Constant Charge Density Research Articles

AC-driven electro-osmotic flow in charged nanopores

In this paper we report the theory describing the electro-osmotic flow in charged nanopores with constant radius and charge density driven by alternating current. We solve the ion and solution transport in unsteady conditions as described by the Navier-Stokes and Nernst-Planck equations considering the electrical potential inside the charged nanopore uniform in the radial direction (Uniform Potential model approximation). We derive the transport equation system in the case in which the pore is connected to two boundary diffusion layers and the cations and anions have different diffusion coefficients. This approach allows the theoretical description of the characteristic frequency dependence of the phase shift between the applied current density and the electro-osmotic flow. Additionally we show how the analysis of the dynamic response of the electro-osmotic coupling factor vs. the AC frequency allows us to quantify the apparent ion diffusion coefficient in membranes. Notably, the frequency window where the phase shift is predicted is well inside the commonly used sampling rate for electro-osmotic flow experiments Hz, hence the proposed method can be useful to determine the apparent diffusion coefficient of ions (such as redox complex for flow batteries) in charged membranes.

Probing the structures and properties of Ti2Si20−/0 clusters by density functional theory calculations

We present a theoretical investigation on the structures and properties of Ti2Si20−/0 clusters using density functional theory calculations. The results showed that the global minima of both anionic and neutral Ti2Si20 adopt a C2h symmetric double hexagonal prisms stacked structure with the two Ti atoms encapsulated inside the silicon cage. Bond length, Wiberg bond order, constant electronic charge density, and molecular orbital analyses suggest that the TiTi interactions in Ti2Si20−/0 are weak. Interestingly, Ti2Si20−/0 exhibit significant 3D aromaticity, which play important roles in their structural stability.

Probing the geometric structures and electronic properties of anionic and neutral Pt3C2 clusters by density functional calculations

We present a theoretical investigation on the geometric structures and electronic properties of Pt3C2−/0. The results showed that both anionic and neutral Pt3C2 adopt PtC2 planar triangular structures with each C atom coordinating with two Pt atoms. Additionally, Pt3C2-− anion and Pt3C2 neutral both show aromaticity. Bond lengths, bond orders, and constant electronic charge densities of Pt3C2−/0 suggest that the strong interactions between the two C atoms play vital roles in their structural stability. NPA charge distributions of anionic and neutral Pt3C2 are different. Furthermore, both Pt3C2− anion and Pt3C2 neutral have σ plus π double bonding patterns.

Determination of charge transport activation energy and injection barrier in organic semiconductor devices

Charge carrier transport in organic semiconductor devices is thermally activated with characteristic activation energies in the range of 0.2–0.6 eV, leading to strongly temperature-dependent behaviour. For designing efficient organic semiconductor materials and devices, it is therefore indispensable to understand the origin of these activation energies. We propose that in bilayer organic light-emitting diodes (OLEDs) employing a polar electron transport layer, as well as in metal-insulator-semiconductor (MIS) devices, the hole injection barrier Einj and the hole mobility activation energy Eμ can be decoupled from each other if temperature-dependent capacitance-frequency (C-f-T) and MIS-CELIV (charge extraction by linearly increasing voltage) experiments are combined. While the C-f-T signal contains information of both injection and transport, the CELIV current is expected to be insensitive to the electrode injection properties. We employ numerical drift-diffusion simulations to investigate the accuracy of this analytical parameter extraction approach and to develop criteria for its validity. We show that the implicit assumption of constant charge density and field profiles leads to systematic errors in determining the activation energies. Thus, one should be aware of the intrinsic limitations of the analytical Arrhenius fit, and for more accurate parameter determination a full drift-diffusion modelling is advised. Applying the analytical method to a standard bilayer OLED, we find that the total activation energy of 0.5 eV for the hole current can be split into contributions of ≈0.25 eV each for injection barrier and mobility. Finally, we also discuss the broader applicability of this method for other device stacks and material combinations.

