Ion Concentration Profiles Research Articles

Multicomponent ion transport in a mono- and bilayer cation-exchange membrane at high current density

This work describes a model for bilayer cation-exchange membranes used in the chlor-alkali process. The ion transport inside the membrane is modeled with the Nernst–Planck equation. A logistic function is used at the boundary between the two layers of the bilayer membrane to describe the change in the properties of each membrane layer. The local convective velocity is calculated inside the membrane using the Schlögl equation and the equation of continuity. The model calculates the ion concentration profiles inside the membrane layers. Modeling results of mono- and bilayer membranes are compared. The changes in membrane voltage drop and sodium selectivity are predicted. The concentration profile of sodium ions in the bilayer membrane is significantly different from the monolayer membrane. Without the applied current, a linear change in the sodium concentration is observed in the monolayer membrane and in each layer of the bilayer membrane. With an increase in current density, the stronger electromotive force in the carboxylate layer causes a decrease in the sodium concentration in the sulfonate layer, down to the fixed ionic group concentration. This significant decrease of sodium ion concentration in the sulfonate layer results in low concentrations of counter ions and as a consequence a higher permselectivity of the bilayer membrane is obtained when compared to the single-layer membrane. As a drawback, the resistance in the bilayer membrane increases.Graphical

Measurements of Positively Charged Ions in Premixed Methane-Oxygen Atmospheric Flames

ABSTRACTCations and anions are formed as a result of chemi-ionization processes in combustion systems. Electric fields can be applied to reduce emissions and improve combustion efficiency by active control of the combustion process. Detailed flame ion chemistry models are needed to understand and predict the effect of external electric fields on combustion plasmas. In this work, a molecular beam mass spectrometer (MBMS) is utilized to measure ion concentration profiles in premixed methane–oxygen argon burner-stabilized atmospheric flames. Lean and stoichiometric flames are considered to assess the dependence of ion chemistry on flame stoichiometry. Relative ion concentration profiles are compared with numerical simulations using various temperature profiles, and good qualitative agreement was observed for the stoichiometric flame. However, for the lean flame, numerical simulations misrepresent the spatial distribution of selected ions greatly. Three modifications are suggested to enhance the ion mechanism and improve the agreement between experiments and simulations. The first two modifications comprise the addition of anion detachment reactions to increase anion recombination at low temperatures. The third modification involves restoring a detachment reaction to its original irreversible form. To our knowledge, this work presents the first detailed measurements of cations and flame temperature in canonical methane–oxygen-argon atmospheric flat flames. The positive ion profiles reported here may be useful to validate and improve ion chemistry models for methane-oxygen flames.

Space charges and defect concentration profiles at complex oxide interfaces

We discuss electronic and ionic defect concentration profiles at the conducting interface between the two wide-band-gap insulators ${\mathrm{LaAlO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ (STO). The profiles are deduced from a thermodynamic model considering a local space charge layer (SCL) originating from charge transfer to the interface region, thus combining electronic and ionic reconstruction mechanisms. We show that the electrical potential confining the two-dimensional electron gas (2DEG) at the interface modifies the equilibrium defect concentrations in the SCL. For the $n$-conducting interface, positively charged oxygen vacancies are depleted within the SCL, while negatively charged strontium vacancies accumulate. Charge compensation within the SCL is achieved by a mixed ionic-electronic interface reconstruction, while the competition between 2DEG and localized ionic defects is controlled by ambient $p{\mathrm{O}}_{2}$. The concentration of strontium vacancies increases drastically in oxidizing conditions and exhibits a steep depth profile towards the interface. Accounting for the low cation diffusivity in STO, we also discuss kinetic limitations of cation defect formation and the effect of a partial equilibration of the cation sublattice. We discuss the resulting implications for low temperature transport.

Effects of electromagnetic field exposure on conduction and concentration of voltage gated calcium channels: A Brownian dynamics study

A three-dimensional Brownian Dynamics (BD) in combination with electrostatic calculations is employed to specifically study the effects of radiation of high frequency electromagnetic fields on the conduction and concentration profile of calcium ions inside the voltage-gated calcium channels. The electrostatic calculations are performed using COMSOL Multiphysics by considering dielectric interfaces effectively. The simulations are performed for different frequencies and intensities. The simulation results show the variations of conductance, average number of ions and the concentration profiles of ions inside the channels in response to high frequency radiation. The ionic current inside the channel increases in response to high frequency electromagnetic field radiation, and the concentration profiles show that the residency of ions in the channel decreases accordingly.

