Tracer diffusion of Co2+ ions in agar gel containing multi-electrolyte systems

Tracer diffusion coefficients of cobalt ions have been measured in the supporting medium containing multi-electrolyte systems of alkali bromides. The electrolyte concentration was varied between 10^–6-0.1M at 25°C and the diffusion coefficients were determined by zone-diffusion technique using agar gel medium. The trend in the theoretical values of diffusion coefficients is accounted for by considering the relative contribution of mobility function, ionic strength as well as ion size parameter to the theoretical value in different systems. While the deviations between theoretical and experimental values of diffusion coefficients are explained on the basis of various co-occurring effects in ion-gel-water system.     

Diffusion of Cesium Ions in Agar Gel Containing Alkali Metal Bromides

The present paper gives an account of different aspects of the tracer diffusion of Cs⁺ ions in alkali metal bromides. We have measured the diffusion coefficients, D, of cesium ions in 1% agar gel medium at 25 ^∘C using a zone-diffusion technique over a concentration range of 5 × 10⁻⁵ to 0.1 mol,dm⁻³. The values of the diffusion coefficients were found to deviate from theory, which are explained on the basis of different types of interactions occurring in the ion-gel-water system. The study is also focused on the effect of alkali metal bromides on the obstruction effect and activation energy for the tracer-diffusion of cesium ions in agar gel medium. It is observed that both parameters, extent of obstruction, ∝, and activation energy, E, decrease with increasing charge density of the cation of the supporting electrolyte. The influence of these trends is explained on the basis of competitive hydration between the ions and agar molecules, and the relative distortion in the water structure that is brought about by these different ions and agar molecules.     

Tracer-diffusion of cobalt ions in sodium and potassium nitrates in agar gel

Tracer-diffusion of Co²⁺ ions is studied in agar gel in the presence of sodium and potassium nitrates at 25°C. The diffusion coefficient values at various electrolyte concentrations are compared with the corresponding theoretical values computed on the basis of Onsager's theory. The deviations from the theory are attributed to the various co-occurring effects in the diffusion medium. The activation energy for the process of diffusion of Co²⁺ ions in presence of above electrolytes is also reported.     

Diffusion studies of cobalt ions and their salts in agar gel medium: Obstruction effect and gel hydration

The obstruction effect in the electrolyte diffusion of cobalt halides and in tracer diffusion of Co²⁺ ions in the presence of different supporting electrolytes at various concentrations has been studied at 25 °C using the zone diffusion technique. It has been observed that obstruction effect expressed in terms of increases with concentration and is higher for electrolyte diffusion than in tracer diffusion. Further, for a given concentration it is found to decrease with increasing charge density of an anion. These observations are explained on the basis of competitive hydration between ions and agar molecules.     

Electrochemical Behavior of Cobaltocene in Ionic Liquids

The electrochemical behavior of cobaltocenium has been studied in a number of room temperature aprotic ionic liquids. Well defined, diffusion controlled, anodic and cathodic peaks were found for the Cc⁺/Cc (cobaltocenium/cobaltocene) reduction/oxidation on gold, platinum and glassy carbon electrodes. Values of the peak separation parameters suggest quasireversibility or even irreversibility for the redox process. The difference between the ferrocene/ferrocenium and cobaltocenium/cobaltocene couples has been evaluated as equal to (1.350 ± 0.020) V. Values of the cobaltocenium (Cc⁺) diffusion coefficients D have been calculated on the basis of the Randles–Sevcik equation.     

Tracer-diffusion of Zn2+ ions in cobalt sulphate

Tracer-diffusion of Zn²⁺ ions in the presence of CoSO₄ is studied at 25°C in 1% agar gel over a concentration range of 10^–5 to 0.25M using a zone-diffusion technique. The deviations observed between experimental and theoretical values of diffusion coefficients are explained by considering different types of interactions occurring in the ion-gel-water system. Further, study of the obstruction, effect in the diffusion of Zn²⁺ ions at different concentrations of CoSO₄ reveals that the -value decreases with increasing concentration of the electrolyte. This observation is accounted on the basis of competitive hydration between ions and agar molecules.     