Steric effects on electroosmotic flow in soft nanochannels

A numerical solution is derived of the mixed pressure and electroosmotically driven flow in a soft nanochannel considering the steric effect within electric double layer. The electric potential and the velocity distributions are described by the modified Poisson–Boltzmann equation and the modified Navier–Stokes equation under the conditions of boundary slip and constant charge density on the walls. By using the finite difference method, the numerical solutions of the electric potential, velocity and the volumetric flow rate are obtained. Results show that the electric potential increases with the steric parameter. The velocity decreases with the steric parameter for lower surface charge density, while opposite trend can be found for higher surface charge density. We compare the results of soft and rigid nanochannels and find the velocity in the soft nanochannel is larger than that in the rigid one except for the case of higher surface charge density.

Generation of electron vortex beams using line charges via the electrostatic Aharonov-Bohm effect

It has recently been shown that an electron vortex beam can be generated by the magnetic field surrounding the tip of a dipole-like magnet. This approach can be described using the magnetic Aharonov-Bohm effect and is associated with the fact that the end of a long magnetic rod can be treated approximately as a magnetic monopole. However, it is difficult to vary the magnetisation of the rod in such a setup and the electron beam vorticity is fixed for a given tip shape. Here, we show how a similar behaviour, which has the advantage of easy tuneability, can be achieved by making use of the electrostatic Aharonov-Bohm effect associated with an electrostatic dipole line. We highlight the analogies between the magnetic and electrostatic cases and use simulations of in-focus, Fresnel and Fraunhofer images to show that a device based on two parallel, oppositely charged lines that each have a constant charge density can be used to generate a tuneable electron vortex beam. We assess the effect of using a dipole line that has a finite length and show that if the charge densities on the two lines are different then an additional biprism-like effect is superimposed on the electron-optical phase.

Equilibria model for pH variations and ion adsorption in capacitive deionization electrodes

Ion adsorption and equilibrium between electrolyte and microstructure of porous electrodes are at the heart of capacitive deionization (CDI) research. Surface functional groups are among the factors which fundamentally affect adsorption characteristics of the material and hence CDI system performance in general. Current CDI-based models for surface charge are mainly based on a fixed (constant) charge density, and do not treat acid-base equilibria of electrode microstructure including so-called micropores. We here expand current models by coupling the modified Donnan (mD) model with weak electrolyte acid-base equilibria theory. In our model, surface charge density can vary based on equilibrium constants (pK's) of individual surface groups as well as micropore and electrolyte pH environments. In this initial paper, we consider this equilibrium in the absence of Faradaic reactions. The model shows the preferential adsorption of cations versus anions to surfaces with respectively acidic or basic surface functional groups. We introduce a new parameter we term “chemical charge efficiency” to quantify efficiency of salt removal due to surface functional groups. We validate our model using well controlled titration experiments for an activated carbon cloth (ACC), and quantify initial and final pH of solution after adding the ACC sample. We also leverage inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography (IC) to quantify the final background concentrations of individual ionic species. Our results show a very good agreement between experiments and model. The model is extendable to a wide variety of porous electrode systems and CDI systems with applied potential.

Effect of RF sputtering power on morphological and electrical properties of calcium copper titanate thin films

Calcium copper titanate (CCTO) thin films were deposited on p-type silicon substrate by RF magnetron sputtering at various RF powers. Post deposition annealing was carried out for all the samples at 950 °C for 1 h in air atmosphere. Various types of surface morphology have been observed for CCTO thin films with the variation of RF power. The evolution of polycrystalline structure of the CCTO thin films was confirmed by XRD studies. The FTIR characteristic absorption band of CCTO was observed in the range of 400–700 cm−1. The capacitance–voltage (C–V) and current–voltage (I–V) characteristics of the films were investigated employing Al/CCTO/Si MOS capacitors. CCTO films with higher dielectric constant, lower oxide and interface charge density were obtained at higher RF power. Leakage current was also found to be minimum for RF power of 105 W.

How big are atoms in crystals?

Atoms and bonds are central concepts in structural chemistry, but neither are concepts that arise naturally from the physics of condensed phases. It is ironic that the internuclear distances in crystals that are readily measured depend on the sizes of atoms, but since atoms in crystals can be defined in many different ways, all of them arbitrary and often incompatible, there is no natural way to express atomic size. I propose a simple coherent picture of Atoms-in-Crystals which combines properties selected from three different physically sound definitions of atoms and bonds. The charge density of the free atom that is used to construct the procrystal is represented by a sphere of constant charge density having the quantum theory of atoms in molecules (QTAIM) bonded radius. The sum of these radii is equal to the bond length that correlates with the bond flux (bond valence) in the flux theory of the bond. The use of this model is illustrated by answering the question: How big are atoms in crystals? The QTAIM bonded radii are shown to be simple functions of two properties, the number of quantum shells in the atomic core and the flux of the bond that links neighbouring atoms. Various radii can be defined. The univalent bonded radius measures the intrinsic size of the atom and is the same for all cations in a given row of the periodic table, but the observed bonded radius depends also on the bond flux that reflects the chemical environment.

