Showing papers in "Zeitschrift für Physikalische Chemie in 2007"

Theoretical trends in particle size effects for the oxygen reduction reaction

Abstract: A simple, first principles-based model of the oxygen reduction reaction (ORR) is used to determine ORR kinetics on the (111), (100), and (211) facets of eleven transition metals (Au, Ag, Pt, Pd, Ir, Cu, Rh, Ni, Ru, Co, Fe). For most metals, the unreconstructed (100) facets are found to have an activity comparable to, or slightly higher than, the (111) facets. In contrast, (211) steps are found to be significantly less active than the terraces, with the exception of the most noble metals. These results are combined with simple models of the geometries of catalytic nanoparticles to estimate the average ORR activity of Pt and Au nanoparticles of various sizes. On Pt, a modest decrease in the activity with decreasing particle size is predicted, while for Au, the opposite trend is found.

Modeling of Solid-Oxide Fuel Cells

Abstract: This article presents a brief review on modeling philosophies of solid-oxide fuel cells (SOFCs) including an introduction to SOFC components and their functions. While a plethora of numerical models is available for SOFC modeling and simulation, this paper focuses on a general overview on mathematical model equations that represent the physico-chemical processes occurring in SOFCs and their boundary conditions. Electrochemical model equations, their simplifications, significance, and solution methods are discussed. Charge transfer chemistry is described in detail from a global as well as from elementary charge transfer perspective. Principles of heterogeneous reforming chemistry on conventional nickel cermet anodes, evaluation of rate expressions, and the implementation of the thermo chemistry into SOFC flow models are described. Results of numerical simulations are presented for configurations ranging from complex stacks to simple zero dimensional electrochemical systems.

A Refined All-Atom Model for the Ionic Liquid 1-n-Butyl 3-Methylimidazolium bis(Trifluoromethylsulfonyl)imide [bmim][Tf2N]

Abstract: A refined flexible all-atom model for a room temperature ionic liquid, 1-n-butyl 3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]), is reported here. Extensive molecular dynamics simulations were performed to check the validity of this model. We found that the thermodynamic and dynamic properties such as density, isobaric expansivity, isothermal compressibility, and self-diffusion coefficient described by the present model are in good agreement with experimental observations. Based on the calculated results, there has been a considerable improvement in the force field with respect to the previous model by Lopes et al. [15,16].

The Electronic Role of the TiO2 Light-Scattering Layer in Dye-Sensitized Solar Cells

Abstract: Reference LPI-ARTICLE-2007-020View record in Web of Science Record created on 2007-05-07, modified on 2017-05-12

An Electrochemical Model for Gold Colloid Formation via Citrate Reduction

Abstract: The mechanism of nanoparticle formation during the reduction of HAuCl 4 with sodium citrate under reflux has been studied. We have found that the redox potential in the nucleating colloid remains very positive, around +0.75 V vs. Ag/AgCl, until the reaction is almost complete. It then shifts suddenly to around +1-0.1 V vs. Ag/AgCl at the end of the reaction, which is close to the final open-circuit potential in the sol. AuCl4 continues to be reduced slowly over the course of the reaction, but is suddenly depleted as the potential shifts near the end of the reaction. The change in potential is correlated to the peptization of aggregates to form the final monodisperse gold particles. There is also a blue-shift of the surface plasmon band associated with charging of the gold particles. © by Oldenbourg Wissenschaftsverlag.

Probing ultrafast purely electronic charge migration in small peptides

Abstract: A pump–probe experiment that can examine a pure charge migration on a time scale short compared to the onset of nuclear motion is discussed. The mass spectrometric studies of Schlag et al. suggest that short peptide terminated by an aromatic amino acid are particularly suitable test compounds. The pump pulse needs to ionize the molecule on a time scale short compared to the period of the electronic motion, typically sub-fs. However, ionization occurs preferentially when the electrical field of the light is maximal so that the duration of the pulse envelope can be somewhat longer. Detection by photoelectron spectrometry of the peptide cation, to produce a dication, is shown to be able to probe the electronic rearrangement.

