Effective Net Charge Research Articles

DFT-based investigation of electronic-structure, magnetic and thermoelectric properties of Dy2CoMnO6 double perovskite

The density functional theory as a computational approach was used to explore the ferrimagnetic semiconducting behavior of Dy2CoMnO6 double perovskite compound in both spin-up and spin-down directions. Thermodynamic stability was confirmed in a wide pressure range set to 30 GPa. Both scalar and full relativistic calculations are used to estimate the spin–orbit effect. The topological distribution of the charge density and the net effective charge of each atom are studied based on the Quantum Theory of Atoms in Molecules (QTAIM) as implemented in Bader code, Besides this, the different QTAIM atomic basins descriptors such as electron density Laplacian of the electron density potential electronic energy density kinetic electronic energy density and density of the total electronic energy at bond critical points (BCPs) are estimated using CRITIC2 software where the ionic type for Dy-O, Co-O, and Mn-O bonds are evaluated. Holes and electrons have different effective masses, their thermoelectric properties appear high figure of merit (ZT) exceeding 0.5 for temperatures greater than 500 K in the negative chemical potential region, suggesting that doping with holes might be more favorable for thermoelectric efficiency than doping with electrons.

Effective charge of PNIPAM microgels determined by conductivity measurements

AbstractUnderstanding the effective charge of colloidal particles is crucial to control the stability and performance of a colloidal system. In particular, the electrostatic interaction potential between the particles determines the stability and thermodynamic properties of electrostatically charged colloidal dispersions. In the case of thermosensitive microgels, which are solvent and ions permeable particles, it is expected that the microgel's size and the number of ions within it are affected by temperature changes. This temperature‐dependent microgel‐solvent ion interchange regulates the microgel net charge and, consequently, the interaction potential. Here, a straightforward experimental method based on conductivity measurements is presented to determine the temperature dependence of the effective net charge of poly‐N‐isopropylacrylamide, PNIPAM, microgels under different salinity conditions. Our results show that the net charge of the microgel decreases with increasing temperature. Specifically, microgels reduce their net charge by around 40%, when the temperature is changed from 25 to 40 °C. This scenario could be explained by the entrance of counterions into the microgel after its collapse to partially neutralize the increase of the electrostatic repulsion due to the closer proximity among the charged groups present in the polymer particle.

Study of the Seismoelectric Effect in Saturated Porous Media Using a Bundle of Capillary Tubes Model

The seismoelectric effect is the fundamental basis for seismoelectric logging. Most of the existing theories for the seismoelectric effect are based on the Pride theory, which adopts the assumption of a thin electric double layer and uses the volume-averaging method to derive the seismoelectric coupling equations; hence, the obtained electrokinetic coupling coefficient is not applicable to large-Debye-length cases. In addition, the Pride theory neglects the change in seepage velocity with the radial position of the pore when calculating the streaming current, which leads to an inaccurate reflection of the influence of pore size on the electrokinetic coupling coefficient. In this study, we proposed a flux-averaging method to solve the effective net residual charge density of porous media and further derived the electrokinetic coupling coefficient expressed by the effective net residual charge density. We also investigated the effect of formation parameters and compared the results with those calculated using the Pride theory. Since the proposed method is not limited by the thin electric double layer assumption, it is suitable for both small- and large-Debye-length cases. Moreover, we also carried out flume experiments to investigate the influence of salinity, where both thin and thick electric double layer cases were studied. The comparison between the results of the experiment and simulation verified the correctness of the proposed method. Furthermore, the proposed method took into account the variation in seepage velocity with pore location when solving for the streaming current; therefore, the influence of the pore size on the electrokinetic coefficient can be described more accurately.

Catalytic Performance of Cobalt(II) Polyethylene Catalysts with Sterically Hindered Dibenzopyranyl Substituents Studied by Experimental and MLR Methods.