Visualization of Steady-State Ionic Concentration Profiles Formed in Electrolytes during Li-Ion Battery Operation and Determination of Mass-Transport Properties by in Situ Magnetic Resonance Imaging.

Accurate modeling of Li-ion batteries performance, particularly during the transient conditions experienced in automotive applications, requires knowledge of electrolyte transport properties (ionic conductivity κ, salt diffusivity D, and lithium ion transference number t(+)) over a wide range of salt concentrations and temperatures. While specific conductivity data can be easily obtained with modern computerized instrumentation, this is not the case for D and t(+). A combination of NMR and MRI techniques was used to solve the problem. The main advantage of such an approach over classical electrochemical methods is its ability to provide spatially resolved details regarding the chemical and dynamic features of charged species in solution, hence the ability to present a more accurate characterization of processes in an electrolyte under operational conditions. We demonstrate herein data on ion transport properties (D and t(+)) of concentrated LiPF6 solutions in a binary ethylene carbonate (EC)-dimethyl carbonate (DMC) 1:1 v/v solvent mixture, obtained by the proposed technique. The buildup of steady-state (time-invariant) ion concentration profiles during galvanostatic experiments with graphite-lithium metal cells containing the electrolyte was monitored by pure phase-encoding single point imaging MRI. We then derived the salt diffusivity and Li(+) transference number over the salt concentration range 0.78-1.27 M from a pseudo-3D combined PFG-NMR and MRI technique. The results obtained with our novel methodology agree with those obtained by electrochemical methods, but in contrast to them, the concentration dependences of salt diffusivity and Li(+) transference number were obtained simultaneously within the single in situ experiment.

Parametrical study of the cementitious materials degradation under external sulfate attack through numerical modeling

Underground concrete structures may be subjected to external sulfate attack and their long-term performance is of concern. A coupled transport/chemical numerical model has been compared to experimental data obtained on a cement paste exposed to external sulfate attack thanks to recent developments in experimental analysis such as scanning electron microscopy and image analysis that provided total ion concentration profiles for a given species in cement paste specimens. As other studies the model succeeded in calculating the position of the ettringite precipitation front and the position of the first decrease due to the portlandite dissolution whereas the leaching of portlandite and ettringite was not accurately assessed in the first millimeters because of the necessity to take into account change of the transport properties in the degraded zone. Then a numerical parametric study was carried on. The influence of permanent solution renewal, pH control, and dissolved CO2 concentration were analyzed. The accelerating effect of permanently renewing the solution was confirmed. The influence of the pH control was found dependent of the renewing of solution. The presence of CO2 did not play a significant role.

Electrokinetic transport of monovalent and divalent cations in silica nanochannels

Electrokinetic transport of aqueous electrolyte solutions in nanochannels and nanopores is considered important toward the understanding of fundamental ion transport in many biological systems, lab-on-chip, and organ-on-chip devices. Despite the overall importance of these systems and devices, detailed calculations showing velocity and concentration profiles for multi-component, multi-valent ionic species are limited. In this paper, molecular dynamics simulations were employed to compute velocity and concentration profiles for an electrolyte mixture containing sodium, magnesium, and chloride ions with water as the solvent in a ~7-nm-deep amorphous silica nanochannel. The results indicate that addition of trace quantities of divalent Mg2+ ions to monovalent (NaCl) electrolyte solutions while preserving overall system electroneutrality increases the maximum electroosmotic velocity of the solution by almost two times. Additionally, analyzing concentration profiles of individual ions revealed that Na+ was found to be preferentially attracted to the negatively charged silica wall in comparison with Mg2+ likely due to the hydrated divalent cation having a larger size compared to the hydrated monovalent cation.

