Concentration Of Charged Species Research Articles

Impact of variable density on electrokinetic transport and mixing in porous media

Electrokinetic (EK) techniques can effectively mobilize amendments and contaminants in porous media and therefore have a great potential for in situ subsurface remediation of a wide variety of organic and inorganic contaminants. However, the fundamental mechanisms governing EK applications in porous media are complex and entail coupling between different flow, transport, electrostatic and geochemical processes. In this study we investigate the interactions between density driven effects and electrokinetic transport in permeable porous media and we compare the observed dynamics with the outcomes of analogous scenarios in which advection and dispersion are the only transport mechanisms. We perform a series of multidimensional experiments and we demonstrate the strong interplay between Coulombic interactions among charged species in pore water and density-driven effects during electrokinetics. Our study illuminates non-trivial interactions between the pore water composition and the macroscopic behavior of variable density dye tracer plumes, showing different sinking and floating patterns, distinct plume shapes and spreading, as well as different extent of mixing depending on the chemical composition of the pore water. A numerical model, based on the Nernst-Planck-Poisson formulation and on the coupling with the geochemical code PHREEQC, was developed to simulate variable density flow and transport and to update the density of the aqueous solution resulting from mixing and electrostatic interactions between all chemical species in the injected plume and in the surrounding pore water electrolyte. The model allowed us to interpret the experiments and to visualize the impact of small density gradients and Coulombic effects on the macroscopic transport of the dye tracer. The outcomes of this study highlight the importance of density-driven flow also during EK transport, where the electrostatic interactions not only influence the concentration of charged species but can also affect the density and result in complex flow patterns, plume shapes and mixing behavior, radically different from the ones generated during advective-dispersive transport.

Ion Dynamics at the Carbon Electrode/Electrolyte Interface: Influence of Carbon Nanotubes Types.

Electrochemical quartz crystal microbalance (EQCM) and AC-electrogravimetry methods were employed to study ion dynamics in carbon nanotube base electrodes in NaCl aqueous electrolyte. Two types of carbon nanotubes, Double Wall Carbon Nanotube (DWCNT) and Multi Wall Carbon Nanotube (MWCNT), were chosen due to their variable morphology of pores and structure properties. The effect of pore morphology/structure on the capacitive charge storage mechanisms demonstrated that DWCNT base electrodes are the best candidates for energy storage applications in terms of current variation and specific surface area. Furthermore, the mass change obtained via EQCM showed that DWCNT films is 1.5 times greater than MWCNT films in the same potential range. In this way, the permselectivity of DWCNT films showed cation exchange preference at cathode potentials while MWCNT films showed anion exchange preference at anode potentials. The relative concentration obtained from AC-electrogravimetry confirm that DWCNT base electrodes are the best candidates for charge storage capacity electrodes, since they can accommodate higher concentration of charged species than MWCNT base electrodes.

An Organic Electrochemical Transistor to Monitor Salmonella Growth in Real‐Time

AbstractOrganic electrochemical transistors (OECTs) are used in research and diagnostic applications due to their facile manufacture, scalability, and biocompatibility. In these devices, the source–drain current upon gate voltage application depends on ion concentration in the electrolyte. This study investigates whether an OECT can be employed to monitor bacterial growth since it is known that the concentration of charged species increases in bacterial cultures during growth. A poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate‐based single‐well OECT, compatible with long‐term incubation of bacterial cultures, is fabricated. It is shown that the growth of Salmonella alters the transfer characteristics of the device and demonstrates how it can be applied to monitor growth in real‐time by recording the source–drain current at gate voltage +0.5 V. The signal can also be measured in filtrates of bacterial cultures, devoid of bacterial cells. This suggests that the signal originates from charged metabolic products. Bacterial biofilm formation does not alter the device response. This proof‐of‐principle study presents OECT recordings as an alternative to optical methods, allowing bacterial growth to be monitored in transparent and opaque media alike. By measuring metabolic products rather than bacterial cell multiplication, insight into the stationary phase and other nondividing states may be obtained in the future.