ELECTRIC POTENTIAL AND FIELD CALCULATION OF CHARGED BEM TRIANGLES AND RECTANGLES BY GAUSSIAN CUBATURE

It is a widely held view that analytical integration is more accurate than the numerical one. In some special cases, however, numerical integration can be more advantageous than analytical integration. In our paper we show this benefit for the case of electric potential and field computation of charged triangles and rectangles applied in the boundary element method (BEM). Analytical potential and field formulas are rather complicated (even in the simplest case of constant charge densities), they have usually large computation times, and at field points far from the elements they suffer from large rounding errors. On the other hand, Gaussian cubature, which is an efficient numerical integration method, yields simple and fast potential and field formulas that are very accurate far from the elements. The simplicity of the method is demonstrated by the physical picture: the triangles and rectangles with their continuous charge distributions are replaced by discrete point charges, whose simple potential and field formulas explain the higher accuracy and speed of this method. We implemented the Gaussian cubature method for the purpose of BEM computations both with CPU and GPU, and we compare its performance with two different analytical integration methods. The ten different Gaussian cubature formulas presented in our paper can be used for arbitrary high-precision and fast integrations over triangles and rectangles.

Influence of Biofluids Rheological Behavior on Electroosmotic Flow and Ionic Current Rectification in Conical Nanopores

Flow through nanopores has received a great deal of attention during the past decade due to its versatile areas of application in biology and engineering. The asymmetrical geometry of conical nanopores has made these devices advantageous in rectification of ionic current for counting and detection of biofluidic entities such as proteins and DNAs. Protein solutions exhibit non-Newtonian rheological behavior which mathematically alludes to a nonlinear relation between shear stress and shear rate. In this study, the electroosmotic flow (EOF) of non-Newtonian solutions through a conical nanopore with a constant charge density on the wall is investigated numerically. Using the assumption of continuity, the ionic transport in the EO flow is modeled by combining the Poisson, Nernst–Plank, and Navier–Stokes equations for potential field, ionic concentration, and velocity distributions, respectively. The biofluid is assumed to behave as a non-Newtonian power-law fluid with constant physical properties. For both ov...

Side Chain Degradable Cationic-Amphiphilic Polymers with Tunable Hydrophobicity Show in Vivo Activity.

Cationic-amphiphilic antibacterial polymers with optimal amphiphilicity generally target the bacterial membranes instead of mammalian membranes. To date, this balance has been achieved by varying the cationic charge or side chain hydrophobicity in a variety of cationic-amphiphilic polymers. Optimal hydrophobicity of cationic-amphiphilic polymers has been considered as the governing factor for potent antibacterial activity yet minimal mammalian cell toxicity. However, the concomitant role of hydrogen bonding and hydrophobicity with constant cationic charge in the interactions of antibacterial polymers with bacterial membranes is not understood. Also, degradable polymers that result in nontoxic degradation byproducts offer promise as safe antibacterial agents. Here we show that amide- and ester (degradable)-bearing cationic-amphiphilic polymers with tunable side chain hydrophobicity can modulate antibacterial activity and cytotoxicity. Our results suggest that an amide polymer can be a potent antibacterial agent with lower hydrophobicity whereas the corresponding ester polymer needs a relatively higher hydrophobicity to be as effective as its amide counterpart. Our studies reveal that at higher hydrophobicities both amide and ester polymers have similar profiles of membrane-active antibacterial activity and mammalian cell toxicity. On the contrary, at lower hydrophobicities, amide and ester polymers are less cytotoxic, but the former have potent antibacterial and membrane activity compared to the latter. Incorporation of amide and ester moieties made these polymers side chain degradable, with amide polymers being more stable than the ester polymers. Further, the polymers are less toxic, and their degradation byproducts are nontoxic to mice. More importantly, the optimized amide polymer reduces the bacterial burden of burn wound infections in mice models. Our design introduces a new strategy of interplay between the hydrophobic and hydrogen bonding interactions keeping constant cationic charge density for developing potent membrane-active antibacterial polymers with minimal toxicity to mammalian cells.