Effect of the mobility of oxygen in perovskite catalyst on the dynamics of oxidative coupling of methane

The effect of the diffusion of oxygen from the volume of the catalyst to its surface on the dynamics of the oxidative coupling of methane was assessed on the basis of a mathematical model of the reaction of methane with the oxidized surface of KNaSrCoO_3–x perovskite. It was shown that the possible values of the diffusion coefficient lie in the range of 10^-18-10^-16 cm²/s characteristic of the diffusion of oxygen in oxide catalysts.     

Electrolyte-diffusion of cobalt sulphate in agar gel medium at 25 °C

The diffusion of cobalt sulphate is studied in 1% agar gel over a concentration range of 10^–5 to 0.2M at 25 °C. The experimental values of the diffusion coefficient at various concentrations determined by the zone-diffusion technique are compared with the theoretical values of diffusion coefficinets computed on the basis of Onsager-Fuoss theory. The observed deviations are interpreted in terms of relative contributions of diffusion-enhancing and diffusion-retarding interactions occurring in the water-gel-electrolyte system.     

Tracer diffusion of Co2+ ions in multielectrolyte systems: Activation energy and obstruction effect

The activation energy for the tracer diffusion of Co²⁺ ions in multielectrolyte systems containing alkali bromides has been determined in agar gel medium over the temperature range of 25–45°C. The decrease in the value of the Arrhenius parameters, E and D_o, with gel percentage is explained on the basis of the transition state theory. Further, studies of the influence of electrolyte concentration on activation energy and obstruction effect reveal that both parameters decrease with the former. The decrease in activation energy is explained by considering the changes in physical properties of the solution with concentration at microscopic level, while the decrease in the extent of obstruction effect is attributed to competitive hydration between ions and agar molecules in a diffusion system.     

Permeation of aromatic compounds in aqueous solutions through thin,dense cellulose acetate membranes

The partition and diffusion coefficients of aqueous solutions of aromatic compounds through a thin, dense cellulose acetate membrane were measured at 20°C. The water content and the thickness of the prepared membranes varied from 0.121 to 0.610 by volume fraction and from 17 to 88 μm, respectively. The aromatic solutes used were phenol, aniline, hydroquinone and p-chlorophenol. The solute concentration ranged between 9.0 x 10^-5 and 1.0 x 10^-3 mol/l. The partition coefficients had the following order: p-chlorophenol, phenol, aniline, hydroquinone; they were experimentally correlated with the water content of the swollen membranes.The dependence of the diffusion coefficients on the water content of the membrane was examined using as basis a pore model and a free volume model, respectively. The diffusion coefficients were adequately correlated with the water content of the membrane according to the relation given by the free volume model.     

Interfacial reactions in chromium-carbon multilayers

Thermal modifications due to interfacial compound formation in magnetron sputtered Cr-C multilayers have been investigated in the temperature range between 450° C and 580° C and the formation of interfacial carbides has been discussed on the basis of thermodynamic and kinetic considerations. Grain boundary diffusion accompanied by grain growth proportional to the reacted layer thickness has been assumed to account for the observed t^1/3 time dependence of the growth rate. The compound layer growth has been thermally activated with an activation energy of Q_G=0.85 eV.     

Glucose diffusivity and porosity in silica hydrogel based on organofunctional silanes

The silica hydrogels prepared at physiological conditions were characterized with respect to the glucose diffusion properties and the porosity by employing various approaches. A diffusion coefficient of glucose in silica hydrogel in the range of 2 × 10⁻¹⁰ m² s⁻¹ was determined by two complementary techniques based on the glucose ingress and egress, respectively. The confocal laser scanning microscopy in a time-lapse imaging mode was employed to measure the ingress of fluorescently labeled glucose analogue inside the hydrogel. In addition, a method for direct glucose release from the hydrogel was established. The simple diffusion model based on the Fickian diffusion and Ritger–Peppas theory were employed for evaluation of diffusion coefficients, respectively. The BET analysis and permeation of fluorescently labeled dextrans of various molecular weights were used to characterize the porosity of silica hydrogel. The radius of pores accessible for diffusion of dextran molecules in prepared silica hydrogel ranges between 1 and 6 nm.     