Study on the electrical response of small ethanol‐air diffusion flame under the uniform electric field

Ethanol is a renewable alternative fuel in many application areas. The external electric field is applied to assist the small ethanol diffusion flame in this study. The electrical response of flame under the uniform electric field is discussed. A reduced chemical mechanism of ethanol is used to establish a numerical model corresponding to the experimental system. The characteristics of current-electric field strength, equivalent resistance, distribution of charged species, velocity field, and reaction rate are studied. The results show that the electric field brings significant effects on the combustion and electrical characteristics of flame. As the electric field strength increases, the current through the circuit and the equivalent resistance of the flame show a three-stage variation. The current increases first (0-40 kV/m), then stabilizes (40-70 kV/m) and then continues to increase (70-100 kV/m) with the increase of the electric field strength. It shows that the flame has certain electric resistance characteristic. The number density of charged species in the flame increases, and the distributions of charged species tend to move upstream by the effect of electric field. When the electric field strength is high as 100 kV/m, the charged species density increases by 10%. Meantime, the gas velocity near the flame front is accelerated and the chemical reaction rate also increases by the effect of electric field. When the electric field strength reaches 100 kV/m, the corresponding reaction rate and the corresponding flow velocity are increased by about 40% and 20%, respectively, compared with those without electric field. The calculated ionic wind velocity increases greatly with the electric field strength and shows a nearly linear increasing trend. The combined effects of the enhanced ionic wind effect, the intensified chemi-ionization process, and the increase of the concentration of charged species are considered as the main reason for the effect of electric field.

Effect of salinity, Mg2+ and SO42− on “smart water”-induced carbonate wettability alteration in a model oil system

HypothesisWe present a systematic study of the “smart water” induced wettability alteration. This process is believed to be greatly affected by the brine salinity and the presence of Mg2+ and SO42− in the brine. Experiments and modellingTo characterize the wettability alteration, we perform spontaneous imbibition measurement using Indiana limestone cores and a model oil with added naphthenic acid. Both single-electrolyte-based and seawater-based “smart water” are tested to investigate the effect of Mg2+, SO42− and salinity on wettability alteration. Rock/brine and oil/brine zeta potentials are measured, and the electrostatic component of disjoining pressure is calculated to understand the role of electrostatics in the wettability alteration. The surface concentration of charged species on the limestone surface is analyzed based on a natural carbonate surface complexation model (SCM). FindingsBoth the reduction of Na+ and addition of SO42− are found to contribute to wettability alteration. Mg2+ is found to be unfavorable for wettability alteration. Ca2+ is believed to facilitate SO42− with wettability alteration based on the comparison between the single-electrolyte-based and seawater-based brines. The reduction of the Na+ surface complexation (>CaOH⋯Na+0.25) in low salinity brines is believed to be a critical mechanism responsible for wettability alteration based on the SCM calculations.

Measurement of ultralow radiation-induced charge densities using picosecond mid-IR laser-induced breakdown

We demonstrate that avalanche ionization breakdown of air with picosecond mid-infrared (mid-IR) laser pulses is an exceptionally sensitive and quantitative probe of extremely low concentrations of charged species. By exponentially increasing the electron density in the vicinity of a single seed atom or molecule to detectable levels, mid-IR electron avalanche is an analogue of single photon detection in photomultiplier tubes and can be useful in a range of applications. We apply the technique to meter-scale standoff detection of a radioactive source, sensitive to extremely low concentrations of radiation-induced negative ions down to ∼103 cm−3, limited only by background. By imaging the location of spatially isolated avalanche breakdown sites, we directly measure these low densities and benchmark the performance of standoff detection diagnostics. We discuss implementation of this radiation detection scheme at ranges of 10–100 m and adapting the avalanche probe to detection of other low-density plasmas.

Charged Species Concentration in Combusting Mixtures Using Equilibrium Chemistry

Ionization in flames is of interest in the design and development of modern combustion devices. The identity and concentration of various charged species in reacting mixtures can play an important role in the diagnostics and control of such devices. Simplified chemistry computations that provide good estimates of ionic species in complex flow-fields can be used to model turbulent reacting flows in various combustion devices, greatly reducing the required computational resources for design and development studies. A critical assessment of the use of the equilibrium chemistry method to compute charged species concentration in combusting mixtures under various temperatures, pressures, and thermal disequilibrium conditions is presented. The use of equilibrium chemistry to compute charged species concentrations in propane-air mixtures performed by Calcote and King has been extended. A more accurate computational methodology that includes the effect of negative ions, chemi-ions (H3O+ and CHO+), and thermal nonequilibrium was investigated to evaluate the suitability of equilibrium computations for estimating charged species concentrations in reacting mixtures. The results show that equilibrium computations which include the effects of H3O+ and elevated electron temperatures can indeed explain the levels of ion concentrations observed in laboratory flame experiments under lean and near-stoichiometric conditions. Furthermore, under engine-like conditions at higher temperatures and pressures, equilibrium computations can be used to obtain useful estimates of charged species concentrations in modern combustion devices.

