith dilute solutions have been found to carry a negative charge whereas the charge reversal was observed when the membrane was separating concentrated solutions. The membrane potential data have been used according to the procedure prescribed by Teorell-Meyer-Sievers theory (the TMS theory) to derive the value of effective fixed charge density of membranes. The electrical double layer at the membrane-solution interface has been suggested to control the over all rate of diffusion.     

Calculation of the salt separation by negatively charged gel-filled membranes

The salt separations of negatively charged gel-filled membranes composed of poly(2-acrylamido-2-methylpropanesulfonic acid) gels anchored within a polypropylene microporous substrate have been determined experimentally and modeled theoretically. The separation of these membranes were calculated by both the Teorell, Meyer and Sievers (TMS) model and the Donnan–Steric Pore (DSP) model coupled with the extended Nernst–Planck equation. For modeling, the membrane effective thickness, effective charge density, and pore radius were either directly measured or calculated from theories without the use of fitting procedures. Good agreement between the experimental measurements and the theoretical calculations of salt separation was observed. For the theoretical calculations, the TMS model is suitable for membranes with moderate gel polymer volume fractions, while the DSP model is more suitable for membranes with high gel polymer volume fractions. Moreover, with a calculated constant effective charge density, the salt separation with different salt concentrations could be accurately predicted. The separation of various other salts could also be predicted with good accuracy.     

Studies of transport of strong electrolytes and carboxylic acids through lipid—nitrocellulose membranes on the basis of a lipid multibarrier channel model

Lipid-dispersed nitrocellulose membranes (L-N membranes) were prepared by casting mixed solutions composed of L-α-lecithin, β,γ-dipalmitoyl and nitrocellulose in a glass vessel. Their lipid contents varied from 0 to 0.2 in the weight ratio of lipid to nitrocellulose, and their water contents were adjusted to 60-65% w/w under experimental conditions depending on the composition of the casting solutions. Various strong electrolytes (LiCl, NaCl, KCl, MgCl₂ and CaCl₂) and carboxylic acids (acetic acid, propionic acid, butyric acid and valeric acid) were permeated through the membranes in a two-compartment cell at 25°C while stirring two bulk solutions at 650 rpm. The results were analyzed quantitatively by a lipid multibarrier channel model, i.e., a membrane model which consists of water channels, WC, and lipid barrier channels, LC. The membrane parameters, such as the fractional areas of WC and LC and tortuosity, were obtained from the water content, the lipid content and the permeability of strong electrolytes, and the effects of the lipid content on these parameters were then examined. The permeability coefficients, P, of L-N membranes were theoretically shown to be a function of the ratio of the diffusivity of a permeant through the water channel to the diffusivity through the lipid barrier. In the case of weak electrolytes the effeccts of their degree of dissociation on the values of P were analyzed. Furthermore, the permeability coefficients of carboxylic acids through the lipid barrier phase in the channel were obtained from the values of P.     

Selective Solid–Liquid and Liquid–Liquid Extraction of Lithium Chloride Using Strapped Calix[4]pyrroles

LiCl is a classic “hard” ion salt that is present in lithium‐rich brines and a key component in end‐of‐life materials (that is, used lithium‐ion batteries). Its isolation and purification from like salts is a recognized challenge with potential strategic and economic implications. Herein, we describe two ditopic calix[4]pyrrole‐based ion‐pair receptors ( 2 and 3 ), that are capable of selectively capturing LiCl. Under solid–liquid extraction conditions, using 2 as the extractant, LiCl could be separated from a NaCl/KCl salt mixture containing as little as 1 % LiCl with circa 100 % selectivity, while receptor 3 achieved similar separations when the LiCl level was as low as 200 ppm. Under liquid–liquid extraction conditions using nitrobenzene as the non‐aqueous phase, the extraction preference displayed by 2 is KCl>NaCl>LiCl. In contrast, 3 exhibits high selectivity towards LiCl over NaCl and KCl, with no appreciable extraction being observed for the latter two salts.