Given the great importance of cobalt catalysts supported by benchmark bis(imino)pyridine in the (oligo)polymerization, a series of dibenzopyran-incorporated symmetrical 2,6-bis(imino) pyridyl cobalt complexes (Co1–Co5) are designed and prepared using a one-pot template approach. The structures of the resulting complexes are well characterized by a number of techniques. After activation with either methylaluminoxane (MAO) or modified MAO (MMAO), the complexes Co1–Co4 are highly active for ethylene polymerization with a maximum activity of up to 7.36 × 106 g (PE) mol−1 (Co) h−1 and produced highly linear polyethylene with narrow molecular weight distributions, while Co5 is completely inactive under the standard conditions. Particularly, complex Co3 affords polyethylene with high molecular weights of 85.02 and 79.85 kg mol−1 in the presence of MAO and MMAO, respectively. The 1H and 13C NMR spectroscopy revealed the existence of vinyl end groups in the resulting polyethylene, highlighting the predominant involvement of the β-H elimination reaction in the chain-termination process. To investigate the mechanism underlying the variation of catalytic activities as a function of substituents, multiple linear regression (MLR) analysis was performed, showing the key role of open cone angle (θ) and effective net charge (Q) on catalytic activity.

Interactions between conducting surfaces in salt solutions.

In this work, we simulate interactions between two perfectly conducting surfaces, immersed in a salt solution. We demonstrate that these forces are quantitatively different from those between (equally charged) non-conducting surfaces. There is, for instance, a significant repulsion between net neutral surfaces. On the other hand, there are also qualitative similarities, with behaviours found with non-conducting surfaces. For instance, there is a non-monotonic dependence of the free energy barrier height, on the salt concentration, and the minimum essentially coincides with a flat profile of the apparent surface charge density (i.e. the effective net surface charge density, some distance away from the surface, when accounting for ion neutralization), outside the so-called Stern layer. These conditions can be described as "perfect surface charge neutralization". Despite observed quantitative differences, we demonstrate that it might be possible to mimic a dispersion containing charged colloidal metal particles by a simpler model system with charged non-conducting particles, using modified particle-ion interactions.

The Quantitative Influence of Coordinated Halogen Atoms on the Catalytic Performance of Bisiminoacenaphthylnickel Complexes in Ethylene Polymerization.

In experiments, nickel bromine complexes usually show a better catalytic performance in ethylene polymerization compared to their nickel chlorine analogues. Therefore, the present modeling study has been performed to investigate the effect of coordinated halogen atoms on the catalytic performances of two bisiminoacenaphthyl nickel systems, namely, Ni-Br and Ni-Cl. By using the multiple linear regression analysis (MLRA), the catalytic activity can be well predicted by the descriptors of effective net charge (Qeff ) and bite angle (β), with correlation coefficient R2 values over 0.91. Meanwhile, the molecular weights of polyethylene are predicted by the descriptors of Qeff and open cone angle (θ). The calculated contributions of each descriptor show that the electronic effect is the predominant factor in Ni-Br system, while the steric effect becomes the dominant factor in Ni-Cl system. The different determined effect is expected to the main reason for the different catalytic performance between two Ni systems.

Separation and zeta-potential determination of proteins and their oligomers using electrical asymmetrical flow field-flow fractionation (EAF4)

Electrical asymmetrical flow field-flow fractionation (EAF4) is an interesting new analytical technique that separates proteins based on size or molecular weight and simultaneously determines the electrical characteristics of each population. However, until now, the research using EAF4 has not been published except for the proof-of-concept in the original publication by Johann et. al. in 2015 [1]. Hence the methods capabilities and optimized conditions need to be further investigated, such as composition of the carrier liquid, pH stability and effect of the electric field strength.The pH instability was observed in the initial method of EAF4 due to the electrolysis products when applied electric field. Therefore, we have investigated and provided a modified method for rapid pH stabilization through additional focusing step with the electric field. Then, the electrical properties such as the zeta-potential and effective net charge of the monomer and oligomers of three different proteins (GA-Z, BSA, and Ferritin) were determined based on their electrophoretic mobility from EAF4. The results showed that there were limitations to the applicability of separation by EAF4 to proteins. Nevertheless, this study shows that EAF4 is an interesting new technique that can examine the zeta-potential of individual proteins in mixtures (or monomers and oligomers) not accessible by other techniques.