An improved reaction mechanism for predicting the charged species in ethanol-air flame

To predict the charged species distributions in the ethanol-air flame, an improved reaction mechanism is proposed. The detailed reaction mechanism involving neutral species for the combustion of ethanol is combined with a set of ionic reactions to account for the charged species chemistry. The new chemical-kinetic mechanism consists of 64 species and 423 reactions. Simulation of ethanol-air flame involving charged species is conducted by using the model based on the proposed mechanism. The present model is validated by the experimental results. The ethanol-air flame temperature, neutral and charged species distributions are reasonably predicted by the present model. Although the concentrations of charged species are much lower than that of neutral species, the charged species have an important effect on the flame characteristics, especially under the electric field. Electrons are the main negative charged species and H3O+ is the main positive charged species in the ethanol-air flame. The results of pathway analysis and sensitivity analysis show the crucial roles of H3O+ and the reactions involving it for the charged species chemistry. When the electric field is applied, the present model has a better performance in predicting the diffusion ethanol-air flame compared with the model without charged species.

Numerical investigation of Ar–NH3 mixture in homogenous DBDs

In this paper, a study of homogenous dielectric barrier discharge Ar–NH3 was made in order to investigate the electrical and the physical characteristics of homogenous discharge at atmospheric pressure. The discharge model includes the electrical module and the chemical kinetic module. The results obtained by the present model were compared with experimental work. The evolution of the plasmas voltage, the species densities, and the concentration of charged species are analyzed and discussed.

A photochemical model of the dust‐loaded ionosphere of Mars

AbstractThe ionization of the lower Martian atmosphere and the presence of charged species are fundamental in the understanding of atmospheric electricity phenomena, such as electric discharges, large‐scale electric currents, and Schumann resonances. The present photochemical model of the lower ionosphere of Mars (0–70 km) is developed to compute the concentration of the most abundant charged species (cluster ions, electrons, and charged aerosols) and electric conductivity, at the landing site and epoch of the ExoMars 2016 mission. The main sources of ionization are galactic cosmic rays (during daytime as well as nighttime) and photoionization of aerosols due to solar UV radiation during daytime. Ion and electron attachment to aerosols is another major source of aerosol charging. The steady state concentration of charged species is computed by solving their respective balance equations (also known as continuity equations), which include the source and sink terms of their photochemical reactions. Since the amount of suspended dust can vary considerably and it has an important effect on atmospheric properties, several dust scenarios, in addition to the day‐night variations, are considered to characterize the variability of the concentration of charged species. It has been found that during daytime, aerosols tend to become positively charged due to electron photoemission and, during nighttime, tend to charge negatively due to electron attachment. The most dominant day‐night variability in ion and electron concentration occurs when the amount of suspended dust is the largest. The electric conductivity has been found to vary in the 10−13–10−7 S/m range, depending on the altitude, dust scenario, and local time.

Harvesting energy of interaction between bacteria and bacteriophage in a membrane-less fuel cell

When a fuel and oxidant flow in laminar contact through a micro-fluidic channel, a sharp interface appears between the two liquids, which eliminate the need of a proton exchange membrane. This principle has been used to generate potential in a membrane-less fuel cell. This study use such a cell to harvest energy of interaction between a bacteria having negative charge on its surface and a bacteriophage with positive and negative charges on its tail and head, respectively. When Klebsiella pneumoniae (Kp6) and phage (P-Kp6) are pumped through a fuel cell fitted with two copper electrodes placed at its two sides, interaction between these two charged species at the interface results in a constant open circuit potential which varies with concentration of charged species but gets generated for both specific and non-specific bacteria and phage system. Oxygenation of bacteria or phage however diminishes the potential unlike in conventional microbial fuel cells.