Hydrogen Bonding of Water Confined in Controlled-Pore Glass 10-75 Studied by ¹H-Solid State NMR

Abstract: In this paper a possible explanation for an unexpected ortho/para-water ratio in the gas clouds of comets is given. The description is based on the quantum-mechanical density matrix formalism and the spin temperature concept. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a four spin system, created by two nearest neighbour water molecules, spin eigenstates and their dynamics under the influence of their mutual dipolar interactions are studied. It is shown that a fast conversion between ortho- and para-states occurs on a msec time scale, caused by the intermolecular homonuclear magnetic dipolar interaction. Moreover the spin eigenstates of water in an ice crystal are determined by magnetic dipolar interactions and are not given by normal ortho- and para-H₂O states of gaseous water. As a result of this the spin temperature of gaseous water evaporated from ice depends strongly on its evaporation history and the ortho/para-ratio of water molecules are only an indirect measure of the temperature of ice crystals from where they descend. This result could explain the unexpected experimentally observed ortho/para-ratios in the clouds of comets.

Pd3Fe and Pt Monolayer-Modified Pd3Fe Electrocatalysts for Oxygen Reduction

Abstract: In a search for electrocatalysts based on other metals with comparable attributes to very scarce and expensive Pt, we studied the kinetics of the oxygen reduction reaction (ORR) on carbon-supported Pd3Fe alloy nanoparticles in HClO4 and NaOH solutions. In acid solution, the electrocatalyst's activity for the ORR is slightly higher than that of commercial Pt/C. The reaction kinetics involves predominantly a four-electron reduction with the first charge-transfer step being the rate-determining one. The synthesis of a Pd overlayer on the Pd3Fe alloy's surface at elevated temperatures is due to strong segregation effects. The major cause of this electrocatalyst's high activity appears to be the downshifting of the d-band center of Pd overlayer, resulting in a weaker interaction between the oxygen-containing species and the Pd surface. We demonstrated a further enhancement of the ORR kinetics with a Pt monolayer-covered Pd3Fe/C electrocatalyst. In HClO4 solution, the mass-specific activity of Pt/Pd3Fe/C was about five times higher than that of commercial Pt/C, surpassing the Pt/Pd/C. In situ XANES data indicate that this elevated ORR activity may be due to the decreased formation of PtOH, and weaker oxygen adsorption on Pt/Pd3Fe/C.

Photothermal Properties of Gold Nanoparticles

Abstract: This paper describes our recent time-resolved spectroscopy studies of the properties of gold particles at high laser excitation levels. In these experiments, an intense pump laser pulse rapidly heats the particle, creating very high lattice temperatures — up to the melting point of bulk gold. These high temperatures can have dramatic effects on the particle and the surroundings. The lattice temperature created is determined by observing the coherently excited the vibrational modes of the particles. The periods of these modes depend on temperature, thus, they act as an internal thermometer. We have used these experiments to provide values for the threshold temperatures for explosive boiling of the solvent surrounding the particles, and laser induced structural transformations in non-spherical particles. The results of these experiments are relevant to the use of metal nanoparticles in photothermal therapy, where laser induced heating is used to selectively kill cells.

Synthesis of Monodisperse PbS Nanoparticles and Their Assembly into Highly Ordered 3D Colloidal Crystals

Abstract: We describe the synthesis of highly monodisperse PbS nanocrystals using an organometalic approach, whereby oleic acid and tri-n-octylphosphine serve as stabilizing ligands. The optical bandgap of the particles is tunable as a function of crystal-size throughout the near-infrared region from 800 to 2200 nm. The particles were characterized using X-ray diffraction, optical spectroscopy and transmission electron microscopy. Applying the three layer super saturation technique we were able to grow colloidal crystals of various shapes as shown by scanning electron microscopy. Small-angle X-ray scattering experiments indicate fcc lattice structure.

Thermal Rate Constants for Polyatomic Reactions: First Principles Quantum Theory

Abstract: The truly accurate knowledge of molecular dynamics phenomena is generally achieved through a combination of detailed experiments and first principle theory. The complexity of such a level of description had until recently restricted accurate studies to rather small systems. However, the sophistication of theoretical methods and massive technological developments have provided remarkable progress in the detailed knowledge of reactive events during the past three decades. Moreover, significant progress towards the detailed understanding of polyatomic reaction has been made in recent years. Detailed experimental and accurate theoretical studies of reactions involving more than only three or four atoms are becoming increasingly available. In this work, aspects of the theoretical work aiming at the accurate description of polyatomic reactions are reviewed. The present article focuses on the development of the first principle theory of reaction rates. It reviews theoretical developments and benchmark applications to reactions as CH4 + H → CH3 + H2 and CH4 + O → CH3 + OH. The importance of quantum effects for the thermal rate constants in different temperature regimes is discussed in detail. The accuracy of the classical transition state theory and of different approximate quantum theories is investigated in detail. A quantum transition state concept which facilitates accurate reaction rate calculations for polyatomic reaction is described. Benchmark results for the CH4 + H → CH3 + H2 reaction are shown which demonstrate that the accuracy of thermal rate constants calculated by first principle theory can rival the accuracy of available experimental data. The perspectives offered by these developments are discussed.

