Ion-exclusion Effects Research Articles

Construction of three-dimensional polyethyleneimine incorporated graphene oxide aerogels as high-capacity electrode for enhanced uranium(VI) electrosorption performance

Electrosorption technique is paid more attention in recovery of uranium(VI) due to its low energy consumption, green process and easy regeneration, but still is hindered by the limited accessible active sites on the electrode material, poor surface wettability and inherent ion exclusion effects. Herein, novel there dimensional porous polyethyleneimine/graphene oxide aerogel (PEI/GOA) electrode was constructed by incorporating polyethyleneimine into graphene oxide and using chitosan as binder for uranium(VI) electrosorption. The synthesized PEI/GOA had a stable three-dimensional porous structure, high specific capacitance (54.06F/G) and good charge–discharge stability. The electrosorption process of uranium(VI) by PEI/GOA electrode was affected by applied voltage and pH, fitted by the pesuo-second-order kinetics and Langmuir model. The maximum theoretical electrosorption capacity of uranium(VI) by PEI/GOA electrode at − 1.2 V was 419.71 mg/g, which was 2.2 times higher than that of pure graphene oxide electrode. PEI/GOA electrode exhibited less than 20 % loss after five cycles using 0.1 M NaHCO3 as the eluent. The efficient U(VI) electrosorption by PEI/GOA electrode was not only attributed to the formation of the electric double layer, but also due to the interaction and synergy of the –NH2, –CONH- and –OH on the PEI/GOA electrode. This research offers a new approach for the efficient removal of uranium (VI) from uranium-containing wastewater.

Theoretical Analysis of Constant Voltage Mode Membrane Capacitive Deionization for Water Softening.

Water softening is desirable to reduce scaling in water infrastructure and to meet industrial water quality needs and consumer preferences. Membrane capacitive deionization (MCDI) can preferentially adsorb divalent ions including calcium and magnesium and thus may be an attractive water softening technology. In this work, a process model incorporating ion exclusion effects was applied to investigate water softening performance including ion selectivity, ion removal efficiency and energy consumption in a constant voltage (CV) mode MCDI. Trade-offs between the simulated Ca2+ selectivity and Ca2+ removal efficiency under varying applied voltage and varying initial concentration ratio of Na+ to Ca2+ were observed. A cut-off CV mode, which was operated to maximize Ca2+ removal efficiency per cycle, was found to lead to a specific energy consumption (SEC) of 0.061 kWh/mole removed Ca2+ for partially softening industrial water and 0.077 kWh/m3 removed Ca2+ for slightly softening tap water at a water recovery of 0.5. This is an order of magnitude less than reported values for other softening techniques. MCDI should be explored more fully as an energy efficient means of water softening.

A polydopamine-coated polyamide thin film composite membrane with enhanced selectivity and stability for vanadium redox flow battery

The aim of the study is to increase the stability and selectivity of a polyamide (PA) thin film composite (TFC) membrane (MT) used in a vanadium redox flow battery (VRB). After immersion for different periods, different concentrations of polydopamine (PDA) are successfully self-polymerized on the surface of the PA TFC membrane to prepare an optimized MDx-y (x and y denote the dopamine concentration and reaction time, respectively) membrane that is used in a VRB. The structure and physicochemical properties of MDx-y membranes are thoroughly evaluated using SEM (scanning electronic microscope), AFM (atomic force microscope), FT-IR (Fourier transform infrared spectroscopy), XPS (X-ray photoelectron spectroscopy), and TG (thermogravimetry) and compared to a pristine substrate (M0) and MT membranes. The PDA layer significantly improves both the selectivity and stability of the pristine MT membrane. After coating the MT membrane with PDA, the average pore radius of the MD membrane is estimated to be 0.24–0.25 nm, which is ideal for separate protons (<0.24 nm) from hydrated vanadium ions (>0.6 nm) in terms of ion exclusion effect. As a result, the PDA coated MT membranes show very low vanadium ion permeability and high proton to vanadium selectivity. The coulombic efficiency of the optimized PDA-coated TFC membrane (MD2.0-10) reaches 99.3% at 80 mA cm−2, which is higher than both pristine TFC and commercial Nafion 115 (N115) membranes. Even after 158 charge-discharge tests, the energy efficiency of the MD2.0-10 membrane remains stable at greater than 80% at 80 mA cm−2, a value that is superior to the pure TFC membrane. In addition, the MD2.0-10 membrane also exhibits excellent chemical stability, increased mechanical properties and high discharge capacity retention rate, suggesting that it has great prospects in VRB applications.