Spin physics and biological membranes

Formula for calculating the concentration profile of ions in biological membranes has been obtained. It is assumed that ions are moving in a viscous medium under the action of the electric field and a concentration gradient. The problem is that ions are coated with shells consisting of water dipoles. These dipoles copy the form of the ions and in a strong electric field they can acquire the shape of an ellipsoid which changes the effective ion radius in the membrane. Calculation of the Na+1 and K+1 profiles leads to a conclusion that active and passive transport of ions is closely associated with the shape of the hydrated shells. The work was performed at the Veksler and Baldin Laboratory of High Energy Physics, JINR, Dubna.

Confocal detection of Rayleigh scattering for residual stress measurement in chemically tempered glass

A new optical method is presented for evaluation of the stress profile in chemically tempered (chemically strengthened) glass based on confocal detection of scattered laser beam. Theoretically, a lateral resolution of 0.2 μm and a depth resolution of 0.6 μm could be achieved by using a confocal microscope with high-NA immersion objective. The stress profile in the 250 μm thick surface layer of chemically tempered lithium aluminosilicate glass was measured with a high spatial resolution to illustrate the capability of the method. The confocal method is validated using transmission photoelastic and Na+ ion concentration profile measurement. Compositional influence on the stress-optic coefficient is calculated and discussed. Our method opens up new possibilities for three-dimensional scattered light tomography of mechanical imaging in birefringent materials.

Time and Temperature Dependent Study of Zn and Ni Codiffused LiNbO<sub>3</sub> Waveguides

Zn and Ni codiffused lithium niobate planar and channel waveguides are fabricated at various temperatures for different times. Compositional and optical properties of the waveguides are studied. Single-mode channel waveguides are measured for 2-D refractive index profiling using a modified end-fire coupling method at excitation wavelengths of both 633 and 1550 nm. Planar waveguides with the same metallic thicknesses as those of channel waveguides are analyzed using secondary-ion-mass-spectrometry to obtain the diffused metal ion concentration profiles. The relationship between the diffusion depths and index depths is discussed based on the completeness of the diffusion process.

The influence of nitrogen implantation on tribological properties of AISI H11 steel

Nitrogen ion implantation can be used to improve the surface mechanical properties (hardness, wear) of steel by modifying the near-surface layers of these materials. In this research, AISI H11 hot work tool steel was implanted with 120 keV nitrogen ions at the fluence of (1 ÷ 10) × 1016 N+/cm2. The concentration profiles of nitrogen ions implanted into H11 steel were measured by RBS and these were compared with the profiles created using SATVAL and SRIM computer simulation codes. The surface morphology was examined and elemental analysis was performed using SEM, AFM, EDS, and RBS. Wear tests were conducted with the use of the pin-on-disc method. The worn surfaces of samples were observed by means of SEM, EDS, WDS and CEMS. The surface layers were characterised using a nano-hardness test. A significant improvement in nano-hardness and wear resistance has been found with increasing implantation fluences. The decreased wear rate and increased friction coefficient caused by the long-range effect extended to a 19-fold greater depth than the range of the implanted nitrogen ions. The investigations have shown that the long-range effect is also induced by nitrogen diffusion during a local increase in temperature in the friction process. Nitrogen implantation modified the nature of surface wear, decreasing adhesive wear and increasing oxidizing wear. The oxidizing wear predominated after the implantation at the fluence of 1 × 1017 N+/cm2.

Transport and Exchange Behavior of Ions in Bentonite During Electro-Osmotic Consolidation

AbstractElectro-osmotic consolidation is considered to be an efficient technique for dewatering and consolidation of soft soil. In the present study, four experiments were conducted on a Na-rich bentonite using two reactive electrodes (copper and iron) and two inert electrodes (graphite and stainless steel) to study the transport and exchange behavior of ions during electro-osmotic consolidation. The results showed that the changes in pH and ion contents were limited to the zone close to the electrode due to the buffering capacity of bentonite and the significant reduction in electric current density. The ion concentration profiles indicated that Na+ ions were largely responsible for carrying the pore water to the cathode. The reactive electrodes are better at transporting Na+ ions and therefore induce better drainage than inert electrodes. Ion-exchange reactions occurred between the Cu2+ and Fe2+/Fe3+ ions released and pre-existing Na+ ions in the electrical double layer, causing decreased water adsorption capacity and plasticity index. The swelling and shrinkage characteristics of the bentonite were thus reduced, and electroosmotic consolidation may therefore provide a new way to improve the stability of expansive soils and slopes.

