Development of urethane acrylate composite ion-exchange membranes and their electrochemical characterization

With the objective of introducing antifouling characteristics into interpolymer types of cation and anion exchange membranes, the surface of these membranes was coated with a 12-microm-thick urethane acrylate layer and was cured by UV radiation of wavelengths 308 and 172 nm under a complete inert atmosphere. Different urethane acrylate composite ion exchange membranes developed were characterized in NaCl solution by measuring their ion-exchange capacity, volume fraction of water, contact angle with water, membrane conductance, and membrane potential. It was found that the electrochemical transport properties of urethane acrylate composite cation-exchange membranes were increased due to resonance stabilization of the urethane group, which acts as a weak acid and dissociates as a negatively charged urethane ion and a positively charged proton. This contributes toward the net charge density of the membrane matrix responsible for enhanced selectivity and conductivity, while for urethane acrylate composite anion-exchange membranes reduction in net charge density was responsible for reduction in electrochemical transport properties. Counterion transport number, permselectivity, and counterion diffusion coefficient values for these membranes were also estimated. Experiments were also carried out in higher homologs of sodium carboxylate solutions in order to observe the fouling tendencies of these membranes. It was concluded that it is possible to obtain antifouling characteristics of ion-exchange membranes by coating and curing thin hydrophilic layers of urethane acrylate on their surfaces without sacrificing their electrochemical transport properties.     

Some considerations concerning the technical use of narrow pores

Carrier transport and diffusion through narrow pores (single-file diffusion) are basic transport mechanisms that have been proposed to explain the specific passive permeation properties of biological membranes. Unlike carrier diffusion, the concept of single-file diffusion has been scarcely considered in connection with technical purposes. Taking into account, however, that extensive studies have been made in recent years on artificial lipid membranes with single-file pores and that cylindrical pores with a very narrow diameter distribution can be produced in thin plates by etching nuclear tracks, it seems useful to discuss more carefully the specific properties and conceivable applications of the single-file pore. As properties of obvious technical interest one finds high permselectivity and high transport rate. In addition, the voltage-dependent block of narrow pores provides an intriguing possibility of ionic flux control. A therapeutic system functioning as an artificial suprarenal gland is briefly outlined as an example.     

Electrolysis of sodium chloride using composite poly(styrene-co-divinylbenzene) cation exchange membranes

Composite cation exchange membranes are prepared from cross-linked styrene-divinylbenzene copolymers for the electrolysis of sodium chloride to produce sodium hydroxide and chlorine by selective removal of sodium ions. It is prepared from a syrup of the polymer using dual initiating system and is modified with chloroacetic acid to introduce acid functional groups (COO⁻) on its surface. The effect of the modification is confirmed by FTIR, SEM, contact angle, water content, and ion exchange capacity measurements. The performance of the membrane has been evaluated in terms of current efficiency and power consumption and the effect of current density, salt concentration and flow rate on efficiency has been studied. Our membrane has an ion exchange capacity of 0.833 meq./g which is close to that of the commercially available Nafion-117 membrane having an ion exchange capacity 0.9 meq./g. The Nafion-117 used for electrodialysis of sodium sulfate has a current efficiency of around 90% and specific energy consumption of 0.1 kW/mol at 2N concentration of the salt at 1000 A/m². Our membrane used for electrodialysis of sodium chloride has a current efficiency of 93% and a power consumption of around 0.3122 kW/mol at the same concentration of salt and at a current density of 254 A/m². The two-dimensional space-charge model in cylindrical coordinates has been solved semi-analytically to obtain the effective wall potential and pore size of the membrane which are difficult to measure directly. The experimentally obtained solute flux and current density have been fitted to the model and optimum values of effective wall potential and pore diameter have been determined to be 98.5 mV and 0.8 nm, respectively.     