A radiochemical study of the kinetics of ion exchange on hydrous titanium dioxide

This paper reports a radiochemical study of the kinetics of ion exchange of Na⁺ and Cs⁺ with H⁺ on hydrous titanium dioxide. The experimental conditions are set to favor the particle diffusion mechanism only, and this is confirmed by the Bt versus t plots. On the basis of these studies the various physical parameters such as the effective diffusion coefficients, activation energies and entropies of activation have been evaluated. Tentative explanations are given for these results, from which some conclusions are drawn.     

Untersuchung der Reaktivität und Sinterungsaktivität von Eisen(III)-oxid mittels der Diffusion radioaktiver Edelgase

Various methods for introduction of radioactive noble gases into solids are reviewed and also the diffusion of radioactive noble gases in α-iron(III)oxide, labelled by different methods. The HAHN emanation and ion bombardment methods have been used. Different noble gas diffusion processes have been shown to occur during heating the type of which is influenced by the kind of labelling conditions. The values of the activation energy of the noble gas diffusion are compared. Lattice defects including the amorphous state of Fe^IIIoxide are produced by ion bombardment. On the basis of the diffusion behaviour of radon, being studied by the emanation method, the “activity” of Fe^III oxide prepared by heating of different iron salts and sintered at various temperatures has been estimated. It is shown that the emanation method can be used for the study of the sintering of ferric oxide. On the basis of the solid state reaction ZnO + Fe₂O₃ the reactivity of ferric oxide in dependence on its thermal history has been investigated.     

Short-time mobility of spherical particles in concentrated aqueous dispersions determined by diffusing wave spectroscopy

Summary The diffusion of optically dense, charge stabilized, polystyrene latex spheres of 500 nm radius in water at 25°C was determined by diffusing wave spectroscopy in the backscattering mode. The diffusion coefficient of the latex spheres had a value of 4.8×10^–13 m² s^–1 at infinite dilution and monotonically decreased as the number density of latex spheres increased. The diffusion coefficient dropped by a factor of 3 at 38 volume % latex and a factor of 20 at 59 volume % latex. Comparison was made to diffusion of sterically stabilized poly(methyl methacrylate) latex in refractive index matched mixed organic fluids as determined by dynamic light scattering. The concentration dependence determined by diffusing wave spectroscopy matched closely to the short-time diffusion determined by dynamic scattering and not to the long-time diffusion. Although the difference in repulsive potential between the charge and the sterically stabilized lattices must be large, it has a surprisingly small effect on the translational diffusion of the latex over distances small compared to the intersphere distance.     

Diffusion in surfactant solutions

The diffusion coefficients in water of Triton X-100 and sodium dodecyl sulfate were measured as a function of concentration using the Taylor dispersion technique. For Triton X-100, a nonionic surfactant, the diffusion coefficient drops from 7.4 × 10^-7 cm²/sec at 0.45 g/liter to 6.45 x 10^-7 cm²/sec at 5 g/liter. The diffusion coefficient of methyl yellow solubilized in Triton X-100 is close to that of the surfactant. This behavior is quantitatively consistent with a chemical equilibrium between monomer and micelle. For sodium dodecyl sulfate, an anionic surfactant, the diffusion coefficient increases from 1.76 x 10^-6 cm²/sec at 0.01 M to 4.53 x 10^-6 cm²/sec at 0.125 M. The increase is less when 0.1 M NaCl is added. The diffusion coefficient of the methyl yellow solubilized by the SDS is significantly less than that of the surfactant, particularly at low ionic strength. This behavior can be quantitatively explained by including electrostatic coupling between monomer, micelle, and counterion.     

