Statistical atomic-topographic model of the active dissolution of metals with point lattice defects. The effect of point defects (vacancies and impurity atoms) on the metal dissolution rate

A physicochemical model is put forward for the dissolution (corrosion) of a metal with point lattice defects, i.e., vacancies and impurity atoms with corrosion stability radically different from that of the base metal. The model takes into account the substantial difference in the dissolution rates of the base metal atoms from positions with different numbers of neighboring atoms, which leads to the formation of characteristic nanofragments of the atomic surface relief. These fragments determine the dissolution rate. The point defects substantially affect the dissolution rate of the base metal exclusively due to their active involvement in the formation of the atomic relief. The equations describing this model allow the polarization curves of active dissolution of the base metal to be calculated as a function of the defect concentration.     

Thermal decomposition of lignocellulosic materials: comparison of the results obtained in different experimental systems

An experimental system that allows the use of large particle sizes and the simulation of different operating conditions was built to study the thermal decomposition of lignocellulosic materials. The values of solid conversion and of temperature obtained at different points using spherical particles of pine wood 2 cm in diameter are shown. The conversion values are compared with those calculated from the equations obtained in a thermobalance for small particle sizes.     

Silver based mercury film electrode: Part IV. Comparison of theoretical and experimental voltammetric results obtained for cadmium

Using the silver based mercury film electrode (SBMFE) and cyclic and stripping voltammetric techniques the behaviour of cadmium has been compared with the behaviour of lead as well as with the predictions of the theory of de Vries and van Dalen. At the SBMFE with film thickness between 0.1 and 2 μm lead behaves with good agreement with theoretical predictions and only at thicknesses higher than 2 μm some deviations occur due to collecting of the excess mercury at the bottom of the wire electrode. On the other hand even at thin film electrodes the behaviour of cadmium deviates significantly from the predictions of the theory. The height of the anodic peak decreases and its width increases; also the displacement of the potentials of both cathodic and anodic peaks is smaller than the values predicted theoretically. The deviations are caused neither by the formation of intermetallic compounds in the bulk of mercury phase nor by the formation of heterogenous cadmium amalgam; they reflect the interaction between cadmium dissolved in mercury and the solid silver amalgam which is the substrate of the mercury film. Owing to the effect discussed the stripping determination of cadmium at SBMFE is characterized by a lower sensitivity and reproducibility.     

A comparison of the electrode kinetics of dissolution of Pb and Hg

Mercury dissolution in HCl and Pb in H₂SO₄ have been investigated using automated equipment of new design. The characteristic electrochemical parameter curves derived from the steady-state impedance and current-potential curves are reported. The data spans the active-passive transition and the passive region. An interpretation of the data in the passive region, based on a reaction kinetic model, is discussed.     

Acidic and reductive dissolution of magnetite in aqueous sulfuric acid: Site-binding model and experimental results

The open circuit dissolution of ionic metal oxides in mineral acids is modelled assuming that the rate is controlled by the transfer of metal ions in hydrolytic equilibrium with bulk metal ions, from the metal oxide surface to the Stern plane. The site-binding model of the double layer metal oxide/electrolyte solution is used to obtain the pH dependence of surface and Stern potentials. The nature of the active sites is discussed and their surface concentration is assumed to be proportional to suface charge σ₀. Again, the site-binding model is used to detemine the pH dependence of σ₀. It is thus shown that the rate order in c_H⁺ is essentially defined by the potential dependence of the charge transfer process, for oxides with points of zero charge near neutrality that dissolve in mildly or strongly acidic solutions. The role of surface complexation is also discussed in terms of the site-binding model and the difficulties in interpreting dissolution experiments under constant external applied potential are discussed in terms of the complexity of the semiconductor oxide/electrolyte solution interfacial region in magnetite.An experimental study of the open circuit dissolution of magnetite in sulfuric acid is presented and interpreted according to the proposed model.The reductive dissolution of magnetite is modelled by extension of the Valverde-Wagner model of oxide dissolution. Experimental results are presented to demonstrate that the reductive dissolution rate of magnetite in ferrous containing solutions is controlled by the rate of electron transfer from adsorbed Fe(II) to Fe(III) surface states of magnetite.     