Aggregation Behavior of Imidazolium-Based Ionic Liquids in Water

Abstract: The UV absorption spectra of aqueous solutions of [C4mim][Br], [C4mim][BF4], and [C8mim][Br], along with the differential UV spectra of aqueous [C8mim][Br] solutions, were measured at room temperature in this work. It is found that three transformations, denoted respectively as α, β and γ, can occur over the interested concentration range of solutions due to ionic liquid aggregation. Further, the content of ionic liquids and the wavelength of absorption maximum at these transformations are determined, and the implications at each transformation are analyzed. As a result, this paper discusses the effects of imidazolium cation, alkyl chain length and counterion type on the aggregation behavior and the contribution of interactions (such as hydrogen bonds, Coulombic, aliphatic and π–π stacking interactions) to the aggregation of 1-alkyl-3-methylimidazolium salts in water. Moreover, the possible structures of both aggregate and bulk phase in the aqueous solutions of ionic liquids are predicted also.

Oxidation of N-(4-Hydroxyphenyl) Acetamide (Paracetamol) Drug by Diperiodatocuprate(III) in Aqueous Alkaline Medium by Stopped Flow Technique

Abstract: The kinetics of oxidation of anti-pyretic drug, paracetamol (PAM) by diperiodatocuprate (III) (DPC) in alkaline medium at a constant ionic strength of 0.10 mol dm−3 was studied spectrophotometrically. The reaction between DPC and paracetamol in alkaline medium exhibits 1:4 stoichiometry (paracetamol: DPC). The reaction is of first order in [DPC] and has less than unit order in [PAM] and negative fractional order in [alkali]. Intervention of free radicals was observed in the reaction. The oxidation reaction in alkaline medium has been shown to proceed via a DPC-paracetamol complex, which decomposes slowly in a rate determining step followed by other fast steps to give the products. The main products were identified by spot test, IR, NMR and GC-MS. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to slow step of the mechanism are computed and discussed and thermodynamic quantities are also determined.

Adsorption of Collagen Fragments on Titanium Oxide Surfaces: A Molecular Dynamics Study

Abstract: The adsorption of flexible collagen I triple helices with lengths of 8 nm in aqueous solution on a partially hydroxylated rutile layer (pH = 7.4) is modelled by classical molecular dynamics simulation with trajectories of up to 5 ns lengths. The carboxyl groups of glutamic and aspartic acids form hydrogen bonds with surface protons, which result in stable contact points at 300 K, if the bond length is smaller than 2 Å. Lysine side chains bind to surface hydroxyl groups. In spite of rotational motion around the C–N figure axis and opening and closing of hydrogen bonds, the amino groups are immobilized at the surface at N–O distances below 4 Å. In all runs the proteins have few contact points to the surface even though it is perfectly planar in our simulation. We suggest that the experimentally observed increase of adsorption energy and area per molecule over time may be associated to an increase of the number of contact points, which induces spreading of the adsorbate.

Primary Processes During the Photodeposition of Ag Clusters on TiO2 Nanoparticles

Abstract: In-situ transient laser flash photolysis absorption studies were carried out on transparent colloidal TiO2 suspensions in the presence of Ag+ ions and polyvinyl alcohol irradiated by an XeF laser (351nm wavelength) to investigate the early stages of Ag photodeposition on the TiO2 nanoparticles in the nanosecond and millisecond timescales. Photon fluxes of 5 × 10-5 and 1 × 10-6 mol photon/L were used; these were equivalent to approximately 60 and 2 photons/TiO2 particle, respectively. Metallic silver clusters were detected immediately after the laser pulse using both photon fluxes, with calculated quantum yields of 0.3 and 0.5, respectively. In both cases, the metallic silver clusters formed had the same characteristic absorption maxima, assigned to silver clusters containing ≥12 silver atoms. The formation of these larger than expected silver clusters, even at low photon fluxes, is believed to be a result of an electron transfer within the TiO2 aggregates, similar to an antenna effect. It is proposed that the silver deposits form an Ohmic contact on the TiO2 particle thus allowing the continuous electron transfer through the metallic silver nuclei and hence the further reduction of adsorbed Ag+ ions. This study reports the first observation of an antenna type effect for a photocatalytic reduction reaction.