Characterization of non-uniformly charged ion-exchange membranes prepared by plasma-induced graft polymerization

Cation-exchange membranes were prepared by plasma-induced grafting of sulfonated glycidyl methacrylate (GMA) and porous polypropylene (PP) membranes. The chemical and physical structures of the prepared membranes were investigated using Fourier transform infrared spectroscopy (FTIR), field emission-scanning electron microscopy (FESEM) and electron-probe micro-analyzing (EPMA). The membranes were also characterized in terms of their electrochemical properties. A non-uniform distribution of fixed charges across the membrane matrix was detected by EPMA analysis. This non-uniform distribution of the fixed charges is the result of using water as solvent for the monomer which led to a fast reaction on the membrane surface and a slow diffusion of the monomer into the pores of the membrane. The prepared membranes exhibited moderate ion-exchange capacities (2.53–3.30 mmol/g dry membrane) and electrical resistances (0.349–0.589 Ω cm²) and an ion permselectivity comparable to that of the commercial membrane CM-1 (Tokuyama Corp.), while the water content of the membranes was significantly higher than that of the commercial membrane. The higher water content of the membranes is the result of water occupying the pores in the bulk of the support membrane after the dense layers with fixed charges are formed on the membrane surfaces by the grafting reaction. The relatively high ion permselectivity in spite of the high water sorption of the membranes is the result of the high fixed charge density in the layers on the membrane surfaces. Current versus voltage curves and the chronopotentiometric measurements revealed that the sulfonated GMA-g-PP membranes can be operated effectively at high current density.     

Template-synthesized nanotubes for chemical separations and analysis

We have developed a new class of synthetic membranes that consist of a porous polymeric support that contains an ensemble of gold nanotubes that span the thickness of the support membrane. The support is a commercially-available microporous polycarbonate filter with cylindrical nanoscopic pores. The gold nanotubes are prepared by electroless deposition of Au onto the pore walls, that is, the pores acts as templates for the nanotubes. We have shown that by controlling the Au deposition time, Au nanotubes that have effective inside diameters of molecular dimensions (<1 nm) can be prepared. These nanotube membranes can be used to cleanly separate small molecules on the basis of molecular size. Furthermore, use of these membranes as a novel electrochemical sensor is also discussed. This new sensing scheme involves applying a constant potential across the Au nanotube membrane and measuring the drop in the transmembrane current upon the addition of the analyte. This paper reviews our recent progress on size-based based transport selectivity and sensor applications in this new class of membranes.     

Molecular sieving and sensing with gold nanotube membranes

We have developed a new class of synthetic membranes that consist of a porous polymeric support that contains an ensemble of gold nanotubes that span the thickness of the support membrane. The support is a commercially available microporous polycarbonate filter with cylindrical nanoscopic pores. The gold nanotubes are prepared via electroless deposition of Au onto the pore walls; i.e., the pores act as templates for the nanotubes. We have shown that by controlling the Au deposition time, Au nanotubes that have effective inside diameters of molecular dimensions (<1 nm) can be prepared. These nanotube membranes can be used to cleanly separate small molecules on the basis of molecular size. Furthermore, use of these membranes as a novel electrochemical sensor is also discussed. This new sensing scheme involves applying a constant potential across the Au nanotube membrane and measuring the drop in the transmembrane current upon the addition of the analyte. This paper reviews our recent progress on size-based transport selectivity and sensor applications in this new class of membranes.     

Electric mass transport of sodium chloride through cation-exchange membrane MK-40 in dilute sodium chloride solutions: A rotating membrane disk study

The polarization properties of an electromembrane system consisting of an MK-40 membrane and a dilute sodium chloride solution are investigated with an experimental apparatus, which includes a rotating membrane disk with a horizontally positioned membrane. For the electrochemical systems of MK-40/0.01 M NaCl and MK-40/0.001 M NaCl, effective ion transport numbers and partial current-voltage curves are determined for sodium and hydrogen ions, and limiting-current densities and the diffusion-layer thickness are calculated as functions of the rotation rate of the membrane disk. The space-charge distribution in the diffusion layer and in the membrane is calculated for various current densities and rotation rates of the membrane. It is shown that when electric-current densities are greater than the limiting value, ion fluxes of the salt increase as a result of a decrease in the effective thickness of the diffusion layer. This decrease is caused by the development of space charge, electroconvection, water dissociation, and the exaltation effect in the region near the membrane. It has been established that in dilute solutions the limiting current is not purely electrodiffusive in nature.     