Nonlinear electrophoresis of a tightly fitting sphere in a cylindrical tube

We investigate electrophoresis of a tightly fitting sphere of radius $R-h_{0}$ on the axis of a circular tube of radius $R$, using lubrication theory and ideas due to Schnitzer &amp; Yariv (Phys. Fluids, vol. 26, 2014, 122002). The electrical charge clouds on both the cylindrical wall and the surface of the sphere are assumed thin compared to the gap between the sphere and cylinder, so that charge clouds do not overlap and ion exclusion effects are minimal. Nevertheless, non-uniform pumping of counter-ions within the charge clouds leads to a change in the ionic concentration outside the charge clouds in the narrow gap between sphere and cylinder. The electro-osmotic slip velocities at the two surfaces are modified, leading to a decrease in the electrophoretic velocity of the sphere at low Péclet numbers and an increase in the velocity at high Péclet numbers. When the field strength $E_{0}$ is low, it is known that the electrophoretic velocity $U_{0}$ is proportional to $E_{0}(\unicode[STIX]{x1D701}_{s}-\unicode[STIX]{x1D701}_{c})$ which is zero when the zeta potential $\unicode[STIX]{x1D701}_{s}$ on the sphere surface is equal to the zeta potential $\unicode[STIX]{x1D701}_{c}$ on the cylinder. The perturbation to the above low field strength electrophoretic velocity (at high Péclet number) is predicted to be proportional to $E_{0}^{3}(\unicode[STIX]{x1D701}_{s}+\unicode[STIX]{x1D701}_{c})^{2}(\unicode[STIX]{x1D70E}_{s}+\unicode[STIX]{x1D70E}_{c})$, where $\unicode[STIX]{x1D70E}_{s}$ and $\unicode[STIX]{x1D70E}_{c}$ are the surface charge densities on the sphere and cylinder. The choice of materials with similar or identical zeta potentials (and surface charge densities) for the cylinder and sphere should therefore facilitate the observation of velocities nonlinear in the field strength $E_{0}$, since the reference linear electrophoretic velocity will be small.

Mechanism of ion stacking in aqueous partition chromatographic processes.

It has been reported that ion enrichment phenomena are observed in liquid chromatographic processes with an aqueous mobile phase on the columns packed with nonionic materials. However, the mechanism of the ion enrichment is not at all well understood. In this study, we investigated the retention and enrichment behaviors of simple inorganic anions on a C18-bonded silica column and a cross-linked hydroxylated methacrylic polymer gel column with pure aqueous mobile phases containing various electrolytes. We show that the stacking of ionic solutes can successfully be accounted for by the ion partition model, and it takes place due to the effect of the background coion in the eluent and/or sample solution on the distribution of the ions between the bulk water and the water incorporated in the packing material, which acts as the stationary phase. Using the ion exclusion effect of fixed anionic charges on a packing material as well as the ion stacking by partition, we developed a simple and versatile method for effective enrichment of anionic solutes in aqueous solutions. The enrichment factor and the elution time of the stacked ion zone can be predicted by the ion partition model.

Study of formation of metal–polymer complexes between copper (I) and polyamic acids by HPLC

ABSTRACTStructural, molecular, and hydrodynamic characteristics of thermostable functionalized polymers [polyamic acids (PAAs) with biquinoline chelate groups in the main chain] were studied by high-performance liquid chromatography. Size exclusion mechanism of chromatography in conditions including suppression of polyelectrolyte swelling and ion-exclusion effects was proved. The influence of molecular and structural characteristics of PAAs on their ability to form metal–polymer complexes (MPCs) with copper(I) chloride was studied. The critical polymer concentration at which binding of two PAA macromolecules with Cu+ ion takes place to form MPCs was estimated.