Stability of Complexes of Alkali Metal Cations with Tetraethyl p-tert-Butylcalix[4]arene Tetraacetate in the Water–Nitrobenzene Extraction System

Abstract: From extraction experiments and γ-activity measurements, the exchange extraction constants corresponding to the general equilibrium M+(aq) + NaL+(nb) ⇔ ML+(nb) + Na+(aq) taking place in the two-phase water–nitrobenzene system (L = tetraethyl p-tert-butylcalix[4]arene tetraacetate; aq = aqueous phase, nb = nitrobenzene phase) were evaluated. Further, the stability constants of the ML+ complexes in water saturated nitrobenzene were calculated; they were found to increase in the cation order Cs+ < Rb+ < K+ < Li+ < Na+.

A First-Principles Investigation of the Electronic Structure of Trivalent Rare Earth Ions in Gallium Nitride

Abstract: Using first-principles electronic structure methods, we have investigated the electronic structure and associated properties of Pr3+, Nd3+, Eu3+, Tb3+, Dy3+ and Er3+ substituting for Ga3+ in GaN. It is found that these rare earth ions (RE3+) are stable at the Ga site in tetrahedral coordination with nitrogen atoms. The nitrogen ligands move away from the RE3+ site to allow for the large ionic radii of the RE3+ ions. The equilibrium bond lengths of RE–N are found to vary between 2.15 to 2.30Å in good agreement with structural data available for Eu3+, Tb3+ and Er3+. Using the calculated energy bands, excitation energies for the 4f→5d transition have also been predicted. Most of these energies are larger than the band gap of GaN and cannot be exploited in transferring host excitation energies to the RE3+ ions.

Controlling the Properties of Manganese-Zinc Ferrites by Substituting In3+ and Al3+ Ions

Abstract: The effect of substitution of In3+ and Al3+ ions on the electrical and magnetic properties of Mn-Zn ferrites has been investigated. The substitution of In3+ ions for Fe3+ ions resulted in an increase of lattice parameter, owing to the larger size of In3+ ions, whereas lattice parameter was found to decrease on substituting Al3+ ions in place of the Fe3+ ions, owing to the smaller size of Al3+ ions. The dc resistivity was found to increase with the substitution of In3+ and Al3+ ions in the Mn-Zn ferrite system. The improvement in the dc resistivity has been observed at the expense of deterioration in the magnetic properties of Al3+ substituted Mn-Zn ferrites. A significant reduction in the values of initial permeability, saturation magnetization and Curie temperature was observed with successive increase of Al3+ ions. The saturation magnetization and initial permeability were found to increase with incorporation of In3+ ions. A marked increase in the value of initial permeability was found for the Mn0.4Zn0.6In0.5Fe1.5O4 ferrite. Curie temperature was found to decrease with an increase of In3+ ion concentration. These changes in the properties are explained on the basis of their magnetic interactions, a modified cation distribution and various models.

Controlled Release of Proteins Bound to Spherical Polyelectrolyte Brushes

Abstract: We discuss the interaction of proteins dissolved in aqueous solution with spherical polyelectrolyte brushes (SPB). The SPB consist of a solid core particle of colloidal dimensions (ca. 100 nm in diameter) onto which long polyelectrolyte chains have been grafted. Immersed in aqueous solution of proteins these SPB will take up high amounts of protein if the ionic strength is low. At high ionic strength, however, virtually no protein will enter into the brush layer attached to the surface of the core particles. We show that bovine serum albumin (BSA) bound at low ionic strength will gradually be released upon raising the salt concentration in the solution in a well-controlled manner: For each raise of the ionic strength in solution there is a well-defined amount of protein that is released. We show that BSA adsorbed to a conventional carboxylated latex will not be released if treated in the same manner. All findings, namely the uptake of protein as well as the controlled release can be explained by the “counterion release force”: Patches of positive charge on the surface of the proteins which are immersed in the brush layer become multivalent counterions of the polyelectrolyte chains thus releasing a concomitant number of counter- and coions. Release of counterions as induced by the adsorption of proteins is hence the main driving force for the polyelectrolyte-mediated protein adsorption (PMPA).

