Ion diode logics for pH control

Electronic control over the generation, transport, and delivery of ions is useful in order to regulate reactions, functions, and processes in various chemical and biological systems. Different kinds of ion diodes and transistors that exhibit non-linear current versus voltage characteristics have been explored to generate chemical gradients and signals. Bipolar membranes (BMs) exhibit both ion current rectification and water splitting and are thus suitable as ion diodes for the regulation of pH. To date, fast switching ion diodes have been difficult to realize due to accumulation of ions inside the device structure at forward bias--charges that take a long time to deplete at reverse bias. Water splitting occurs at elevated reverse voltage bias and is a feature that renders high ion current rectification impossible. This makes integration of ion diodes in circuits difficult. Here, we report three different designs of micro-fabricated ion bipolar membrane diodes (IBMDs). The first two designs consist of single BM configurations, and are capable of either splitting water or providing high current rectification. In the third design, water-splitting BMs and a highly-rectifying BM are connected in series, thus suppressing accumulation of ions. The resulting IBMD shows less hysteresis, faster off-switching, and also a high ion current rectification ratio as compared to the single BM devices. Further, the IBMD was integrated in a diode-based AND gate, which is capable of controlling delivery of hydroxide ions into a receiving reservoir.     

Dielectric relaxation on the intermediate layer in a bipolar membrane under the water splitting phenomenon. I. Shift of the dielectric properties by means of time-dependent impedance measurements

In this study, we examined the dielectric properties of an intermediate layer in a bipolar membrane, which is composed of a negatively charged layer and a positively charged layer joined in series. As a result of the time-dependent impedance measurements of charged membranes, the negative increment in electric conductivity and the positive increment in electric capacity were observed only in the case of a bipolar membrane under the application of reverse-biased voltages, which were quite different from the behavior of both monopolar membranes and of a bipolar membrane under forward-biased voltages. Further, the observed shifts showed a nearly constant value against the reverse-biased voltage. It is concluded that these characteristics coincide with the process of ion exclusion in the intermediate layer and are attributed to the water splitting mechanism.     

多羧基酞菁铜、乙酰基二茂铁改性CMC/CS双极膜的制备及表征

柔性链高分子聚乙二醇(PEG)分别与阳离子交换膜层羧甲基纤维素钠(CMC)、阴离子交换膜层壳聚糖(CS)共混以增强界面的相容性和膜的机械性能.以金属有机高分子多羧基酞菁铜(CuPc)改性CMC、乙酰基二茂铁改性CS,采用流延法制备了CuPc-mCMC/mCS双极膜.双极膜溶胀性和热重分析结果表明膜改性后稳定性能得到提高.膜交流阻抗、I-V工作曲线的测定结果表明该双极膜阻抗及工作电压均较小.双极膜经改性具有较高的离子渗透性能。     

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.     

Limiting current density and water dissociation in bipolar membranes

The behaviour of bipolar membranes in NaCl and Na₂SO₄ solutions is discussed. The membranes are characterized in terms of their limiting current densities. Below the limiting current density the electric current is carried by salt ions migrating from the transition region between the anion and the cation exchange layer of the bipolar membrane. In steady state these ions are replaced by salt ions transported from the bulk solutions into the transition region by diffusion and migration due to the fact that the ion-exchange layers are not strictly permselective. When the limiting current density is exceeded, the salt transport from the transition region can no longer be compensated by the transport into the region and a drastic increase in the membrane resistance and enhanced water dissociation is observed. This water dissociation is described as being a combination of the second Wien effect and the protonation and deprotonation of functional groups in the membrane. The limiting current density is calculated from a mass balance that includes all components involved in the transport. The parameters used in the mathematical treatment are the diffusion coefficients of salt ions and water, the ion mobilities in the membrane, the fixed charge densitiy of the membrane, the pK_b values of the functional groups and the solution bulk concentrations.     