The Influence of Divalent Anions on the Rectification Properties of Nanofluidic Diodes: Insights from Experiments and Theoretical Simulations.

During the last decade, the possibility of generating synthetic nanoarchitectures with functionalities comparable to biological entities has sparked the interest of the scientific community related to diverse research fields. In this context, gaining fundamental understanding of the central features that determine the rectifying characteristics of the conical nanopores is of mandatory importance. In this work, we analyze the influence of mono- and divalent salts in the ionic current transported by asymmetric nanopores and focus on the delicate interplay between ion exclusion and charge screening effects that govern the functional response of the nanofluidic device. Experiments were performed using KCl and K2 SO4 as representative species of singly and doubly charged species. Results showed that higher currents and rectification efficiencies are achieved by doubly charged salts. In order to understand the physicochemical processes underlying these effects simulations using the Poisson-Nernst-Planck formalism were performed. We consider that our theoretical and experimental account of the effect of divalent anions in the functional response of nanofluidic diodes provides further insights into the critical role of electrostatic interactions (ion exclusion versus charge screening effects) in presetting the ionic selectivity to anions as well as the observed rectification properties of these chemical nanodevices.

Ion exclusion and electrokinetic effects resulting from electro-osmotic flow of salt solutions in charged silica nanopores

Silica nanopores are the focus of significant scientific interest due to their potential in a wide variety of applications including desalination membranes. In this paper, the results of extensive all-atom molecular dynamics simulations of the electro-osmotic flow of 0.5 M monovalent (NaCl) and divalent (CaCl(2)) ionic solutions through cylindrical charged silica nanopores are presented. The silica nanopores are produced such that they capture the experimentally observed interfacial properties. The results provide an atomistic description of the ion transport through pores of diameters of 1.5 nm, 2.0 nm, 2.5 nm and 3.0 nm. In doing so, the effect of pore size on ion pairing, ion hydration, and water orientation for each ionic solution was investigated. Also, the transport of the ions through the nanopores is studied, and it is found that in the monovalent solutions the Cl(-) ions are excluded from the nanopores of all sizes. Whereas in the divalent solutions, there is no such preferential exclusion of either ion. This is due to the fact that the interfacial charge is fully compensated for by the Ca(2+) ions while it is not the case for the Na(+) ions.

Development of Simultaneous Ion-Exclusion/Anion-Exchange Chromatographic Determination of Ammonium, Nitrite and Nitrate Ions and Its Application to High-Salt Content Sample

Simultaneous chromatographic separation of nitrate ion (NO3−), nitrite ion (NO2−) and ammonium ion (NH4+) was obtained using single injection, single column and a detector (conductivity). The principle is based on selective ion-exclusion effect for NH4+ and anion-exchange effects for NO3− and NO2− on an OH−-formed basic anion-exchange resin column (Tosoh TSKgel Super IC-AZ) with basic eluent (2 mol/m3 KOH). The present system could determine NH4+, NO2− and NO3− in 500-fold diluted artificial seawater sample in 40 min at 0.6 mL/min of flow rate, by standard addition method. Also, this method was applicable to nitrogen analysis in artificial seawater with iron matrix, by pre-column packed with an iminodiacetate chelating resin in order to remove iron or calcium ions.

A Direct Probe of the Interplay between Bilayer Morphology and Surface Reactivity in Polymersomes

Bilayer vesicles self-assembled from amphiphilic poly(ethylene oxide)-b-polybutadiene (PEO-b-PBd) copolymers are cell-like structures whose high stability and tunable membrane properties make them ideal for use as potential drug carriers and cell mimicry templates. Understanding how the surface interactions (reaction, binding, etc.) are governed by the bilayer structure is critical to enable construction of polymersomes with tailored colloidal behavior. Here, we adapt a previously established chemical labeling method by incorporating coumarin functionalized copolymer into the vesicular structure. This allows us to probe the effect of poly(ethylene glycol) (PEG) brush and surface architecture on the bimolecular quenching reaction occurring at the polymersome surface. Using these measurements, we have tracked quenching in free solution, on bare particles, and on two types of vesicle surfaces: one where the functionalized copolymer groups are longer than the surrounding unfunctionalized copolymer, and one where both functionalized and unfunctionalized groups are the same length. We find that quenching in the presence of the PEG brush proceeds at less than half the free solution rate in both vesicle architectures. However, the quenching rate is further reduced when the functionalized and unfunctionalized groups are the same length. The surface reaction appears to be dominated by quencher diffusion, a conclusion supported by conductivity measurements and ion partition studies indicating that these effects arise as a consequence of retarded ion mobility in the presence of the PEG brush rather than ion exclusion effects. These studies reveal the interplay between the vesicle bilayer architecture (copolymer composition, chain length, local concentration surrounding the active site) and the surface reaction rate, thereby providing useful insights that can help guide the design of polymersomes with desired functional properties.