Density functional theory study on ion adsorption and electroosmotic flow in a membrane with charged cylindrical pores

ABSTRACTIon adsorption and electroosmotic flow induced by an external electric field have a variety of practical applications, especially for membrane technology. In this work, a partially perturbative density functional theory (DFT) based on the modified fundamental measure theory was applied to investigate the ion density distributions and partitions in a charged cylindrical pore. Different types of electrolyte solutions, including both charge symmetric and asymmetric, were examined using the proposed theory with various pore diameters, bulk densities, ion valencies and surface charge densities. The ion concentration profiles calculated with the theory exhibit good agreements with the results of the Monte Carlo simulations, while the results of the Poisson–Boltzmann equation deviate greatly especially for the high valence electrolytes in narrow cylindrical pores. Some interesting phenomena discovered in both experiments and simulations, such as the reverse distribution of the ions and charge inversion, can be well reproduced with the DFT. Based on the ion concentration distributions obtained from the DFT, the transient velocity profiles of the electroosmotic flow in the charged cylindrical nanopores were calculated with the Navier–Stokes (NS) equation. The characteristics of the electroosmotic flow were discussed under the different bulk electrolyte concentrations and thickness of the electric double layer inside the nanopore. The enhancement of the velocity near the pore wall, which cannot be described by the traditional theory, was well characterised by the DFT combined with the NS equation.

Thermal Effect on Microchannel Electro-osmotic Flow With Consideration of Thermodiffusion

Electro-osmotic flow (EOF) is widely used in microfluidic systems. Here, we report an analysis of the thermal effect on EOF under an imposed temperature difference. Our model not only considers the temperature-dependent thermophysical and electrical properties but also includes ion thermodiffusion. The inclusion of ion thermodiffusion affects ionic distribution, local electrical potential, as well as free charge density, and thus has effect on EOF. In particular, we formulate an analytical model for the thermal effect on a steady, fully developed EOF in slit microchannel. Using the regular perturbation method, we solve the model analytically to allow for decoupling several physical mechanisms contributing to the thermal effect on EOF. The parametric studies show that the presence of imposed temperature difference/gradient causes a deviation of the ionic concentration, electrical potential, and electro-osmotic velocity profiles from their isothermal counterparts, thereby giving rise to faster EOF. It is the thermodiffusion induced free charge density that plays a key role in the thermodiffusion induced electro-osmotic velocity.

Long-term performance of concrete surface coatings in soil exposure of marine environments

The present research addresses the performance of coated concrete samples after 88months of natural exposure to coastal soil of the Persian Gulf. A concrete mixture proportioned with water-to-cement ratio of 0.5 was used for casting the reference and all coated specimens. Five types of surface coatings were investigated. Samples were extracted at different ages of 9, 36, and 88months to monitor carbonation depth and chloride ion penetration into concrete. Slight signs of carbonation are observed for the reference concrete and specimens coated with cementitious coatings. Chloride ion concentration profiles are analyzed during the exposure period. Chloride ion diffusivity is investigated for each sampling time. Surface chloride build-up is also monitored. Time to corrosion initiation is calculated and incorporated as a means of ranking coating efficiencies. Among all studied coatings, aliphatic acrylic and polyurethane are proved to reduce chloride ion concentration most effectively. This resulted in increasing the simulated time required for corrosion initiation in soil exposure at marine environments.