Statistical evaluation of accelerated stability data obtained at a single temperature. I. Effect of experimental errors in evaluation of stability data obtained

Accelerated stability data obtained at a single temperature is statistically evaluated, and the utility of such data for assessment of stability is discussed focussing on the chemical stability of solution-state dosage forms. The probability that the drug content of a product is observed to be within the lower specification limit in the accelerated test is interpreted graphically. This probability depends on experimental errors in the assay and temperature control, as well as the true degradation rate and activation energy. Therefore, the observation that the drug content meets the specification in the accelerated testing can provide only limited information on the shelf-life of the drug, without the knowledge of the activation energy and the accuracy and precision of the assay and temperature control.     

Comparison of lattice and continuum treatments of n-alkanes with experimental data

Using a homologous series of n-alkanes as a model system, we compare the predictions of a lattice Born-Green-Yvon (BGY) theory and a continuum BGY theory with experimental results. We find that both theories are capable of describing the fluid properties and critical points of alkanes ranging from heptamers (n-C7) to nonadecamers (n-C19). We probe the connection between the lattice and continuum BGY models and extend our discussion to include a sampling of other lattice and continuum treatments.     

Determination of interfacial tension from crystallization and dissolution data: a comparison with other methods

Methods for the determination of interfacial tension between a solid and a liquid are reviewed including solubility/particle size, crystallization and dissolution kinetics. The use of solubility as a function of particle size, originally put forward by Ostwald and later corrected by Freundlich, may be unjustified for determining interfacial tension at solid-liquid interfaces. The interfacial tension values between solutions and sparingly soluble minerals such as hydroxyapatite, fluorapatite, brushite, octacalcium phosphate, calcium oxalate monohydrate, barium sulfate, calcium sulfate, calcite, and divalent metal fluorides are discussed. A comparison of these results is made with contact angle or wetting measurements. The interfacial tension values obtained from constant composition reaction kinetics are of the same order of magnitude as those determined using a contact angle method involving thin layer wicking techniques.     

Development of master curves for discriminating the mechanism of solid state reactions from experimental data obtained by using the cyclic and controlled rate thermal analysis

The cyclic and Controlled Rate Thermal Analysis method (CRTA) has been used. The two rates automatically selected in the cyclic curve are small enough to allow the two states of the sample to be compared have nearly the same reacted fraction. Thus, the activation energy can be calculated without previous knowledge of the actual reaction mechanism. Provided that the activation energy,E, is known, a procedure has been developed for determining the kinetic law obeyed by the reaction by means of master curves that represent the values of the reacted fraction, α, as a function of?E/R(1/T-1/T _0.5),T _0.5 being the temperature at which α=0.5. This procedure has been tested by studying the thermal decomposition reaction of BaCO₃.     

Measurement and analysis of results obtained on biological substances with d.s.c.

The differential scanning calorimeter (d.s.c.) has been widely used to determine the thermodynamics of phase transitions and conformational changes in biological systems including proteins, nucleic acid sequences, and lipid assemblies. The d.s.c monitors the temperature difference between two vessels, one containing the biological solution and the other containing a reference solution, as a function of temperature at a given scan rate. Recommendations for d.s.c. measurement procedures, calibration procedures, and procedures for testing the performance of the d.s.c. are described. Analysis of the measurements should include a correction for the time response of the instrument and conversion of the power versus time curve to a heat capacity versus temperature plot. Thermodynamic transition models should only be applied to the analysis of the heat capacity curves if the model-derived transition temperatures and enthalpies are independent of the d.s.c. scan rate. Otherwise, kinetic models should be applied to the analysis of the data. Application of thermodynamic transition models involving two states, two states and dissociation, and three states to the heat capacity versus temperature data are described. To check the operating performance with standard d.s.c.s, samples of (1 to 10) mg · cm³solutions of hen egg white lysozyme in 0.1 mol · dm ^− 3HCl–glycine buffer at pH  =  (2.4  ±  0.1) were sent to six different d.s.c. laboratories worldwide. The values obtained from proper measurements and application of a two-state transition model yielded an average unfolding transition temperature for lysozyme of 331.2 K with values ranging between T =  329.4 K and 331.9 K, and an average transition enthalpy of 405 kJ · mol ^− 1with values ranging from 377 kJ · mol ^− 1to 439 kJ · mol ^− 1. It is recommended that the reporting of d.s.c. results be specific with regard to the composition of the solution, the operating conditions and calibrations of the d.s.c. determination of base lines that may be model dependent, and the model used in the analysis of the data.     