The dissociation rate of water molecules in systems with cation- and anion-exchange membranes

Within the framework of the mathematical model of Nernst-Planck-Poisson, an attempt is undertaken to theoretically describe the electrodiffusion of ions in the system diffusion layer/monopolar ionexchange membrane, which is accompanied by dissociation of water molecules. The formulas for estimating the current density transferred through a monopolar membrane by hydrogen or hydroxyl ions formed in dissociation of water in the space-charge region are derived. The rate constants and other parameters of dissociation of water molecules in the space-charge region of monopolar membranes under conditions of stabilization of the diffusion layer thickness are calculated. Their comparative analysis with the similar characteristics of bipolar membranes is carried out. For the phosphoric-acid heterogeneous membrane MK-41 in which the polarization conditions in the current density range under study are not so severe and the reaction layer is not being depleted as in the bipolar membrane MB-3 (contains the same phosphoric-acid groups), it is shown that only single-charged phosphoric-acid groups are involved in the water dissociation reaction. For MK-41, the calculated constants of the heterolytic reaction of water molecule dissociation are lower than for the heterogeneous membrane MA-40 containing ternary and quaternary amino groups. It is confirmed that the nature of ionogenic groups in membranes is a factor that determines the rate of water dissociation in systems with ion-exchange membranes.     

Influence of the nature of membrane ionogenic groups on water dissociation and electrolyte ion transport: A rotating membrane disk study

Polarization properties of electromembrane systems (EMS) consisting of a heterogeneous membrane, either the MK-41 phosphonic acid membrane or the MK-40 sulfonic acid membrane, and dilute sodium chloride solutions are investigated with the rotating membrane disk method. For the MK-41/0.01 M NaCl and MK-41/0.001 M NaCl EMS, effective ion transport numbers and partial current-voltage curves (CVC) are measured for sodium and hydrogen ions, and limiting-current densities and the diffusion-layer thickness are calculated as functions of the rotation rate of the membrane disk. With the theory of the overlimiting state of EMS, internal parameters of the systems under investigation—the diffusion-layer thickness, the space-charge distribution, and electric-field strengths in the diffusion layer and in the membrane—are calculated from experimentally obtained CVC and the dependence of effective transport numbers on current density. The catalytic influence of ionogenic groups on the dissociation rate of water is analyzed quantitatively. Partial CVC for H⁺ ions are calculated for the space-charge region in MK-40 and MK-41 membranes. Analogous CVC for bipolar membranes containing sulfonic acid and phosphonic acid groups are compared. The dissociation mechanism of water is the same in all EMS and is independent of the membrane type and the nature of the functional groups.     

Ionic conductance of nanopores in microscale analysis systems: where microfluidics meets nanofluidics

