Adsorption of NO on the Rh-13, Pd-13, Ir-13, and Pt-13 Clusters: A Density Functional Theory Investigation

The adsorption of NO on transition-metal (TM) surfaces has been widely studied by experimental and theoretical techniques; however, our atomistic understanding of the interaction of nitrogen monoxide (NO) with small TM clusters is far from satisfactory, which compromises a deep understanding of real catalyst devices. In this study, we report a density functional theory study of the adsorption properties of NO on the TM13 (TM = Rh, Pd, Ir, Pt) clusters employing the projected augmented wave method. We found that the interaction of NO with TM13 is much more complex than that for NO/TM(111). In particular, for low symmetry TM13 clusters, there is a strong rearrangement of the electronic charge density upon NO adsorption and, as a consequence, the adsorption energy shows a very complex dependence even for adsorption sites with the same local effective coordination. We found a strong enhancement of the binding energy of NO to the TM13 clusters compared with the TM(111) surfaces, as the antibonding NO states are not occupied for NO/TM13, and the general relationship based on the d-band model between adsorption energy and the center of gravity of the occupied d-states does not hold for the studied TM13 clusters, in particular, for clusters with low symmetry. In contrast with the adsorption energy trends, the geometric NO/TM13 parameters and the vibrational N-O frequencies for different coordination sites follow the same trend as for the respective TM(111) surfaces, while the changes in the frequencies between different surfaces and TM13 clusters reflect the strong NO-TM13 interaction.

Circularly Polarized Photoluminescence as a Probe of Density of States in GaAs/AlGaAs Quantum Hall Bilayers

Polarized magnetophotoluminescence is employed to study the energies and occupancies of four lowest Landau levels in a couple quantum Hall GaAs/AlGaAs double quantum well. As a result, a magnetic field-induced redistribution of charge over the Landau levels manifesting to the continuous formation of the charge density wave and direct evidence for the symmetric-antisymmetric gap shrinkage at v = 3 are found. The observed interlayer charge exchange causes depolarization of the ferromagnetic ground state.

Complexes of polyelectrolytes with defined charge distance and different dendrimer counterions

Complexes of polyelectrolytes with defined charge distance and different dendrimer counterions Magdalena Chelmecka Max Planck Institute for Polymer Research; Ackermannweg 10; D-55128 Mainz ; Tel.: (+49) 06131- 379 – 226 A study of complexes in solution is of interest to investigate whether the formation of well-defined assemblies like in classical surfactant systems is possible. Aim of this thesis is to investigate the electrostatic self-assembly of linear polycations of varying charge distance with “large” counterions of varying architecture. We especially investigate the morphology of objects formed, but also their stability under salt free condition and after low molecular mass salt addition. As polycations, Poly(dialkylimino)-alkylene salts (Ionenes) I65MeBr and I25MeBr were chosen. Ionenes are synthesized via Menschutkin reaction and characterized by standard methods. Counterions are Polyamidoamine (PAMAM) dendrimers of generations G2.5, G5.5, G7.5 with -COONa surface groups and shape-persistent, Polyphenylene dendrimers of generation G1 with surface -COOH groups. A complex interplay of interactions is expected to direct the self assembly via electrostatic interaction, geometric factors, hydrophobic interaction or hydrogen bonds. Methods used for the investigation of complexes are: UV-spectroscopy, pH-metric techniques, dynamic and static light scattering, small angle neutron scattering,  potential measurements and potentiometric titration. Under certain conditions, (i.e. charge ratio of compounds, charge density of ionene and dendrimer also concentration of sample) polyelectrolyte systems composed of ionenes and dendrimers build complexes in solution. System compounds are typical polyelectrolytes, but structures which they build behave not usual for typical polyelectrolytes. In a one diffusion mode regime aggregates of about 100 nm hydrodynamic radius have been found. Such aggregates are core-shell or anisotropic core shell structures in the case of ionenes/PAMAM dendrimers complexes. These complexes are stable even at high ionic strength. In case of ionenes with poly(phenylene) dendrimers, hard sphere-like objects or spherical objects with hairy-like surface have been found in a one diffusion mode regime. Their stability at high ionic strength is lower. For the ionenes/poly(phenylene) dendrimers systems one transition point has been found from one to two diffusion processes, towards increasing ionene concentration, i.e. for the samples with fixed dendrimer concentration towards increasing ionic strength. For the diffusion profile of ionene/PAMAM dendrimers in most cases two transition regimes are observed. One at very low ionene concentration, the second one at high ionene concentrations, which again means for the samples with fixed dendrimer concentration, also at higher ionic strength. Both two mode regimes are separated by the one mode regime. As was confirmed experimentally, the one diffusion mode regime is caused by the motion of well defined assemblies. The two diffusion mode regimes are caused by the movement of different sized species in solution, large aggregates and middle-size aggregates (oligoaggregates). The location and also the number of transition points in the diffusion profiles is dependent on the ionene to dendrimer charge ratio, charge density of the compounds and concentration. No influence of the molecular mass of the ionene has been found. The aggregates are found to be charged on the surface, however this surface charge does not significantly influence the diffusion properties of the system.