Diagrammatic kinetic theory for a lattice model of a liquid. II. Comparison of theory and simulation results

We compare the predictions of the mean field, the two site multiple scattering, and the simple mode coupling approximation developed in the previous paper for the dynamics of a tagged particle in an excluded volume lattice gas with the results of computer simulations. The tagged particle has a transition rate of gamma while the background particles have transition rates of alphagamma. We consider the tracer diffusion coefficient and the incoherent intermediate scattering function (IISF) for low, intermediate, and high concentrations of particles and for simple square and cubic lattices. In general, the approximate kinetic theories are more accurate in predicting simulations results at low concentrations, high dimensions, and large alpha. For the tracer diffusion coefficient, the mean field approximation is the least accurate, the two site multiple scattering approximation is more accurate, and the simple mode coupling approximation is the most accurate; all three approximate theories overestimate the simulation results. For the IISF, the mean field approximation is quantitatively accurate in the limit of small concentration and large alpha but in general decays too quickly. The two site multiple scattering approximation is quantitatively accurate at low and intermediate concentrations for large wave vectors; it is always more accurate than the mean field approximation and always decays more quickly than the simulation results. The simple mode coupling approximation is the most accurate of the three approximations in most cases and especially so for small wave vectors, high concentration, and small alpha; unfortunately, its predictions are not quantitatively accurate in these highly nonmean field regimes. We discuss the implications of these results for developing diagrammatic kinetic theories.     

A slender-body micromechanical model for viscoelasticity of magnetic colloids: comparison with preliminary experimental data

The storage modulus, G', together with the yield stress, is an essential quantity characterizing the rheological properties of magnetic field-responsive suspensions (magnetorheological fluids or MRF). In this work, we present both experimental and theoretical results on the viscoelastic properties of MRFs. Two MRFs are used: In one the solid phase consists of cobalt ferrite particles + silica gel, with silicone oil as liquid phase. The second system is formed by carbonyl iron + silica gel also dispersed in silicone oil. The cobalt ferrite particles are synthesized as monodisperse colloidal spheres with an average diameter of 850 nm. We describe a new model based on the slender-body approach for hydrodynamic interactions. The predictions of the model are compared to preliminary experimental G' data obtained in a controlled stress plate-plate rheometer. It is found that the model gives the correct order of magnitude for the highest fields in iron suspensions, but underestimates the experimental results obtained in ferrite ones. In the case of high permeability materials such as carbonyl iron, by the inclusion of high-order multipolar interactions and saturation effects we also predict the order of magnitude of the experimental results. When dealing with low permeability cobalt ferrite based MRFs, other effects, such as remanence (at low fields) and saturation (at high fields), must be considered.     

The electrocrystallisation of nickel: Part II. comparison of models with the experimental data

Theoretical equations derived for general models of the electrocrystallisation of nickel are fitted to the deposition transients. The formation of one or more monolayers is shown to be followed by three-dimensional nucleation and growth of the bulk phase; synchronised “death” and “rebirth” processes are shown to be operative.     