The Catalytic Activities of Carbocyclic Fused Pyridineimine Nickel Complexes Analogues in Ethylene Polymerization by Modeling Study

In this work, two carbocyclic fused pyridineimine nickel analogue systems (Ni1 and Ni2) with different fused member rings were investigated to reveal the relationship between catalyst structure and reaction activity. Multiple linear regression analysis was performed by means of five electronic descriptors and two steric descriptors, including the Hammett constant (F), effective net charge (Qeff), energy difference (ΔE), HOMO–LUMO energy gap (Δε1, Δε2), open cone angle (θ), and bite angle (β). Very good values of correlation coefficient (R2) over 0.938 were obtained by using a combination of effective net charge (Qeff) and open cone angle (θ) for both individual analysis and comparisons between analogue systems. By analyzing the contribution of descriptors, it indicates that the dominant descriptor is effective net charge (Qeff) in the Ni1 system and open cone angle (θ) in Ni2 systems, respectively. This may explain the different variation trends of catalytic activities in two Ni complexes systems as a function of substituents.

Geometric and electronic structures of 5,10,15,20-tetraphenylporphyrinato Palladium(II) and Zinc(II): Phenomenon of Pd(II) complex

The structure of PdTPP and ZnTPP molecules have been studied at first time by a combined gas-phase electron diffraction and mass spectrometry (GED/MS) complemented by quantum-chemical calculations. Four conformers have been found in which four phenyl fragments are turned in different directions from their perpendicular position relative to the porphyrin plane. The structures with turned orientation of phenyl fragments demonstrate an energy balance between orbital conjugation of phenyls with porphyrin framework and steric repulsion of the phenyl and pyrrole fragments. Comparison of PdTPP and unsubstituted PdP as well as ZnTPP and ZnP complexes within the framework of NBO analysis shows that characteristics of the MN coordination bond do not practically change when meso-substituents are introduced. In contrast, the Cα-Cm bond lengths at the meso-substituent, the Cα-Cm-Cα bond angle, and the energy of the frontier orbitals are changed significantly. The change in these parameters indicates the electron-donor nature of the phenyl fragments. Strong donor-acceptor interactions between LP(N) and both 4dx2-y2 and 5s AO (Pd) causes a significant transfer of the electron density from the donor atom N on Pd resulting the extremely low effective net charge on the palladium atom and determining the high donor and acceptor properties of PdP and PdTPP complexes.

Catalytic Activities of Bis(pentamethylene)pyridyl(Fe/Co) Complex Analogues in Ethylene Polymerization by Modeling Method.

The catalytic activities of α,α'-bisimino-2,3:5,6-bis(pentamethylene)pyridyl(Fe/Co) chloride analogue complexes are quantitatively investigated by the multiple linear regression analysis (MLRA) method. From the point of view of the electronic and steric effects, seven structural descriptors are selected and calculated, including the Hammett constant ( F), effective net charge ( Qeff), energy difference (Δ E), HOMO-LUMO energy gap (Δε1, Δε2), open cone angle (θ), and bite angle (β). In order to get better model, the fitting analyses are carried out by using the combinations of four, three, two, and single descriptors. The calculation results show quite good correlation results. By using two descriptors ( Qeff, β), the catalytic activities for both the Fe and Co complexes individually and also the variation between Fe and Co (Fe-Co) analogue system can be well predicted with correlation coefficient values over 0.934. It is found that the effective net charge ( Qeff) plays the dominant role in determining the catalytic activities for Fe and Co complexes. Furthermore, the lower values of catalytic activities in Co complexes are mainly attributed to the decreasing values of Qeff.