Determination of effective charge of flexible polyelectrolytes by light scattering

The most important property controlling the physicochemical behaviour of polyelectrolytes and their applicability in different fields is the charge density on the macromolecular chain. A polyelectrolyte molecule in solution may have an effective charge density which is smaller than the actual charge density determined from its chemical structure. In the present work an attempt has been made to quantitatively determine this effective charge density of a model polyelectrolyte by using light scattering techniques. Flexible linear polyelectrolytes with a Poly(2-Vinylpyridine) (2-PVP) backbone are used in the present study. The polyelectrolytes are synthesized by quaternizing the pyridine groups of 2-PVP by ethyl bromide to different quaternization degrees. The effect of the molar mass, degree of quaternization and solvent polarity on the effective charge is studied. The results show that the effective charge does not vary much with the polymer molar mass or the degree of quaternization. But a significant increase in the effective charge is observed when the solvent polarity is increased. The results do not obey the counterion condensation theory proposed by Manning. Based on the very low effective charges determined in this study, a new mechanism for the counterion condensation phenomena from a specific polyelectrolyte-counterion interaction is proposed

From the chiral magnetic wave to the charge dependence of elliptic flow

The quark-gluon plasma formed in heavy ion collisions contains charged chiral fermions evolving in an external magnetic field. At finite density of electric charge or baryon number (resulting either from nuclear stopping or from fluctuations), the triangle anomaly induces in the plasma the Chiral Magnetic Wave (CMW). The CMW first induces a separation of the right and left chiral charges along the magnetic field; the resulting dipolar axial charge density in turn induces the oppositely directed vector charge currents leading to an electric quadrupole moment of the quark-gluon plasma. Boosted by the strong collective flow, the electric quadrupole moment translates into the charge dependence of the elliptic flow coefficients, so that $v_2(\pi^+) < v_2(\pi^-)$ (at positive net charge). Using the latest quantitative simulations of the produced magnetic field and solving the CMW equation, we make further quantitative estimates of the produced $v_2$ splitting and its centrality dependence. We compare the results with the available experimental data.

Experimental and Theoretical Electron Density Analysis of Copper Pyrazine Nitrate Quasi-Low-Dimensional Quantum Magnets

The accurate electron density distribution and magnetic properties of two metal-organic polymeric magnets, the quasi-one-dimensional (1D) Cu(pyz)(NO3)2 and the quasi-two-dimensional (2D) [Cu(pyz)2(NO3)]NO3·H2O, have been investigated by high-resolution single-crystal X-ray diffraction and density functional theory calculations on the whole periodic systems and on selected fragments. Topological analyses, based on quantum theory of atoms in molecules, enabled the characterization of possible magnetic exchange pathways and the establishment of relationships between the electron (charge and spin) densities and the exchange-coupling constants. In both compounds, the experimentally observed antiferromagnetic coupling can be quantitatively explained by the Cu-Cu superexchange pathway mediated by the pyrazine bridging ligands, via a σ-type interaction. From topological analyses of experimental charge-density data, we show for the first time that the pyrazine tilt angle does not play a role in determining the strength of the magnetic interaction. Taken in combination with molecular orbital analysis and spin density calculations, we find a synergistic relationship between spin delocalization and spin polarization mechanisms and that both determine the bulk magnetic behavior of these Cu(II)-pyz coordination polymers.