Modeling the effect of cell-associated polymeric fluid layers on force spectroscopy measurements. Part II: experimental results and comparison with model predictions

In this paper, experimentally obtained force curves on Staphylococcus aureus are compared with a previously developed model that incorporates hydrodynamic effects of extracellular polysaccharides together with the elastic response of the bacterium and cantilever. Force-displacement curves were predicted without any adjustable parameters. It is demonstrated that experimental results can be accurately described by our model, especially if viscoelastic effects of the extracellular polysaccharide layer are taken into account. Polysaccharide layer viscoelasticity was treated by means of a multimode Phan-Thien/Tanner (PTT) constitutive equation. Typical maximum relaxation times range from 0.2 to 2 s, whereas the corresponding zero-shear-rate viscosities are 6-9 Pa.s, based on published, steady-state rheological measurements on Staphylococcus aureus polysaccharide extracted from its native environment. The bacterial elastic constant is found to be in the range 0.02-0.4 N/m, corresponding to bacterial wall Young's moduli in the range of a few hundred MPa. Repeatability of measurements performed on different bacteria is found to be only fair, due to large individuum variability, whereas repetitions of measurements on the same bacterium showed high reproducibility. Improved force-indentation curve predictions are expected if transient rheological characterization of extracellular polysaccharides is available. More desirable however is the direct, in vivo rheological characterization of the extracellular polysaccharide. A model-based analysis of experimental force-indentation curves shows that appreciable further experimental improvements are still necessary to achieve this goal.     

Differential thermometric kinetic analysis. a new general technique of interpreting the data obtained in kinetic analyses

The concentration-time data obtained in kinetic analyses may be easily and conveniently interpreted by multiparametric curve-fitting. A new technique of kinetic analysis, in which the overall extent of reaction is monitored by following the variation with time of the difference between the temperature of a reaction mixture and that of a reference solution, is described. Both these techniques are illustrated with-data obtained in oximations of mixtures of propanal-and cyclohexanone. The results confirm the existence of the synergic effect reported by Siggia and Hanna.     

Electrode reaction of Pu3+/Pu couple in LiCl-KCl eutectic melts: comparison of the electrode reaction at the surface of liquid Bi with that at a solid Mo electrode.

The electrode reaction of Pu3+/Pu couple at the interface between LiCl-KCl eutectic melt containing PuCl3 and liquid Bi phase was investigated by cyclic voltammetry at 723, 773 and 823 K. For comparison, the system was also studied using a solid Mo electrode in place of the liquid Bi electrode. It was found that the electrode reaction of Pu3+/Pu couple at the Bi electrode was almost reversible. The redox potentials of Pu3+/Pu couple at the liquid Bi electrode in the LiCl-KCl eutectic melt containing 0.87 wt% PuCl3 (0.0014 mole fraction) at 723, 773 and 823 K were observed to be more positive by 0.575, 0.572 and 0.566 V, respectively, than those at the Mo electrode. These differences in potential were thermodynamically analyzed by assuming a lowering of the activity of Pu in Bi phase according to the alloy formation of PuBi2.     

Uranium ore processing: Kinetics of the growth and dissolution of a potential host solid from a model surface

As part of an investigation concerning the fate of²²⁶Ra during uranium ore milling and long-term tailings storage, we have investigated the growth of potential host solids-barium sulfate and lead sulfate-on mica, a model surface. The kinetics of dissolution of barium sulfate-and of a barium sulfate solid containing²²⁸Ra as a trace component into 10^–3M sulfuric acid were subsequently investigated using a flow-cell and radiotracer techniques. It was found that ther dissolution of both the barium sulfate host and the²²⁸Ra trace component was congruent. The dissolution reaction appears to be surface controlled.     

Functional transformation of experimental data and the chemical model of equilibrium for polynuclear systems-1. Family of auxiliary concentration functions curves with common cross-over point

The differential relationships between concentration variables and some of the auxiliary concentration functions for systems with complex formation reactions have been obtained. Systems which give a family of concentration functions with a common cross-over point are discussed in detail. The derived equations were shown to be useful in the determination of the chemical model of equilibrium as well as for evaluating the accuracy of available data. The proposed approach was applied for the determination of composition and stability of complexes in some real systems.