Study of pH correction process of chloride-bicarbonate dilute solutions by electrodialysis with bipolar membranes

The pH of a dilute chloride-hydrocarbonate solution and the concentrations of chloride ions and carbonic acid anions at the outlet of the alkaline and acid chambers of the electrodialysis cell formed by bipolar and anion-exchange membranes were determined. The decrease in the concentration of hydrocarbonate ions in the alkaline chamber with growth of current density was not equal to its increase in the acid chamber. This disbalance was caused by two concurrent processes: the electromigration ion transport through the anion-exchange membrane and the chemical reactions of hydrocarbonate ions with the water dissociation products formed on the bipolar and anion-exchange membranes. A mathematical model was suggested to describe the electrodialysis correction of the pH of a dilute chloride-hydrocarbonate solution. The experimental data on the correction of pH of the chloride-hydrocarbonate solution were well approximated by both the model that takes into account water dissociation on the anion-exchange membrane and the simplified model that neglects water dissociation. The experimental data agreed well with the results of calculations by the model in which the effective anion transport numbers were calculated only from ion concentrations and diffusion coefficients in solution. This reflects the outer diffusion character of the kinetics of ion transport through the anion-exchange membrane, with pH of dilute solutions corrected by electrodialysis.     

Electrochemical Parameters of Heterogeneous Bipolar Membranes: Dependence on the Structure and Nature of Monopolar Layers

The current–voltage curves, transport numbers of ions, and electrochemical impedance spectra are obtained for pilot samples of heterogeneous bipolar membranes synthesized from cation- and anion-exchange membranes. In the synthesis, the composition of the monopolar layers of membranes was varied or a catalyst was introduced into them. The voltage drop and the frequency spectra at the pilot membranes are close to those for industrial membrane MB-3. The transport numbers of co-ions in the pilot membranes are smaller and those of the hydrogen and hydroxyl ions are larger than in MB-3. The effective water dissociation constants in the pilot membranes, calculated from the impedance spectra, are close to those of MB-3.     

Copper-exchanged LTA zeolite membranes with enhanced water flux for ethanol dehydration

Phase-pure and well-intergrown Cu-LTA membranes are developed through copper ions exchange of sodium ions in Na-LTA framework. For pervaporation of 90.0 wt% ethanol/10.0 wt% water mixtures, the Cu-LTA membrane shows much higher water flux than Na-LTA membranes due to the enhancement of the pore size after ions exchange.     

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.     

Asymmetric bipolar membrane: A tool to improve product purity

Bipolar membranes (BPMs) are catalytic membranes for electro-membrane processes splitting water into protons and hydroxyl ions. To improve selectivity and current efficiency of BPMs, we prepare new asymmetric BPMs with reduced salt leakages. The flux of salt ions across a BPM is determined by the co-ion transport across the respective layer of the membrane. BPM asymmetry can be used to decrease the co-ion fluxes through the membrane and shows that the change of the layer thickness and charge density of the corresponding ion exchange layer determines the co-ion flux. The modification of a commercial BP-1 with a thin additional cation exchange layer on the cationic side results in a 47% lower salt leakage. Thicker layers result in water diffusion limitations. In order to avoid water diffusion limitations we prepared tailor made BPMs with thin anion exchange layers, to increase the water flux into the membrane. Therefore a BPM could be prepared with a thick cation exchange layer showing a 62% decreased salt ion leakage through the cationic side of the membrane.     

A novel ion trap that enables high duty cycle and wide m/z range on an orthogonal injection TOF mass spectrometer

Although TOF analyzers with orthogonal ion injection provide the whole spectrum without scanning, their duty cycle is low compared with scanning analyzers in single ion monitoring mode. Typical duty cycle is in the range of 5% to 30% depending on the instrument geometry and ion m/z value. We present here a novel trapping/releasing setup, which offers the duty cycle near 100% over a wide range. Operation in the mass range from m/z 120 to almost 2000 is demonstrated. Ions are trapped in a short linear ion trap at the end of the collision cell in an axial pseudopotential well created by additional rf (“AC”) voltage applied to all four rods of the trap with the same amplitude and phase. The pseudopotential created by AC field is mass dependent, and by ramping down the AC voltage, ions can be released from the trap sequentially from high m/z to low, while all ions are gaining the same kinetic energy. Upon entering the TOF accelerator, ions with lower m/z catch up with heavier ions, and the AC ramp parameters can be selected to make all ions meet in the center of the TOF extraction region, resulting in sensitivity gains from 3 to 14 without loss of mass accuracy or resolution.     