Monte Carlo Simulation of Kinetically Limited Electrodeposition on a Surface with Metal Seed Clusters

Abstract: Kinetic Monte Carlo (KMC) simulations were carried out to simulate kinetic-limited electrodeposition of a metal (M) onto an array of pre-existing metal clusters on a substrate (S) of a second conducting material. Electrochemical reaction and surface diffusion were accounted for in a KMC code which tracked deposit growth with a (2+1)-dimensional approach. Beginning with various arrangements of ten-atom metal seed clusters on a substrate platform of 300×300 fcc lattice sites, KMC simulations were carried out to investigate the evolution of the surface morphology. The influence of the number (spacing) of pre-existing seed clusters, the applied potential, and the metal–substrate surface diffusion energy barrier were investigated. It was found that when 16 or fewer seed clusters were present on the surface prior to electrodeposition, the resulting nucleation distribution was dominated by secondary nuclei formed during deposition. For substrates with a metal– substrate surface diffusion energy barrier greater than 3.5×10−20 J, it was more difficult to control the uniform growth of the seed clusters owing to the nucleation of secondary clusters. At lower applied potentials it was found that larger nuclei could be grown with a more controlled size distribution because fewer secondary nuclei were formed. Furthermore, it was found that larger clusters with a more controlled size distribution can be grown when more clusters are seeded onto the surface because the deposited atoms were more likely to attach to existing clusters, than to form secondary nuclei.

Hydrophobic Interactions of Xenon by Monte Carlo Simulations

Abstract: Hydrophobic interactions of xenon atoms dissolved in liquid water were studied by NpT Monte Carlo simulations in the temperature range 298.15 to 333K and at ambient pressure. Structural properties of dilute xenon solutions were calculated and compared to those of bulk water in order to show the influence of the hydrophobic solute. It was found that the xenon atoms tend to aggregate with increasing temperature. At low temperatures the aggregates are predominantly solvent-separated pairs; at higher temperatures the quota of contact pairs increases. Furthermore, the residual chemical potentials of xenon and water were calculated with different methods; it was found that the Widom insertion methods works best for this system. For the thermodynamic conditions of this work, the residual chemical potential of water in the presence of xenon was found to be a linear function of temperature.

Processing of High Density Manganese Zinc Nanoferrites by Co-Precipitation Method

Abstract: Nanoparticles of spinel ferrites MnxZn1-xFe2O4 with x = 0.0 to 1.0 were prepared by co-precipitation method. The ferrite samples were pre-sintered at 200°C at the rate of 200°C/h for 15h. Finally, one set of samples was sintered at 400°C and the other set at 500°C at the rate of 200°C/h for another 15h. The spinel structure was confirmed by X-ray diffraction (XRD) pattern. The particle size was calculated by using Debye–Scherrer's equation for Lorentzian peak. The calculated average particle size lies between 19–53 nm. The particle size was also investigated by using scanning electron microscopy (SEM). Both the results were found to be in good agreement. The resistivity data was taken by two probe method and resistivity, activation energy and porosity were calculated. There was an increase in the resistivity and activation energy when the ferrite samples were sintered at a higher temperature. Possible models, theories and mechanisms contributing to these processes have been discussed.

Voltammetry and Electrocatalysis of Achromobacter Xylosoxidans Copper Nitrite Reductase on Functionalized Au(111)-Electrode Surfaces