ACIDIC REMOVAL OF METALS FROM FLUIDIZED CATALYTIC CRACKING CATALYST WASTE ASSISTED BY ELECTROKINETIC TREATMENT

Abstract - One of the main uses of catalysts in the oil industry is in the fluidized catalytic cracking process, which generates large quantities of waste material after use and regeneration cycles and that can be treated by the electrokinetic remediation technique, in which the contaminant metals are transported by migration. In this study, deactivated FCC catalyst was characterized before and after the electrokinetic remediation process to evaluate the amount of metal removed, and assess structural modifications, in order to indicate a possible use as an adsorbent material. The analyses included pH measurement and the concentration profile of vanadium ions along the reactor, X-ray microtomography, X-ray fluorescence, BET analysis and DTA analysis. The re-sults indicated that 40% of the surface area of the material was recovered in relation to the disabled material, showing an increase in the available area for the adsorption. The remediation process removed nearly 31% of the vanadium and 72% of the P

Coupling Bulk and Near-Electrode Interfacial Nanostructuring in Ionic Liquids

The efficiency of charge transfer in electrochemical devices is largely determined by the ion concentration profile near the electrode surface, i.e., the electrical double layer (EDL). Room temperature ionic liquids (RTILs) are attractive for electrochemical applications due to their high charge density as well as for their tunable anion/cation design, low vapor pressure, and wide electrochemical window. The EDL structure in RTILs is profoundly different from that in traditional (dilute) electrolytes in that opposite charges tend to layer in a spatially alternating, segregated structure that decays toward the bulk region. Such charge layering becomes crucial for applications that require confinement of RTILs into narrow spaces, where RTILs are interfaced with nanostructured electrodes. Layering in the EDL is frequently explained by electrostatic interactions of the ions with the electrode, assuming that RTILs are homogeneous liquids made of ions only. However, a growing evidence points to the presence of ...

Development of Melting System for Measurement of Trace Elements and Ions in Ice Core

We present a titanium (Ti) melting head divided into three zones as an improved melting system for decontaminating ice‐core samples. This system was subjected to performance tests using short ice‐core samples (4 × 4 cm2, ~5 cm long). The procedural blanks (PBs) and detection limits of ionic species, with the exception of , were comparable with published values, but for elements the experimental procedures should be refined to obtain valid Zn concentrations due to the PB of ~90.0 ± 16.2 ng/L. The improved melting system efficiently decontaminated the samples, as verified by the concentration profiles of elements and ions in the melted samples from the three melting‐head zones. The recovery of trace elements in ice‐core samples was ~70–120% at ~100 ng/L in artificial ice cores. Because of the memory effects between ice‐core samples melted in series, the melting system should be rinsed at least 5–6 times (in a total volume of ~2.5 mL deionized water) after each melting procedure. Finally, as an application of this technique, trace elements were measured in ice‐core samples recovered from the East Rongbuk Glacier, Mount Everest, (28°03′N, 86°96′E, 6518 m a.s.l.), and the concentrations of trace elements following mechanical chiseling and the melting method were compared.

Estimating the parameters of a positive column of the halogen-containing glow discharge at moderate pressures

The methodical recommendations for estimating the plasma parameters of an electronegative glow discharge containing halogens at moderate pressures (up to 40 Torr) with the use of simple analytic formulae and without numerical modeling are given. The initial data are easily measureable discharge parameters such as a discharge current, a voltage and a gas mixture pressure and composition as well. It is shown how one can easily consider such important plasma features as non-Maxwellian electron energy distribution function and halogen molecules dissociation by electron impact. As a result, such plasma parameters as the absolute degree of electronegativity, the value of border coordinate between ion–ion and electron–ion plasmas, and the forms of transversal profiles of electron and negative ion concentrations can be evaluated. The comparison of the results with the ones given by a global numerical model shows the suitability of said analytic approach to estimate plasma parameters of real discharges.

STEADY STATE AND PSEUDO-TRANSIENT ELECTRIC POTENTIAL USING THE POISSONBOLTZMANN EQUATION

A method for analysis of the electric potential profile in saline solutions was developed for systems with one or two infinite flat plates. A modified Poisson-Boltzmann equation, taking into account nonelectrostatic interactions between ions and surfaces, was used. To solve the stated problem in the steady-state approach the finite-difference method was used. For the formulated pseudo-transient problem, we solved the set of ordinary differential equations generated from the algebraic equations of the stationary case. A case study was also carried out in relation to temperature, solution concentration, surface charge and salt-type. The results were validated by the stationary problem solution, which had also been used to verify the ionic specificity for different salts. The pseudo-transient approach allowed a better understanding of the dynamic behavior of the ion-concentration profile and other properties due to the surface charge variation.