Controlling surface charge and spin density oscillations by Dirac plasmon interaction in thin topological insulators

We study the selective excitation at infrared and THz frequencies of optical and acoustic plasmonic modes supported by thin topological insulators. These modes are characterized by effective net charge or net spin density, respectively, and we study their excitation by combining many-body and electromagnetic calculations. We first show that non-locality can significantly modify the plasmonic response: it changes the energy of propagating plasmons up to tens of percent. We then discuss how, by changing the distance between a dipolar source and a semi-infinite 10 nm thin film, it is possible to control the excitation of acoustic and optical propagating plasmons, which can propagate over a distance of several plasmonic wavelengths. Furthermore, we consider 10 nm thin TI nanodisks and study the excitation of acoustic and optical localized plasmon modes by a point dipole source and plane wave illumination, respectively. The resonant plasmonic modes appear at frequencies that strongly depends on the size of the disk, and that can be potentially tuned by applying electrostatic gating to modify the Fermi Energy of the conductive 2-dimensional layer that forms at the interfaces of the TI. We observe a spectral shift from ~29 $\mu$m to ~34 $\mu$m by changing the Fermi Energy from 250meV to 350meV. Last, the electromagnetic energy of these plasmonics modes can be confined to very small regions, of effective volume ~(120 nm)^3 for the smaller disk considered, much less than the free-space wavelength cubed $\lambda$^3 ~(35000 nm)^3. The strong confinement is desirable for achieving very efficient coupling with nearby systems. Our detailed study thus shows that thin topological insulators are a promising system to control both the spin and charge oscillations associated with the plasmonic resonances, with possible applications to fast, compact and electrically-controlled spintronics devices.

The Effect of Cation Size on DNA Thermal Stability

Capillary electrophoresis has been used to examine the effect of cation size on DNA thermal stability, using hairpins with 6 base pair-stems and 4 nucleotide-loops as the reporter system. The midpoint melting temperatures observed in solutions containing the tetrabutylammonium ion (TBA+) are consistently 16°C lower than observed in solutions containing the same concentration of Na+, even though no preferential interaction with either cation is observed. The thermal melting temperatures decrease progressively with increasing cation size, due primarily to the effect of cation size on solution viscosity. We postulate that the viscosity of the solution affects the hairpin ↔ coil equilibrium because of through-solvent effects on hairpin renaturation, as observed in other studies. The mobilities of the various hairpins are independent of sequence in Na+ but depend on sequence in TBA+ solutions. In particular, the observed mobilities increase with the increasing number of thymine bases in the loop, possibly because of the partial ionization of thymine in TBA+ which would increase the effective net charge of the hairpin.

Force spectroscopy unravels the role of ionic strength on DNA-cisplatin interaction: Modulating the binding parameters

In the present work we have gone a step forward in the understanding of the DNA-cisplatin interaction, investigating the role of the ionic strength on the complexes formation. To achieve this task, we use optical tweezers to perform force spectroscopy on the DNA-cisplatin complexes, determining their mechanical parameters as a function of the drug concentration in the sample for three different buffers. From such measurements, we determine the binding parameters and study their behavior as a function of the ionic strength. The equilibrium binding constant decreases with the counterion concentration ([Na]) and can be used to estimate the effective net charge of cisplatin in solution. The cooperativity degree of the binding reaction, on the other hand, increases with the ionic strength, as a result of the different conformational changes induced by the drug on the double-helix when binding under different buffer conditions. Such results can be used to modulate the drug binding to DNA, by appropriately setting the ionic strength of the surrounding buffer. The conclusions drawn provide significant new insights on the complex cooperative interactions between the DNA molecule and the class of platinum-based compounds, much used in chemotherapies.

Distinct Effects of Multivalent Macroion and Simple Ion on the Structure and Local Electric Environment of a Weak Polyelectrolyte in Aqueous Solution.