Abstract: A long-standing issue in protein film voltammetry (PFV), particularly electrocatalytic voltammetry of redox enzyme monolayers, is the variability of protein adsorption modes, reflected in distributions of catalytic activity of the adsorbed protein/enzyme molecules. Use of well-defined, atomically planar electrode surfaces is a step towards the resolution of this central issue. We report here the voltammetry of copper nitrite reductase (CNiR, Achromobacter xylosoxidans) on Au(111)-electrode surfaces modified by monolayers of a broad variety of thiol-based linker molecules. These represent positively charged and electrostatically neutral, hydrophobic and hydrophilic, aliphatic and aromatic, and variable-length micro-environments, as well as their combinations. Optimal conditions for enzyme function seems to be a combination of hydrophobic and hydrophilic surface linker properties, which can lead to close to complete non-catalytic monolayer interfacial electron transfer function and electrocatalysis with activity approaching enzyme activity in homogeneous solution. Thiophenol (combined hydrophobic stacking and interdispersed water molecules), 4-methyl-thiophenol (hydrophobic and water molecules), and 3- and 4-aminothiophenol (hydrophilic, hydrophobic) offer the overall most efficient micro-environments. Subtle differences with even small structural linker differences, however, lead to widely different electrocatalytic properties, strikingly illuminated by the ω-mercaptoamines. CuNiR thus shows highly efficient, close to ideal reversible electrocatalytic voltammetry on cysteamine-covered Au(111)-electrode surfaces, most likely due to two cysteamine orientations previously disclosed by in situ scanning tunnelling microscopy. Such a dual orientation exposes both a hydrophobic and a positively charged, hydrophilic surface feature. In contrast, the higher cysteamine homologues expose only the hydrophilic component with no electrocatalytic activity on these surfaces. These results offer a basis for rational surface design in forthcoming biological electrocatalysis useful both fundamentally and in novel biosensor technology.

Carbon Nanotubes and Electrochemistry

Abstract: Rigid carbon nanotube (CNT) polymer composites were characterized as bare electrochemical transducers and as support for (bio)functionalization. In order to accomplish that, cyclic voltammetry and impedance spectroscopy in presence of benchmark redox systems were performed. The response and sensitivity can be enhanced if the density of edges is increased in the CNT systems. Bamboo-like CNTs seem to be the best choice for composite electrodes. Additionally, the CNT electrodes result in a very robust support for chemical functionalization. Direct electron transfer of myoglobin has been verified without the need of electrochemical mediators and without compromising the biomolecule activity. Finally, some other exciting applications of CNT electrochemistry at the nanoscale regime are also highlighted.

Adsorption of amyloid beta (1-40) peptide at liquid interfaces

Abstract: The folding of amyloid β (1-40) peptide into β-sheet containing fibrils, which are the main components of deposits and plaques in some neurodegenerative diseases, is thought to play a causative role in Alzheimer's disease. The peptide is amphiphilic and therefore surface active. Interactions with surfaces can play an important role in the secondary structure changes. Langmuir monolayers of zwitterionic (DMPC, DPPC, DMPE, DPPE) as well ionic (DMPG, DPPG, DPTAP, DSTAP) phospholipids have been used to study the influence of the peptide on the lipid packing and, vice versa, the influence of phospholipid monolayers on the peptide secondary structure by infrared reflection absorption spectroscopy and grazing incidence X-ray diffraction. The peptide adsorbs at the air/water (buffer) interface and penetrates into uncompressed phospholipid monolayers. After a special pre-treatment, the peptide forms predominantly a random-coil secondary structure in solution but adopts a β-sheet conformation, which is almost parallel oriented to the surface in the adsorbed state. The peptide does not influence the condensed phospholipid monolayer structure. In contrast to Aβ insertion into zwitterionic or anionic phospholipid monolayers, a non β-sheet structure was detected during the first stage of Aβ insertion into positively charged monolayers. At high lateral pressure, the peptide is squeezed out of the monolayer. It desorbs completely from zwitterionic monolayers and charged monolayers on buffer, but remains adsorbed in β-sheet conformation at charged monolayers on water. In contrast, Aβ remains in a fluid (disordered) and charged monolayer on buffer even above 30 mN m-1. This observation shows that a combination of hydrophobic and electrostatic interactions is necessary to keep the peptide in the adsorbed state at high pressure on buffer.

Fluorescence Quenching of Xanthene Dyes by TiO2

Abstract: The fluorescence quenching of xanthene dyes by TiO2 has been investigated using steady state technique. The dyes used are eosin, rhodamine B, rhodamine 6G. The quenching was found to obey Stern-Volmer equation and the corresponding Stern-Volmer plots were linear in the range of quencher concentration used [0–5 × 10--5] M. The quenching rate constant (kq) is in the range of 0.77–8.30 × 1012 M-1 s-1. This study reveals effective quenching of xanthene dyes with TiO2. The quenching of xanthene dyes followed electron transfer mechanism.