Analysis of the Current Signature in a Constant-Volume Combustion Chamber

Identifying charged species and their relative concentrations as a function of initial mixture composition, temperature, and pressure can play an important role in the diagnostics and control of combustion devices. This article aims to identify important charged species and their temporal variation in a spark-ignited methane/air mixture using the equilibrium constant method. The equilibrium composition of 20 neutral and seven charged species [(e−), CHO+, H3O+, NO+, OH−, , O−] was obtained using a novel adaptation of the Newton–Raphson method for highly stiff nonlinear system of equations. A two-zone combustion model was used to study the effect of air–fuel ratio (AFR) on the temporal variation of charged species concentration. Temporal variation of the current computed from the equilibrium concentration of charged species was found to compare well with experimental data for several AFRs.

Electrical and kinetic model of an atmospheric rf device for plasma aerodynamics applications

The asymmetrically mounted flat plasma actuator is investigated using a self-consistent two-dimensional fluid model at atmospheric pressure. The computational model assumes the drift-diffusion approximation and uses a simple plasma kinetic model. It investigated the electrical and kinetic properties of the plasma, calculated the charged species concentrations, surface charge density, electrohydrodynamic forces, and gas speed. The present computational model contributes to understand the main physical mechanisms, and suggests ways to improve its performance.

Electrochromatographic retention of peptides on strong cation‐exchange stationary phases

We analyze the systematic and substantial electrical field-dependence of electrochromatographic retention for four counterionic peptides ([Met5]enkephalin, oxytocin, [Arg8]vasopressin, and luteinizing hormone releasing hormone (LHRH) ) on a strong cation-exchange (SCX) stationary phase. Our experiments show that retention behavior in the studied system depends on the charge-selectivity of the stationary phase particles, the applied voltage, and the peptides' net charge. Retention factors of twice positively charged peptides ([Arg8]vasopressin and LHRH at pH 2.7) decrease with increasing applied voltage, whereas lower charged peptides (oxytocin and [Met5]enkephalin at pH 2.7, [Arg8]vasopressin and LHRH at pH 7.0) show a concomitant increase in their retention factors. The observed behavior is explained on the basis of electrical field-induced concentration polarization (CP) that develops around the SCX particles of the packing. The intraparticle concentration of charged species (buffer ions, peptides) increases with increasing applied voltage due to diffusive backflux from the enriched CP zone associated with each SCX particle. For twice charged and on the SCX phase strongly retained peptides the local increase in mobile phase ionic strength reduces the electrostatic interactions with the stationary phase, which explains the decrease of retention factors with increasing applied voltage and CP intensity. Lower charged and weaker retained peptides experience a much stronger relative intraparticle enrichment than the twice-charged peptides, which results in a net increase of retention factors with increasing applied voltage. The CP-related contribution to electrochromatographic retention of peptides on the SCX stationary phase is modulated by the applied voltage, the mobile phase ionic strength, and the peptides' net charge and could be used for selectivity tuning in difficult separations.

Interaction of a glow discharge with a rarefied hypersonic flow in a curvilinear channel

A two-dimensional simulation model is used to study the gasdynamic structure of a rarefied hypersonic flow of molecular nitrogen in a curvilinear channel in which the lower surface carries the cathode section of the discharge gap whereas the upper surface serves as the anode. The electrodynamic structure of the glow discharge (the distribution of concentrations of charged species, current density, and electric potential) is examined. It was demonstrated that the burning of a glow discharge in a rarefied hypersonic flow makes it possible to effectively modify the shock wave structure of the flow.

Ab initio simulation of the equation of state and kinetics of shocked water

We report herein first principles simulations of water under shock loading and the chemical reactivity under these hot, compressed conditions. Using a recently developed simulation technique for shock compression, we observe that water achieves chemical equilibrium in less than 2 ps for all shock conditions studied. We make comparison to the experimental results for the Hugoniot pressure and density final states. Our simulations show that decomposition occurs through the reversible reaction H(2)O <--> H(+) + OH(-), in agreement with experiment. Near the approximate intersection of the Hugoniot and the Neptune isentrope, we observe high concentrations of charged species that contribute electronic states near the band gap.