Modelling reversible cyclic voltammetry at the one-dimensional junction established by three phases: an organic liquid containing a neutral electroactive solute,an aqueous electrolyte solution,and an electronic conductor

Scholz et al. (Electrochem. Commun. 2 (2000) 112) recently performed cyclic voltammetry on droplets of a solution of decamethylferrocene (dmfc) in nitrobenzene (nb) lying between graphite and an aqueous solution of potassium chloride. Those authors hypothesized that the reaction dmfc(nb)+Cl⁻(aq)→dmfc⁺(nb)+Cl⁻(nb)+e⁻ took place at the hoop where the three phases meet. We have modelled this hypothesis on the basis that the cyclovoltammetric current is controlled by Nernst’s law and by the cylindrical diffusion of dmfc to, and of dmfc⁺Cl⁻ from, the three-phase junction. Model and experiment do not match.     

Redox coupled ion exchange in cobalt oxide films

Ion exchange coupled to oxidation–reduction cycling of electrochromic cobalt oxide films was studied using Probe Beam Deflection (PBD) techniques. The films were deposited by potentiodynamic cycling of glassy carbon electrodes in aqueous Co⁺² solution. The cyclic voltammograms of cobalt oxide films present three peaks in basic (pH=13) solution. The PBD signal is positive during oxidation indicating insertion of hydroxide ions. The results are confirmed by monitoring the PBD signal during pulse potential experiments at varying distances of the beam from the electrode. Using the theory of PBD behaviour for discontinuous reactions, a diffusion coefficient for the mobile ion of 1.68×10⁻⁵ cm²/s was measured, which agreed with that observed for NaOH diffusion in similar systems. The results are explained on the basis of a scheme of solid state oxidations from Co^II to Co^IV in three redox steps, coupled to hydroxide ion insertion.     

Effect of anodic potential on process of formation of polyporphyrin film in solutions of tetrakis(p-aminophenyl)porphin in dichloromethane

The electrode impedance spectroscopy technique was used to study the process of formation of a conducting polyporphyrin film on a Pt electrode from a 10^?3 M solution of tetrakis(p-aminophenyl)porphin in dichloromethane. An equivalent circuit is suggested for simulation of interface impedance in a wide range of working electrode potentials. It is shown that regions with a different mechanism of film formation are observed at an increase in potential from 0.0 to +1.0 V. The kinetics of film formation are studied at the potentials of +0.40, +0.60, and +0.80 V. It is found that good agreement is observed between the model and experimental data when the growing film is simulated using a Warburg element with a finite diffusion length. Conductivity and the diffusion coefficient of charge carriers in it are estimated on the basis of the suggested model for a film obtained at the potential of +0.40 V. It is shown that conductivity of a polyporphyrin film grows by more than an order of magnitude at an increase in deposition potential from +0.40 to +0.80 V.     

Binary diffusion coefficients of l-histidine methyl ester dihydrochloride in aqueous solutions

The Taylor dispersion technique has been used for measuring mutual diffusion coefficients of l-histidine methyl ester as its dihydrochloride at T = 298.15 K and finite concentrations from (0.001 to 0.100) mol · dm⁻³. On the basis of experimental mutual diffusion coefficients, the hydrodynamic radii, R_h, the diffusion coefficient at infinite dilution D⁰ and the dependence of thermodynamic factors, F_T, on the concentration, have been estimated using the Onsager–Fuoss equation. Further insight on the diffusion has been obtained from ¹H and ¹³C NMR spectroscopy and DFT calculations, which suggest that the l-histidine methyl ester is present as its dication in acidic solution in a fully extended conformation, with considerable charge delocalization over the imidazolium ring. These experimental and computational results allow us to have a better understanding of the thermodynamic and kinetic behavior of this amino acid derivative in aqueous solutions.