SEC Assay for Polyvinylsulfonic Impurities in 2-(N-Morpholino)ethanesulfonic Acid Using a Charged Aerosol Detector

A quantitative method for measuring polyvinylsulfonic acid (PVS) concentrations in 2-(N-morpholino)ethanesulfonic acid (MES) is described here. Utilizing a TSK-GEL G2500PWxl SEC column, PVS elutes in the void volume as a sharp peak, yielding group separation between PVS and MES. 20 mM formic acid is used as the mobile phase, as its volatility is compatible with a charged aerosol detector. The low pH mobile phase also suppresses residual negative charges on the stationary phase, reducing the ion exclusion effect commonly observed in aqueous SEC. This method was qualified to measure PVS concentration in MES per International Conference on Harmonization guidelines, using multiple MES lots and other Good’s buffers such as N-(2-acetamido)-2-aminoethanesulfonic acid and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid.

Analysis of accordion DNA stretching revealed by the gold cluster ruler

A promising method for measuring intramolecular distances in solution uses small-angle x-ray scattering interference between gold nanocrystal labels [Mathew-Fenn, Science 322, 446 (2008)]. When applied to double-stranded DNA, it revealed that the DNA length fluctuations are strikingly strong and correlated over at least 80 base pair steps. In other words, the DNA behaves as accordion bellows with distant fragments stretching and shrinking concertedly. This hypothesis, however, disagrees with earlier experimental and computational observations. This Rapid Communication shows that the discrepancy can be rationalized by taking into account the cluster exclusion volume and assuming a moderate long-range repulsion between them. The long-range interaction can originate from an ion exclusion effect and cluster polarization in close proximity to the DNA surface.

Characterization of the properties of stationary phases for liquid chromatography in aqueous mobile phases using aromatic sulphonic acids as the test compounds

We investigated the effects of the concentration of naphthalene sulphonic acids (NSAs) as anionic test compounds in the injected sample and of the salt additives to the mobile phase on ion-exclusion. The retention behaviour of NSAs sensitively reflects even minor changes in the ionic and hydrophobic interactions and can be useful for predicting the effects of the stationary phases in reversed-phase chromatography of polar and ionic compounds, both small ones and biopolymers, e.g., oligonucleotides. We studied chromatographic properties of several stationary phases intended for separations in aqueous mobile phases: a C18 column end-capped with polar hydrophilic groups, a densely bonded C8 column doubly end-capped with short alkyl groups, a short alkyl stationary phase designed to keep full pore accessibility in highly-aqueous mobile phases and a Bidentate column with “bridged” C18 groups attached to the silica hydride support. The chemistry and pore structure of various types of column packing materials and of the salt additives to the mobile phase affect the proportion of the pore volume non-accessible to anions due to ion-exclusion and consequently the peak asymmetry and hydrophobic selectivity in reversed-phase chromatography of organic acids. We also addressed the problems connected with the determination of column hold-up volume in aqueous mobile phases. The accessibility of the stationary phase for anionic compounds in contact with the sample zone is affected by ion-exclusion due to repulsive interactions with the negatively charged surface in the pores of the stationary phase. The accessible part of the stationary phase increases and consequently the migration velocity along the column decreases with increasing concentration of the sample in the zone moving along the column. Because of a limited access to the stationary phase, its capacity can be easily overloaded. The combination of the column overload and ion-exclusion effects may result in fronting or tailing peak asymmetry. To explain this behaviour, we proposed a modified Langmuir model, respecting the variation of the column capacity due to the effects of sample concentration on ion-exclusion.