Adding ionic species can critically affect the structure of weak polyelectrolyte (PE) chains, whose charge density in aqueous solution can be greatly regulated by bathing solution conditions such as pH and added ions. Distinct from simple ions that can be treated as point charges, multivalent macroions of finite size, including many charged nanoparticles and biopolymers, could show strong electrostatic coupling with PEs and effectively modify the conformation and assembly of PEs in aqueous solution. In this work, we have compared the effects of hydrophilic multivalent macroion of finite size and simple divalent ion on the conformational transition of a model weak polybase, poly(2-vinylpyridine) (P2VP), in dilute aqueous solution. By using fluorescence correlation spectroscopy combined with photon counting histogram analysis, we have examined the swollen-to-collapsed conformational transition and local electric potential of a P2VP chain in ionic aqueous solution at a single-molecule level. Adding inorganic polytungstate ([W12]) macroion bearing eight negative charges per [W12] of ∼0.8 nm in diameter at increased concentration from 10-9 to 10-5 mM can lead to a shift of the critical conformational transition pH, pHcr, of P2VP to lower pH values, in an opposite trend to the previously reported effect of adding simple monovalent anion. Conversely, adding simple divalent sulfate anion can lead to a nonmonotonic change of pHcr when increasing its concentration from 0.03 to 15 mM. Additionally, at pH < pHcr where P2VP is highly protonated and adopts a swollen conformation, a monotonic decrease of P2VP size is observed with increased sulfate ionic concentration, exhibiting the typical ionic screening effect. In contrast, the size of the P2VP chain shows little change with increasing [W12] concentration before the precipitation of P2VP from water. To investigate the distinct effects of multivalent ion and macroion on the conformational transition of P2VP in aqueous solution, we have also measured the local proton concentration in the vicinity to a P2VP chain by an attached pH-sensitive fluorescence probe. In both cases, we have observed the monotonic reduction of the local electric potential of a swollen P2VP chain with increased ionic concentration, despite the increased protonation degree of P2VP. The results suggest that counterion condensation of multivalent ion and macroion can modify the effective net charge density of P2VP chains in dilute aqueous solution. However, possibly due to its high multivalency and finite size, multivalent [W12] macroion is much more effective in modifying the local electric environment and structure of P2VP chains at 3-7 orders of magnitude lower concentrations than simple sulfate counterion.

Investigation of the static electric field effect of strontium silicate layers on silicon substrates

We studied the effective net charge density (Qeff) of strontium silicate (SrxSiOx+2, x = 1, 2, 3) films grown on silicon (Si) (100) substrates. The SrxSiOx+2 layers were deposited from a Sr2SiO4 polycrystalline target by pulsed laser deposition, and then annealed at 400–600 °C in an oxygen atmosphere with a tube furnace. The Qeff values of the SrxSiOx+2/Si (100) samples were obtained from the shift in the voltage of the flat band state in their capacitance–voltage curves. The SrxSiOx+2/Si (100) samples with a thickness of 15 nm annealed at 400 °C showed the maximum Qeff/q value of 1.03 × 1013 cm−2, where q is the elementary charge. With increasing annealing temperature, the SrxSiOx+2 layer penetrated into the Si (100) substrate. This penetration may degrade the interfacial properties and decrease the Qeff value of the layers. For all the samples, the charges concentrated near the SrxSiOx+2/Si (100) interface. Our results suggest that anion and cation migration plays an important role in charge generation at the SrxSiOx+2/Si (100) interface, consistent with the findings of a previously reported molecular dynamics calculation.

Quantitative Investigation of the Electronic and Steric Influences on Ethylene Oligo/Polymerization by 2‐Azacyclyl‐6‐aryliminopyridylmetal (Fe, Co, and Cr) Complexes

The multiple linear regression analysis is performed to investigate both electronic and steric effects of ligands on the catalytic activities of the transition metal complexes in ethylene oligo/polymerization. The Hammett constants (F) and effective net charge (Q) are used to describe the electronic effect, meanwhile the steric effect is illustrated by open cone angle (θ) and bite angle (β). Four series of 2‐azacyclyl‐6‐aryliminopyridylmetal (Fe, Co, and Cr) analog complexes are calculated and the catalytic activity can be efficiently analyzed through taking both the electronic and steric effects into account. All of the obtained correlation coefficient R 2 values are over 0.93. The electronic effect is dominant in determining the catalytic activities for Fe and Co complexes; however, the steric effect plays a major role in controlling activities of Cr complexes. image

Nature of the Intermicellar Interactions in Ethoxylated Polysorbate Surfactants with High Degrees of Ethoxylation.