Fundamental studies on the intermediate layer of a bipolar membrane V. Effect of silver halide and its dope in gelatin on water dissociation at the interface of a bipolar membrane

The effect of silver ions on the water dissociation of bipolar membranes was first investigated in this paper. To do this, the bipolar membranes were prepared by immersing the anion exchange layers in an AgNO3 solution and then coating a solution of sulfonated polyphenylene oxide (SPPO) on the anion exchange layers. XPS and AES observations indicated that silver at the intermediate layer was in the form of AgCl. The experimental results proved that AgCl has an excellent catalytic function for water dissociation in terms of I-V curves, and the quantity of AgCl played an important role in the behavior of a bipolar membrane. The bipolar membranes with gelatin and the gelatin doped with silver as a catalytic layer were also prepared in the same way, and their I-V behavior and the water dissociation pilot tests were also investigated. The experimental results showed that in the case of gelatin alone, the voltage drop increased slightly at high gelatin concentrations, due mainly to the steric effect and electrostatic interaction, but decreased at low gelatin concentrations due to the hydrophilicity. However, when gelatin was doped with AgCl, the bipolar membranes have an appreciable improvement in both stability and catalytic function, in comparison with those prepared from silver or gelatin.     

Bipolar reverse osmosis membrane for separating mono-and divalent ions

Bipolar reverse osmosis membranes that have both negatively and positively charged layers have been prepared to enhance the selectivity towards mono- and divalent ions in respect of both cations and anions. Positively charged layers are formed on low pressure reverse osmosis membranes having negative charge (NTR-7410 and 7450) by an adsorption method using polyethyleneimine (PEI) or a quaternary ammonium polyelectrolyte (QAP). These layers attach to the membrane's dense layer, which is made of sulfonated polyether sulfone. The selectivity of mono- and divalent ions is proven by experimental results for single electrolytes (NaCl, Na₂SO₄ and MgCl₂). Although negatively charged membranes repulse divalent anions more strongly than cations and monovalent anions, bipolar reverse osmosis membranes reject both divalent cations and divalent anions better than monovalent ions. An optimal preparation method for bipolar membranes showing selectivity towards mono- and divalent ions were developed. The bipolar membranes showed good ion selectivity for artificial sea water.     

Assisted Electromigration of Bipolar Ions through Ion-Selective Membranes in Glycine Solutions

At current densities above the limiting diffusion currents in electromembrane systems, bipolar ions of amino acids react with protons or hydroxyl ions that form during irreversible dissociation of water molecules at membrane–solution interfaces, convert into cations or anions, and then, after a gradient of electric potential is imposed on the system, migrate through cation-selective or anion-selective membranes (assisted electromigration). This phenomenon makes it possible to effectively separate solutions containing bipolar ions and nonelectrolyte molecules using electrodialysis with alternate cation-exchange and anion-exchange membranes.     

Electroconvection in systems with heterogeneous ion-exchange membranes after thermal modification

It is found that the variations in the structure (morphology and microrelief) and chemical composition of surface of heterogeneous ion-exchange membranes as a result of thermal modification have different effects on the current—voltage characteristics and conditions for the generation of electroconvective instability at the membrane/solution interface under intense current modes. After thermal treatment of strongly acidic sulfocation-exchange membrane, which is characterized by a low catalytic activity in the reaction of water dissociation and a high thermal stability of fixed groups, a fraction of conducting surface area increases and the membrane microrelief develops. As a result, the diffusion limiting current density increases and the length of plateau of the current—voltage curve decreases. Therewith, the thickness of the region of electroconvective instability of solution in the near-membrane region increases and the polarization of electromembrane system, at which the mode of unstable electroconvection is reached, decreases. The thermodestruction of strongly basic anion-exchange membranes, conversely, leads to suppression of electroconvection and an increase in the length of plateau of the current—voltage curve due to the formation of fixed weakly basic amino groups, which are catalytically active in the reaction of water dissociation. A linear correlation is found between the dimensions of the region of electroconvective instability and a fraction of weakly basic functional amino groups in the composition of strongly basic membranes.     