Combined electroosmotically and pressure driven flow in soft nanofluidics

The present study is devoted to the analysis of mixed electroosmotic and pressure driven flows through a soft charged nanochannel considering boundary slip and constant charge density on the walls of the slit channel. The sources of the fluid flow are the pressure gradient along the channel axis and the electrokinetic effects that trigger an electroosmotic flow under the influence of a uniformly applied electric field. The polyelectrolyte layer (PEL) is denoted as a fixed charge layer (FCL) and the electrolyte ions can be present both inside and outside the PEL i.e., the PEL–electrolyte interface acts as a semi-penetrable membrane. The Poisson–Boltzmann equation is solved assuming the Debye–Hückel linearization for the low electric potential to provide us with analytical closed form solutions for the conservation equations. The conservation equations are solved to obtain the electric potential and velocity distributions in terms of governing dimensionless parameters. The results for the dimensionless electric potential, the dimensionless velocity and Poiseuille number are presented graphically and discussed in detail.

Diffusiophoresis of a pH-regulated polyelectrolyte in a nanopore of nonuniform cross section

The diffusiophoresis of a pH-regulated polyelectrolyte (PE) along the axis of a charged nanopore having nonuniform cross section is modeled, aimed with improving the performance of a nanopore-based sensing device for single biomolecules such as DNAs and proteins. Previous analysis based on a rigid particle of constant charge density in a solution containing binary ionic species is extended to a porous, pH-regulated PE in a solution containing multiple ionic species so that the conditions considered are much closer to reality. The behavior of the PE under various conditions is simulated by varying its charge, relative size, position in the nanopore, the solution pH, and the background salt concentration. We show that the PE translocation velocity can be regulated by the pore width and its charged conditions. In particular, both the magnitude and the direction of the translocation velocity can be adjusted.

Adsorption of cationic polyacrylamide at the cellulose–liquid interface: A neutron reflectometry study

The layer thickness and density of high molecular weight cationic polyacrylamide (CPAM) adsorbed at the cellulose–water interface was quantified by neutron reflectometry. The thickness of a full monolayer of CPAM of constant molecular weight (13MD) but different charge densities, adsorbed with or without NaCl (10−3M), was studied. Thin cellulose films (40±7Å) of roughness <10Å were produced by spin coating a cellulose acetate–acetone solution and regenerating by alkaline hydrolysis. Film smoothness was greatly improved by controlling the concentration of cellulose acetate (0.13wt%) and the hydrolysis time in sodium methoxide. The adsorption thickness of CPAM (40% charge 13MD) at the solid-D2O interface was 43±4Å on cellulose and 13±2Å on silicon, an order of magnitude smaller than the CPAM radius of gyration. At constant molecular weight, the thickness of the CPAM layer adsorbed on cellulose increases with polymer charge density (10±1Å at 5%). Addition of 10−3M NaCl decreased the thickness of CPAM layer already adsorbed on cellulose. However, the adsorption layer on cellulose of a CPAM solution equilibrated in 10−3M NaCl is much thicker (89±11Å for 40% CPAM). For high molecular weight CPAMs adsorbed from solution under constant conditions, the adsorption layer can be varied by 1 order of magnitude via control of the variables affecting electrostatic intra- and inter-polymer chain interactions.

Current–Voltage Characteristics of ZnO Nanowires Under Uniaxial Loading

The charge transport in zinc-oxide piezosemiconductive nanowires under purely vertical compressive or tensile strains is investigated. For simplicity, only the additional band bending originated by the piezoelectric charges has been accounted for. Moreover, a constant volumetric piezoelectric charge density is assumed, distributed within a maximum distance $\delta_{\rm piezo}$ from the two junctions between the metal ends and the nanowire. Examples demonstrate that the carrier concentration, the energy conduction band profile, and the current–voltage characteristics significantly depend on $\delta_{\rm piezo}$. Therefore, we propose the use of current–voltage measurements to obtain information on $\delta_{\rm piezo}$ in strained piezosemiconductors.