Adsorption of selected ions on hydrous cerium oxide

Hydrous cerium oxide was synthesized and laboratory study was conducted to address its applicability for decontamination of barium, strontium, molybdenum, europium, iodine, samarium and cesium ions from radioactive liquid wastes using radiotracer technique. The adsorption of Ba(II), Sr(II), Mo(II), Eu(II), I(I), Sm(III) and Cs(I) on hydrous cerium oxide has been investigated as a function of pH, concentration and temperature of the adsorptive solution and the result obtained show that these parameters affect the extent of adsorption. The ion-exchange capacity decreased at higher temperatures which could be due to decrease of active sites as well as hydrophilic properties of surface adsorbent. Hydrous cerium oxide shows relatively high adsorption in acidic and neutral media towards the radiotracers studied, which can be attributed to large hydrated radius and ion exclusion effect.

Ion-exclusion/adsorption chromatography of dimethylsulfoxide and its derivatives for the evaluation to quality-test of TiO 2-photocatalyst in water

Ion-exclusion/adsorption chromatography of dimethylsulfoxide (DMSO) and its derivatives, i.e., methanesulfinic acid (MSI), methanesulfonic acid (MSA) and sulfuric acid (SA), was developed in order to clear the decomposition mechanism of DMSO on quality-test of TiO 2-photocatalyst in water. The separation was achieved by the adsorption effect for DMSO and ion-exclusion effect for MSI, MSA and SA under optimum conditions, using a weakly acidic cation-exchange resin column with 20 mM succinic acid as the eluent. In this system, DMSO and MSI with UV at 195 nm and MSA and SA with conductivity detection were consecutively determined by single injection and single separation column. This method was used to monitor the artificial decomposition of DMSO induced by a photocatalyst. The concentration of DMSO by active oxygens (e.g., OH radical) generated from surface of photocatalyst was found to be decreased through the stoichiometric reaction in the order of MSI, MSA and SA.

Retention behavior of C1?C6 aliphatic monoamines on anion-exchange and polymethacrylate resins with heptylamine as eluent

Retention behavior of C1–C6 aliphatic monoamines (methylamine, ethylamine, propylamine, butylamine, amylamine and hexylamine) on columns (150mm×6mm i.d.) packed with various anion-exchange resins (styrene–divinylbenzene (PS–DVB) copolymer-based strongly basic anion-exchange resin: TSKgel SAX, polymethacrylate-based strongly basic anion-exchange resin: TSKgel SuperQ-5PW and polymethacrylate-based weakly basic anion-exchange resin: TSKgel DEAE-5PW) and unfunctionized polymethacrylate resins (TSKgel G5000PW and TSKgel G3000PWXL) was investigated with basic solutions (sodium hydroxide and heptylamine) as the eluents. Due to strongly electrostatic repulsion (ion-exclusion effect) between these anion-exchange resins and these amines, peak resolution between these amines on these anion-exchange resin columns was unsatisfactory with both sodium hydroxide and heptylamine as the eluents. In contrast, these polymethacrylate resins were successfully applied as the stationary phases for the separation of these C1–C6 amines with heptylamine as eluent, because of both small hydrophobicity and small cation-exchange ability of these resins. Excellent simultaneous separation, highly sensitive conductimetric detection and symmetrical peaks for these C1–C6 amines were achieved on the TSKgel G3000PWXL column in 35 min with 5 mM heptylamine at pH 11.1 as the eluent.