Correction of pH of diluted solutions of electrolytes by electrodialysis with bipolar membranes

The current efficiencies of the water dissociation water and the voltage-current characteristics of the bipolar (asymmetric bipolar) membranes were measured in a two-chamber electrochemical cell. The cell was formed of an MB-3 bipolar membrane or an asymmetric bipolar membrane, which is an MA-40 heterogeneous membrane with a thin surface layer in the form of a cation-selective homogeneous film and MA-40 and MA-41 heterogeneous monopolar membranes. The dissociation of water on MA-40 in 0.01 M sodium chloride decreased the current efficiency of the acid and alkali both in the channel with a bipolar membrane and in the channel with an asymmetric bipolar membrane. The effective ion transport numbers across MA-40 and MA-41 at different pH values were determined. The water dissociation rate on MA-40 decreased at pH > 9.5. A kinetic model of the electrodialysis of a dilute solution of sodium chloride in a two-chamber unit cell with a bipolar and anionite membranes was suggested.     

A combined linear ion trap for mass spectrometry

We propose a novel ion cyclotron resonance ion trap capable of confining ions even at high pressure. The trap consists of three capacitively coupled axial sections, each composed of four circular cross-section rods parallel to the magnetic field axis. Ion confinement along the magnetic field direction is provided by applying the same static voltage to each set of “endcap” rods. As for a two-dimensional quadrupole mass filter, a sufficiently high rf frequency (several MHz) leads to an “effective” electrostatic “pseudopotential” well with a minimum on the trap central axis. Ions are confined radially by the combination of an applied axial static magnetic field and a radially inward-directed electric field resulting from differential rf voltages applied to each set of four rods. Ion confinement properties are revealed from a Paul traplike “stability diagram,” whereas ion trajectories are analyzed in terms of Penning-type ion cyclotron rotation, magnetron rotation, and axial oscillation motional modes. Ion cyclotron frequency increases with the strength of the rf trapping field. Ion magnetron motion becomes stable if the rf voltage is high enough. Therefore, ion trajectories can be stable even in the presence of ion-neutral collisions. Adding an ac potential to a Penning trap should dramatically increase the upper mass detection limit.     

Coupling of ion and water fluxes in synthetic membranes*

The behaviour of ion-exchange membranes has, in most circumstances, been interpreted in terms of the fixed charge theory of Teorell, Meyer and Sievers. Generally it has been recognised that some allowance should be made in the theory for coupling between ion and water fluxes. In making approximate corrections for this, it has usually been assumed that close coupling between all ion fluxes and the osmotic or electro-osmotic water flux occurs. In this paper the specific nature of ion/water flux coupling is explored and it is shown that close coupling is a good approximation for co-ion and water fluxes but that the coupling of the counter-ion and water fluxes is less strong. p]The interactions between ions, water and membrane matrix in a cation exchange membrane are revealed by determining the values of the pairwise molecular friction coefficients. These findings support long standing qualitative ideas about the interplay of electrostatic, hydration and hydrophobic interactions in membranes. p]The picture that emerges shows that an exact extension of the usual equations of the fixed charge theory to include flux coupling would be too complex to be useful. An approximate procedure is to divide conceptually the counter-ions into two groups, those balancing the fixed charges and those accompanying the sorbed co-ions, i.e. constituting sorbed electrolyte. If coupling of the former group of counter-ions with the water flux is ignored whilst close coupling is assumed between the sorbed electrolyte and the water flux, a very good correction is achieved for the overall effects of coupling.     

Discrimination between the permeation of H+ and OH− ions in precipitation membranes

The unique property of precipitation membranes, which allows determination of permeation rates of single ion species by measurement of current—voltage relationships, has been used to measure the transport rates of H⁺ and Oil ions. The permeation of OH^? was found to be approximately 4-5 times higher than that of H⁺ in both BaSO₄ and calcium oxalate precipitation membranes.     

Negative chemical ionization in quadrupole ion trap mass spectrometry: Effects of applied voltages and reaction times

The effects of applied voltages and reaction times on negative ion chemical ionization in the quadrupole ion trap are investigated. Mass-selected ejection of undesired reagent ions and selective mass storage of only negative ions are required for practical negative ion chemical ionization. This is achieved by application of rf and dc voltages to the ring electrode to control the mass-to-charge ratios one polarity) of ions stored, as well as by application of a supplemental rf voltage applied across the endcap electrodes to selectively eject ions of a particular mass-to-charge ratio. Even with careful control of these parameters, negative chemical ionization is not as sensitive as electron ionization and positive chemical ionization because of the lack of thermal electrons in the ion trap. Mass selection of the hydroxide anion as a reagent ion and exclusion of all positive ions provide [M ? H]^? ions with little or no fragmentation for a wide variety of compounds.