Density, Apparent and Partial Molar Volumes, and Viscosity of Aqueous Na2CO3 Solutions at High Temperatures and High Pressures

Abstract: Density of five (0.124, 0.334, 0.706, 1.055, and 1.123) mol kg-1 and viscosity of seven (0.124, 0.2918, 0.334, 0.706, 0.8472, 1.055, and 1.123) mol kg-1 binary aqueous Na2CO3 solutions have been measured with a constant-volume piezometer and capillary flow techniques, respectively. Measurements were performed at pressures up to 52MPa for the density and 40MPa for the viscosity. The range of temperature was from 299 to 577K for density and from 293 to 478K for the viscosity. The total uncertainty of density, viscosity, pressure, temperature, and composition measurements was estimated to be less than 0.06%, 1.6%, 0.05%, 15mK, and 0.02%, respectively. Apparent molar volumes were derived using measured values of density for the solutions and for pure water calculated with IAPWS formulation. The effect of the thermodynamic variables (temperature, pressure, and concentration) on density, apparent and partial molar volumes, and viscosity of Na2CO3(aq) solutions was studied. The derived apparent molar volumes have been interpreted in terms of the Pitzer’s ion-interaction model of electrolyte solutions to accurate calculates the values of partial molar volumes at infinite dilution V¯2∞ and the second (BV) and third (CV) virial coefficients for the apparent molar volume as a function of temperature. The viscosity data have been analyzed and interpreted in terms of extended Jones–Dole equation for the relative viscosity (η/η0) of strong electrolyte solutions to accurate calculate the values of viscosity A- and B-coefficients as a function of temperature. The Arrhenius–Andrade parameters ηA and b = Ea/R (where Ea is the flow activation energy) were calculated using present experimental viscosity data. The effective pressures Pe due to the salt (Na2CO3) in water in the TTG (Timmann-Tait-Gibson) model were calculated from present viscosity measurements.

DFT Studies on the nature of coadsorbates on SO42-/Au(111)

Abstract: Density Functional Theory (DFT) studies were performed on the adsorption of sulfate on Au(111). Focusing on the well-known (√3 × √7)R19.1° structure reported by various surface sensitive techniques, the coadsorption of H3O+ and/or H2O has been considered in different combinations. The calculated binding energies show that the coadsorption of a single H3O+ per sulfate is the most stable configuration, which is in agreement with experimental observations. Further, we find that in the case of coadsorption of both H3O+ and H2O along with sulfate, one of the protons of hydronium moves to sulfate, finally leading to bisulfate with two coadsorbed water molecules. Besides the morphology and energetics of the different configurations, we also discuss the nature of the surface bonds by analyzing the charge density distribution.

Electroreduction of nitrate at copper electrodes and copper-PANI composite layers

Abstract: The electroreduction of nitrate ions is investigated in acid and neutral aqueous solutions (HClO4 and NaClO4 as electrolytes) at polycrystalline copper electrodes, copper single crystals and at copper particles deposited in polyaniline (PANI) layers. In the presence of low nitrate concentrations (5 mM), the reduction of nitrate is not significantly different on various copper atomic surface structures but is greatly dependent on the local pH at the electrode. In contrast to strong acidic solutions, two separate waves are detected when nitrate ions are present in neutral solutions irrespective of the use of copper polycrystalline or single crystal electrodes. The first of the two waves leads to the formation of nitrite ions. When copper particles are dispersed in polyaniline layers it is demonstrated that the electrocatalytic activity is strongly dependent on the way of depositing copper in the polymer layer. A clear difference is observed in the current response in absence and presence of nitrate ions for copper deposited in the reduced state of the PANI layer, whereas copper deposited in the oxidized state of the PANI layer remains still electrocatalytically rather inactive. Copper crystalline species act effectively for the investigated reaction only if copper conducting paths are available through the polymer matrix up to the underlying metal surface.

Order–Disorder Transition at the Liquid/Vapor Interface of Imidazolium Based Ionic Liquids: A Surface Light Scattering Study

Abstract: The liquid/vapor interface of imidazolium based ionic liquids, [C6mim][PF6] and [C6mim][BTI], has been probed by capillary wave spectroscopy at various temperatures up to 400K. Separately, the corresponding surface tensions have been measured by the maximum bubble pressure method. Interpretation of the capillary wave spectra with a modified dispersion relation yields the surface dipole moment density γ(T) as a function of temperature. In both systems γ(T) exhibits a characteristic decrease with temperature which indicates an interfacial order–disorder transition. Within the mean field approximation, fits of γ(T) yield the following critical temperatures for this transition: Tc = 364K for [C6mim][BTI], and Tc = 383K for [C6mim][PF6].