Rhombohedral–scalenohedral calcite transition produced by adjusting the solution electrical conductivity in the system Ca(OH) 2–CO 2–H 2O

This work is aimed to investigate the effects of the adjustment of the electrical conductivity ( κ 25) during the semicontinuous carbonation of Ca(OH) 2 suspension (slaked lime) on the morphology of the precipitated calcite (CaCO 3) particles. The experiments were carried out at 30, 45, and 60 °C. A gradual morphological change from rhombohedral to scalenohedral shapes was produced with an increase of κ 25 from 1 to 7 mS/cm at each temperature. The explanation of this morphological change is given in terms of the increase of both the supersaturation and the ratio between concentrations of charged species containing calcium and carbonate ([Ca] ch/[CO 3] ch) in the aqueous phase as the κ 25 set-point increases, prior to the precipitation process. In addition to the rise of the supersaturation this change most probably takes place because the increase of the [Ca] ch/[CO 3] ch ratio affects the growth rate of the rhombohedral {104} and scalenohedral {21-1} faces in a different manner: (i) favoring the equality between the surface coverage of Ca 2+ and CO 3 2− on the stoichiometric {104} face, thus enhancing the formation of CaCO 3 0 growth units and then its growth rate and (ii) inhibiting the growth of the {21-1} face by adsorption of the excess calcium species.

Stabilization of High Ionic Strength Slurries Using Surfactant Mixtures: Molecular Factors That Determine Optimal Stability

The successful formulation of particulate dispersions is often hindered by the presence of charged species dissolved in solution. If present at high enough concentrations, these species can reduce the screening length between particles to near zero, allowing rapid coagulation and destabilization. The addition of a mixture of ionic and nonionic surfactants has been shown in a previous publication (B. J. Palla and D. O. Shah, 2000, J. Colloid Interface Sci. 223, 102) to stabilize alumina particles in the presence of high concentrations of charged species. The mechanism has been explained as enhanced adsorption of a nonionic surfactant using a strongly adsorbing ionic surfactant as a binding agent. In this work, the various factors that influence this stabilization mechanism are examined in detail. The factors are then verified by showing the same results for two different chemical environments, both of which are model slurries for chemical mechanical polishing (CMP) of metal surfaces. The stabilizing ability of the surfactant system is found to increase with increasing hydrophobicity of the nonionic surfactant and increasing hydrophobicity of the ionic surfactant. The effect of surfactant concentration on stabilizing ability is shown to have an optimum concentration range for a number of surfactants. The effect of the ratio of ionic to nonionic surfactant is also investigated for both chemical environments and is shown to have significant effects on stabilizing ability. All of the results are then compiled to present a concise picture of the factors that influence the stabilizing ability of mixed surfactant systems for high-ionic-strength slurries.

Influence of Lipophilic Inert Electrolytes on the Selectivity of Polymer Membrane Electrodes

Lipophilic inert electrolytes, i.e., salts without ion-exchange properties, may influence the selectivity of ionophore-based liquid membrane electrodes by affecting the activity coefficients in the organic phase. It is expected by a theoretical model that the addition of a lipophilic salt renders the ion-selective electrode more selective for divalent over monovalent ions. These predictions are confirmed with Ca(2+)-responsive membranes containing the ionophores ETH 2120, ETH 1001, and ETH 129. The effect is especially pronounced with nonpolar membrane phases containing a low concentration of charged species, where up to 2 orders of magnitude selectivity improvement is observed.

Interaction of Glutathione and Other Low-Molecular-Weight Thiols with DNA: Evidence for Counterion Condensation and Coion Depletion near DNA

The interaction of low-molecular-weight thiols with sonicated DNA was examined using spin filtration to concentrate the DNA. Cationic thiols (WR 1065 and cysteamine) behaved as counterions and were found to have increased concentrations in the DNA retentate relative to the filtrate. Anionic thiols (GSH, 2-mercaptoethanesulfonate, mercaptosuccinate) behaved as coions and were decreased in concentration in the DNA fraction. Concentrations of the uncharged thiol 2-mercaptoethanol were little influenced by DNA. The results demonstrate the importance of counterion condensation and coion depletion in determining the concentrations of charged species near DNA. They provide a rationale for enhanced effectiveness of WR 1065 and cysteamine as radioprotectors compared to neutral and anionic thiols and suggest that anionic thiols such as GSH should be poor radioprotectors of DNA.