In this tutorial review we illustrate the origin and dependence on various system parameters of the ionic conductance that exists in discrete nanochannels as well as in nanoporous separation and preconcentration units contained as hybrid configurations, membranes, packed beds, or monoliths in microscale liquid phase analysis systems. A particular complexity arises as external electrical fields are superimposed on internal chemical and electrical potential gradients for tailoring molecular transport. It is demonstrated that the variety of geometries in which the microfluidic/nanofluidic interfaces are realized share common, fundamental features of coupled mass and charge transport, but that phenomena behind the key steps in a particular application can be significantly tuned, depending on the morphology of a material. Thus, the understanding of morphology-related transport in internal and external electrical potential gradients is critical to the performance of a device. This addresses a variety of geometries (slits, channels, filters, membranes, random or regular networks of pores, etc.) and applications, e. g., the gating, sensing, preconcentration, and separation in multifunctional miniaturized devices. Inherently coupled mass and charge transport through ion-permselective (charge-selective) microfluidic/nanofluidic interfaces is analyzed with a stepwise-added complexity and discussed with respect to the morphology of the charge-selective spatial domains. Within this scenario, the electrostatics and electrokinetics in microfluidic and nanofluidic channels, as well as the electrohydrodynamics evolving at microfluidic/nanofluidic interfaces, where microfluidics meets nanofluidics, define the platform of central phenomena.     

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.     

Insight from molecular modelling: does the polymer side chain length matter for transport properties of perfluorosulfonic acid membranes?

We present a detailed analysis of the nanostructure of the short side chain (SSC) perfluorosulfonic acid membrane and its effect on H(2)O clustering, H(3)O(+) and H(2)O diffusion, and mean residence times of H(2)O near SO(3)(-) groups based on molecular dynamics simulations. We studied a range of hydration levels (λ) at temperatures of 300 and 360 K, and compare the results to our findings in the benchmark Nafion? membrane. The water cluster diameter is nearly the same in the two membranes, while the extent of SO(3)(-) clustering is more in the SSC membrane. The calculated cluster diameter of about 2.4 nm is in excellent agreement with the recently proposed cylindrical water channel model of these membranes. The diffusion coefficients of H(2)O and H(3)O(+) are similar in SSC and Nafion membranes. Raising the temperature of the SSC membrane from 300 to 360 K provides a much bigger increase in proton vehicular diffusion coefficient (by a factor of about 4) than changing the side chain length. H(3)O(+) ions are found to exchange more frequently with SO(3)(-) partners at the higher temperature. Our key findings are that (a) the hydrophobic-hydrophilic separation in the two membranes is surprisingly similar; (b) at all hydration levels studied, the long side chain of Nafion is bent and is effectively equivalent to a short side chain in terms of extension into the water domain; (c) vehicular proton transport occurs mainly between SO(3)(-) groups; and (d) changing the size of the simulation cell does not change the results significantly. The simulations are validated in good agreement with the corresponding experimental values for the simulated membrane density and diffusion coefficients of H(2)O.     

Interaction between anionic polyelectrolytes and anion exchange membranes and change in membrane properties

Electrodialytic transport properties of anion exchange membranes were measured after formation of anionic polyelectrolyte layers on the membrane surfaces: relative transport number of various anions to chloride ions, current efficiency and apparent diffusion coefficients of neutral molecules. The anionic polyelectrolyte layers were formed by immersing the membrane into an aqueous solution of polycondensation product of sodium naphthalene sulfonate and formaldehyde or polystyrene sulfonic acid.

The change in the relative transport number between anions was remarkable in the anion exchange membrane with high ion exchange capacity by forming the layer. Results were: the relative transport number of sulfate ions to chloride ions decreased and those of nitrate ions to chloride ions, fluoride ions to chloride ions and bromide ions to chloride ions increased compared with the corresponding membrane. Although the apparent diffusion coefficient of neutral molecules suggested clogging of the membrane pores by the polyelectrolyte, anions with higher hydrated ionic diameter were able to permeate through the membrane easily. This means that difference of electrostatic repulsion force against two anions is effective on the change in the relative transport number of anions.     