Retention behavior of C 1–C 6 aliphatic monoamines on anion-exchange and polymethacrylate resins with heptylamine as eluent

Retention behavior of C 1–C 6 aliphatic monoamines (methylamine, ethylamine, propylamine, butylamine, amylamine and hexylamine) on columns ( 150 mm×6 mm i.d.) packed with various anion-exchange resins (styrene–divinylbenzene (PS–DVB) copolymer-based strongly basic anion-exchange resin: TSKgel SAX, polymethacrylate-based strongly basic anion-exchange resin: TSKgel SuperQ-5PW and polymethacrylate-based weakly basic anion-exchange resin: TSKgel DEAE-5PW) and unfunctionized polymethacrylate resins (TSKgel G5000PW and TSKgel G3000PW XL) was investigated with basic solutions (sodium hydroxide and heptylamine) as the eluents. Due to strongly electrostatic repulsion (ion-exclusion effect) between these anion-exchange resins and these amines, peak resolution between these amines on these anion-exchange resin columns was unsatisfactory with both sodium hydroxide and heptylamine as the eluents. In contrast, these polymethacrylate resins were successfully applied as the stationary phases for the separation of these C 1–C 6 amines with heptylamine as eluent, because of both small hydrophobicity and small cation-exchange ability of these resins. Excellent simultaneous separation, highly sensitive conductimetric detection and symmetrical peaks for these C 1–C 6 amines were achieved on the TSKgel G3000PW XL column in 35 min with 5 mM heptylamine at pH 11.1 as the eluent.

Mechanistic studies on the separation of cations in zwitterionic ion chromatography

Electrostatic ion chromatography, also known as zwitterionic ion chromatography, has been predominantly used for the analysis of anions. Consequently, separation mechanisms proposed for this technique have been based on anion retention data obtained using a sulfobetaine-type surfactant-coated column. A comprehensive cation retention data set has been obtained on a C 18 column coated with the zwitterionic surfactant N-tetradecylphosphocholine (which has the negatively and positively charged functional groups reversed in comparison to the sulfobetaine surfactants), with mobile phases being varied systematically in the concentration and species of both the mobile-phase anion and cation. A retention mechanism based on both an ion exclusion effect and a direct (chaotropic) interaction with the inner negative charge on the zwitterion is proposed for the retention of cations. Despite the relatively low chaotropic nature of cations compared with anions, the retention data shows that cations are retained in this system predominantly due to a chaotropic interaction with the inner charge, analogous to anions in a system where the C 18 column is coated with a sulfobetaine-type surfactant. The retention of an analyte cation, and the effect of the mobile-phase anion and cation, can be predicted by the relative positions of these species on the Hofmeister (chaotropic) series.

Characterisation of reversed-phase stationary phases for the liquid chromatographic analysis of basic pharmaceuticals by thermodynamic data

This paper describes the characterisation of reversed-phase liquid chromatography (RPLC) columns using thermodynamic measurements. Retention versus 1/ T data were used to construct Van ’t Hoff plots. The slope of these plots indicates the standard enthalpy of transfer of the analyte from the mobile to the stationary phase. The standard entropy can be calculated from the intercept. Van ’t Hoff plots were linear for the investigated RPLC columns, meaning that for basic analytes over the temperature range studied no changes in the retention mechanism occurred. Enthalpies and entropies of transfer of basic analytes from the mobile to the stationary phase revealed information about the types of interaction of protonated and neutral compounds with the stationary phases. However, a clear view using the present set of basic compounds on how these thermodynamic data may explain the observed substantial differences in peak symmetry cannot be given. It is considered that addition of N, N-dimethyloctylamine (DMOA) to the eluent will results in a dynamically coating of the stationary phase. Addition of DMOA to the eluent resulted for protonated basic compounds in a reduction of both enthalpy and entropy. In practice, with DMOA in the eluent symmetrical peaks were obtained. It is assumed that this is due to blocking residual silanols and/or ion exclusion effects.

Continuous preconcentration system for nitrate ions using anionic reverse osmosis tubes coupled to an ion chromatograph.

A continuous preconcentration system for nitrate ions was developed using cation exchange tubing made from Nafion perfluorosulfonic acid membrane. This method is based on ion exclusion effects and reverse osmosis phenomena. The system was evaluated by connecting it to an ion chromatograph. The concentration ratios could be increased by raising the pressures between the two sides of the cation exchange tubing. Twenty-fold concentration of nitrate ion was achieved when the pump pressure was 20 x 10(5) Pa. The relative standard deviations of the preconcentration ratio at four different pump pressures, 5, 10, 15 and 20 x 10(5) Pa were 1.2-2.8% (n = 5).