The change in hydrogen bond strength accompanying charge rearrangement: Implications for enzymatic catalysis

The equilibrium for formation of the intramolecular hydrogen bond (KHB) in a series of substituted salicylate monoanions was investigated as a function of ΔpKa, the difference between the pKa values of the hydrogen bond donor and acceptor, in both water and dimethyl sulfoxide. The dependence of log KHB upon ΔpKa is linear in both solvents, but is steeper in dimethyl sulfoxide (slope = 0.73) than in water (slope = 0.05). Thus, hydrogen bond strength can undergo substantially larger increases in nonaqueous media than aqueous solutions as the charge density on the donor or acceptor atom increases. These results support a general mechanism for enzymatic catalysis, in which hydrogen bonding to a substrate is strengthened as charge rearranges in going from the ground state to the transition state; the strengthening of the hydrogen bond would be greater in a nonaqueous enzymatic active site than in water, thus providing a rate enhancement for an enzymatic reaction relative to the solution reaction. We suggest that binding energy of an enzyme is used to fix the substrate in the low-dielectric active site, where the strengthening of the hydrogen bond in the course of a reaction is increased.

The effect of ionic strength variation and anion competition on the development of nitrate accumulations in variable charge subsoils

The leaching of N fertilisers has led to the formation of nitrate (NO3) accumulations in deep subsoils (>5 m depth) of the Johnstone River catchment. This paper outlines the chemical mechanism by which these NO3 accumulations are formed and maintained. This was achieved via a series of column experiments designed to investigate NO3 leaching in relation to the soil charge chemistry and the competition of anions for exchange sites. The presence of variable charge minerals has led to the formation positive surface charge within these profiles. An increase in the soil solution ionic strength accompanying the fertiliser leaching front acts to increase the positive (and negative) charge density, thus providing adsorption sites for NO3. A decrease in the soil solution ionic strength occurs after the fertiliser pulse moves past a point in the profile, due to dilution with incoming rainwater. Nitrate is then released from the exchange back into the soil solution, thus buffering the decrease in the soil solution ionic strength. Since NO3 was adsorbed throughout the profile in this experiment it does not effectively explain the situation occurring in the field. Previous observations of the sulfate (SO4) profile distribution indicated that large SO4 accumulations in the upper profile may influence the NO3 distribution through competition for adsorption sites. A subsequent experiment investigating the effect of SO4 additions on NO3 leaching showed that NO3 adsorption was minimal in the upper profile. Adsorption of NO3 did occur, though only in the region of the profile where SO4 occupancy was low, i.e. in the lower profile. Therefore, the formation of the NO3 accumulations is dependent on the variable charge mineralogy, the variation of charge density with soil solution ionic strength, and the effects of SO4 competition for adsorption sites.

Effects of electric charge on osmotic flow across periodically arranged circular cylinders

An electrostatic model is developed for osmotic flow across a layer consisting of identical circular cylinders with a fixed surface charge, aligned parallel to each other so as to form an ordered hexagonal arrangement. The expression of the osmotic reflection coefficient is derived for spherical solutes with a fixed surface charge suspended in an electrolyte, based on low-Reynolds-number hydrodynamics and a continuum, point-charge description of the electric double layers. The repulsive electrostatic interaction between the surface charges with the same sign on the solute and the cylinders is shown to increase the exclusion region of solute from the cylinder surface, which enhances the osmotic flow. Applying the present model to the study of osmotic flow across the endothelial surface glycocalyx of capillary walls has revealed that this electrostatic model could account well for the reflection coefficients measured for charged macromolecules, such as albumin, in the physiological range of charge density and ion concentration.