Ethoxylated polysorbate Tween nonionic surfactants are extensively used as foam and emulsion stabilizers and in aqueous solution form globular micelles. The ethoxylated polysorbate surfactants with higher degrees of ethoxylation than the Tween surfactants exhibit some interesting self-assembly properties. Small-angle neutron scattering, SANS, measurements have revealed intermicellar interactions which are more pronounced than the hard-sphere excluded volume interactions normally associated with nonionic surfactant micelles. The interactions are interpreted as arising from the partial charge on the ether oxygen of the ethylene oxide groups. This gives rise to an effective net negative charge on the micelles, which has been determined from the SANS data and zeta potential measurements. For degrees of ethoxylation of ⩽20, the effect is relatively small. The interaction increases with increasing ethoxylation such that for a degree of ethoxylation of 50 the interaction is comparable to that of ionic surfactant micelles. Unlike the intermicellar interaction in ionic surfactant micellar solutions, which results from the charge on the micelle arising from the partial binding of counterions, the interaction between ethoxthylated polysorbate surfactant micelles is unaffected by the addition of electrolyte.

Role of Steric Interactions on the Ionic Permeation Inside Charged Microgels: Theory and Simulations

In this work, we study the effect of the steric excluded-volume interactions between counterions and thermoresponsive ionic heterogeneous microgel particles. With this aim, we perform Monte Carlo simulations to calculate the microgel effective net charge and the conunterion distribution function inside and around the microgel for different degrees of swelling. These results are compared to the ones obtained solving the Ornstein–Zernike integral equations within the HNC approximation. For this purpose, the equilibrium polymer mass and charge distribution inside the microgel resulting from simulations are used as the input for the integral equations. Two different models are considered to quantify the microgel-ion steric interaction. The model that considers polymer fibers formed by spheres demonstrates to be a very reliable way to predict counterion permeation in such microgels. Finally, integral equations are solved ignoring the steric interaction as well, in order to determine to what extent this effect ...

Molecular Mechanisms of ZnO Nanoparticle Dispersion in Solution: Modeling of Surfactant Association, Electrostatic Shielding and Counter Ion Dynamics.

Molecular models of 5 nm sized ZnO/Zn(OH)2 core-shell nanoparticles in ethanolic solution were derived as scale-up models (based on an earlier model created from ion-by-ion aggregation and self-organization) and subjected to mechanistic analyses of surface stabilization by block-copolymers. The latter comprise a poly-methacrylate chain accounting for strong surfactant association to the nanoparticle by hydrogen bonding and salt-bridges. While dangling poly-ethylene oxide chains provide only a limited degree of sterical hindering to nanoparticle agglomeration, the key mechanism of surface stabilization is electrostatic shielding arising from the acrylates and a halo of Na+ counter ions associated to the nanoparticle. Molecular dynamics simulations reveal different solvent shells and distance-dependent mobility of ions and solvent molecules. From this, we provide a molecular rationale of effective particle size, net charge and polarizability of the nanoparticles in solution.

Flanking A·T Basepairs Destabilize the B∗ Conformation of DNA A-Tracts

Capillary electrophoresis has been used to characterize the interaction of monovalent cations with 26-basepair DNA oligomers containing A-tracts embedded in flanking sequences with different basepair compositions. A 26-basepair random-sequence oligomer was used as the reference; lithium and tetrabutylammonium (TBA+) ions were used as the probe ions. The free solution mobilities of the A-tract and random-sequence oligomers were identical in solutions containing <∼100 mM cation. At higher cation concentrations, the A-tract oligomers migrated faster than the reference oligomer in TBA+ and slower than the reference in Li+. Hence, cations of different sizes can interact very differently with DNA A-tracts. The increased mobilities observed in TBA+ suggest that the large hydrophobic TBA+ ions are preferentially excluded from the vicinity of the A-tract minor groove, increasing the effective net charge of the A-tract oligomers and increasing the mobility. By contrast, Li+ ions decrease the mobility of A-tract oligomers because of the preferential localization of Li+ ions in the narrow A-tract minor groove. Embedding the A-tracts in AT-rich flanking sequences markedly alters preferential interactions of monovalent cations with the B∗ conformation. Hence, A-tracts embedded in genomic DNA may or may not interact preferentially with monovalent cations, depending on the relative number of A·T basepairs in the flanking sequences.