Effect of Cetane Improver on Autoignition Characteristics of Low Cetane Sasol IPK Using Ignition Quality Tester1

This paper investigates the effect of a cetane improver on the autoignition characteristics of Sasol IPK in the combustion chamber of the ignition quality tester (IQT). The fuel tested was Sasol IPK with a derived cetane number (DCN) of 31, treated with different percentages of Lubrizol 8090 cetane improver ranging from 0.1 to 0.4%. Tests were conducted under steady state conditions at a constant charging pressure of 21 bar. The charge air temperature before fuel injection varied from 778 to 848 K. Accordingly, all the tests were conducted under a constant charge density. The rate of heat release was calculated and analyzed in detail, particularly during the autoignition period. In addition, the physical and chemical delay periods were determined by comparing the results of two tests. The first was conducted with fuel injection into air according to ASTM standards where combustion occurred. In the second test, the fuel was injected into the chamber charged with nitrogen. The physical delay is defined as the period of time from start of injection (SOI) to point of inflection (POI), and the chemical delay is defined as the period of time from POI to start of combustion (SOC). Both the physical and chemical delay periods were determined under different charge temperatures. The cetane improver was found to have an effect only on the chemical ID period. In addition, the effect of the cetane improver on the apparent activation energy of the global combustion reactions was determined. The results showed a linear drop in the apparent activation energy with the increase in the percentage of the cetane improver. Moreover, the low temperature (LT) regimes were investigated and found to be presented in base fuel, as well as cetane improver treated fuels.

Effect of charge regulation on the stability of electrolyte films.

The stability of a thin liquid film of an electrolyte on a solid substrate is investigated. In the framework of the Debye-Hückel approximation, we show that the commonly used approximation of fixed potential at the solid-liquid interface does not lead to predictions of film rupture. To reconcile the model with experimental observations, we consider the constant charge density approximation for the solid substrate and then proceed to systematically investigate the effects of charge regulation based on a linear relationship between charge density and potential. Stability criteria are formulated in terms of charge regulation parameters and electrolyte properties, resulting in different types of stability diagrams. Critical thickness below which the film ruptures is shown to decrease as the charge regulation at the solid-liquid interface becomes stronger.

Effect of post-deposition annealing on composition and electrical properties of dc reactive magnetron sputtered Al2O3 thin films

We have investigated the effect of post-deposition annealing on the composition and electrical properties of alumina (Al2O3) thin films. Al2O3 were deposited on n-type Si <100> substrates by dc reactive magnetron sputtering. The films were subjected to post-deposition annealing at 623, 823 and 1023 K in vacuum. X-ray photoelectron spectroscopy results revealed that the composition improved with post-deposition annealing, and the film annealed at 1023 K became stoichiometric with an O/Al atomic ratio of 1·49. Al/Al2O3/Si metal–oxide–semiconductor (MOS) structures were then fabricated, and a correlation between the dielectric constant ϵr and interface charge density Qi with annealing conditions were studied. The dielectric constant of the Al2O3 thin films increased to 9·8 with post-deposition annealing matching the bulk value, whereas the oxide charge density decreased to 3·11×1011 cm−2. Studies on current–voltage IV characteristics indicated ohmic and Schottky type of conduction at lower electric fields (<0·16 MV cm−1) and space charge limited conduction at higher electric fields.

Electrokinetic flow in a pH-regulated, cylindrical nanochannel containing multiple ionic species

Considering the wide applications of the electrokinetic flow regulated by pH, we model the flow of an electrolyte solution containing multiple ionic species in a charge-regulated cylindrical nanochannel. This extends previous analyses, where only two kinds of ionic species are usually considered, and a charged surface assumed to maintain at either constant potential or constant charge density, to a case closer to reality. Adopting a fused silica channel containing an aqueous NaCl background salt solution with its pH adjusted by HCl and NaOH as an example, we show that if the density of the functional groups on the channel surface increases (decreases), it approaches a constant potential (charge density) surface; if that density is low, the channel behavior is similar to that of a constant charge density channel at high salt concentration and large channel radius. Several interesting results are observed, for example, the volumetric flow rate of a small channel has a local maximum as salt concentration varies, which is not seen in a constant potential or charge density channel.