Charge effects on the hindered transport of macromolecules across the endothelial surface glycocalyx layer

A fluid mechanical and electrostatic model for the transport of solute molecules across the vascular endothelial surface glycocalyx layer (EGL) was developed to study the charge effect on the diffusive and convective transport of the solutes. The solute was assumed to be a spherical particle with a constant surface charge density, and the EGL was represented as an array of periodically arranged circular cylinders of like charge, with a constant surface charge density. By combining the fluid mechanical analyses for the flow around a solute suspended in an electrolyte solution and the electrostatic analyses for the free energy of the interaction between the solute and cylinders based on a mean field theory, we estimated the transport coefficients of the solute across the EGL. Both of diffusive and convective transports are reduced compared to those for an uncharged system, due to the stronger exclusion of the solute that results from the repulsive electrostatic interaction. The model prediction for the reflection coefficient for serum albumin agreed well with experimental observations if the charge density in the EGL is ranged from approximately -10 to -30 mEq/l.

Conceptualisation of articular cartilage as a giant reverse micelle: A hypothetical mechanism for joint biocushioning and lubrication

Phospholipid (PL) molecules form the main structure of the membrane that prevents the direct contact of opposing articular cartilage layers. In this paper we conceptualise articular cartilage as a giant reverse micelle (GRM) in which the highly hydrated three-dimensional network of phospholipids is electrically charged and able to resist compressive forces during joint movement, and hence loading. Using this hypothetical base, we describe a hydrophilic-hydrophilic (HL-HL) biopair model of joint lubrication by contacting cartilages, whose mechanism is reliant on lamellar cushioning. To demonstrate the viability of our concept, the electrokinetic properties of the membranous layer on the articular surface were determined by measuring via microelectrophoresis, the adsorption of ions H, OH, Na and Cl on phospholipid membrane of liposomes, leading to the calculation of the effective surface charge density. The surface charge density was found to be -0.08 ± 0.002 cm-2 (mean ± S.D.) for phospholipid membranes, in 0.155 M NaCl solution and physiological pH. This value was approximately five times less than that measured in 0.01 M NaCl. The addition of synovial fluid (SF) to the 0.155 M NaCl solution reduced the surface charge density by 30% which was attributed to the binding of synovial fluid macromolecules to the phospholipid membrane. Our experiments show that particles charge and interact strongly with the polar core of RM. We demonstrate that particles can have strong electrostatic interactions when ions and macromolecules are solubilized by reverse micelle (RM). Since ions are solubilized by reverse micelle, the surface entropy influences the change in the charge density of the phospholipid membrane on cartilage surfaces. Reverse micelles stabilize ions maintaining equilibrium, their surface charges contribute to the stability of particles, while providing additional screening for electrostatic processes. © 2008 Elsevier Ireland Ltd. All rights reserved.

Monitoring charged particle and ion emissions from a laser printer

While the emission rate of ultrafine particles has been measured and quantified, there is very little information on the emission rates of ions and charged particles from laser printers. This paper describes a methodology that can be adopted for measuring the surface charge density on printed paper and the ion and charged particle emissions during operation of a high-emitting laser printer and shows how emission rates of ultrafine particles, ions and charged particles may be quantified using a controlled experiment within a closed chamber.

Adsorption of heavy metals by road deposited solids

The research study discussed in the paper investigated the adsorption/desorption behaviour of heavy metals commonly deposited on urban road surfaces, namely, Zn, Cu, Cr and Pb for different particle size ranges of solids. The study outcomes, based on field studies and batch experiments confirmed that road deposited solids particles contain a significantly high amount of vacant charge sites with the potential to adsorb additional heavy metals. Kinetic study and adsorption experiments indicated that Cr is the most preferred metal element to associate with solids due to the relatively high electro negativity and high charge density of trivalent cation (Cr3+). However, the relatively low availability of Cr in the urban road environment could influence this behaviour. Comparing total adsorbed metals present in solids particles, it was found that Zn has the highest capacity for adsorption to solids. Desorption experiments confirmed that a low concentration of Cu, Cr and Pb in solids was present in water-soluble and exchangeable form, whilst a significant fraction of adsorbed Zn has a high likelihood of being released back into solution. Among heavy metals, Zn is considered to be the most commonly available metal among road surface pollutants.