Solvent-extraction and Langmuir-adsorption-based transport in chemically functionalized nanopore membranes

We have investigated the transport properties of nanopore alumina membranes that were rendered hydrophobic by functionalization with octadecyltrimethoxysilane (ODS). The pores in these ODS-modified membranes are so hydrophobic that they are not wetted by water. Nevertheless, nonionic molecules can be transported from an aqueous feed solution on one side of the membrane, through the dry nanopores, and into an aqueous receiver solution on the other side. The transport mechanism involves Langmuir-type adsorption of the permeating molecule onto the ODS layers lining the pore walls, followed by solid-state diffusion along these ODS layers; we have measured the diffusion coefficients associated with this transport process. We have also investigated the transport properties of membranes prepared by filling the ODS-modified pores with the water-immiscible (hydrophobic) liquid mineral oil. In this case the transport mechanism involves solvent extraction of the permeating molecule into the mineral oil subphase confined with the pores, followed by solution-based diffusion through this liquid subphase. Because of this different transport mechanism, the supported-liquid membranes show substantially better transport selectivity than the ODS-modified membranes that contain no liquid subphase.     

银在质子交换膜燃料电池空气阴极内促进氧传递研究

利用离子交换及随后的氢还原,将单质银负载在质子交换膜(Nafion)孔道内.TEM、XRD表征载银Nafion膜的结构,电化学极限电流法测定氧在载银Nafion膜内的扩散系数.结果表明,因银晶颗粒大于Nafion孔道直径,致使Nafion孔道有所扩张;氧在载银Nafion膜内的扩散系数是无银Nafion膜的4倍.据此,把银引入质子交换膜燃料电池空气阴极催化剂表面的Nafion薄层,则电池的性能在高电流密度下有明显的提高,显示了银对该电极内氧传递的促进作用.     

Mathematical model for the overlimiting state of an ion-exchange membrane system

A three-layered mathematical model is proposed for describing the overlimiting state in an ion-exchange membrane system. The model’s prominent feature is the allowance for the space-charge region; the water dissociation reaction, which is catalyzed by active ionogenic groups; and the coupled gravitational and electroosmotic convection, which leads to the emergence of dependence of the effective diffusion layer thickness on the electric current density. The model is used for calculating, on the basis of known initial current-voltage curves and dependences of effective transport numbers on the current density, such internal characteristics of the system as the diffusion layer thickness, distribution of concentration of ions, space charge, and electric-field strength at various current densities.     

Coupled transport membranes II: : The mechanism of uranium transport with a tertiary amine

This paper describes the parameters controlling the coupled transport of uranium anions through liquid membranes. The membranes consist of a microporous polymeric support with a liquid, tertiary amine complexing agent held within the pores by capillary forces. When this liquid membrane is interposed between two aqueous solutions of unequal ion concentrations, the complexing agent can pick up the anion on one side of the membrane and carry it across the membrane by diffusion in the form of a neutral complex. Ions of opposite charge may be carried in the same direction, or ions of like charge may be carried in the opposite direction. We refer to these two modes of transport as “co-transport” and “counter-transport”, respectively. In the coupled transport of uranium, both co-transport and counter-transport can occur. p]The coupling of the flows of two ions permits one of the ions to be pumped against its concentration gradient. We have demonstrated “uphill diffusion” of uranium against substantial concentration gradients, and at significant rates. A number of factors affect uranium flux, principally the concentrations of uranium and the coupled ion in the aqueous solutions. The base strength of the tertiary amine is also an important parameter.     

Transport of ions through an elliptic ion-selective membrane

The rate of transport of ions through an annular ion-selective membrane having an elliptic cross section is analyzed. This is a generalization to a hollow-fiber type of device, and represents a wide class of membranes which are capable of providing a large (surface area/volume) ratio for ion transport. The Nernst–Plank equation governing the transport of ions is solved numerically. The results obtained reveal that, if the concentration of fixed charge is high, its current efficiency is insensitive to fixed charge distribution, and the local electroneutrality can be assumed. On the other hand, if the concentration of fixed charge is low, the distribution of fixed charge becomes significant, and assuming local electroneutrality can be inappropriate. In general, the higher the average concentration (or the greater the total amount) of fixed charge of a membrane, the higher its current efficiency. We show that the results for planar and cylindrical membranes can be recovered from the present model.