Kinetic equations for thermal dissociation processes

Relationships between average degree of transformation and time of dissociation were derived for polydisperse granular materials with account to the type of grain size distribution. It has been checked, under what conditions the kinetic curves obtained by numeric solution of those relationships may be described in terms of equations D1 α²=kt , F2 [1/(1 - α) - 1 =kt] and F3 [1/(1 - α)² - 1=kt]. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetic theory of self-condensing vinyl polymerization

This review introduces the kinetic theory of self-condensing vinyl polymerization (SCVP), including the SCVP of AB* inimers, the SCVP with non-equal reactivity between A* and B* groups, the SCVP in the presence of a small amount of multifunctional initiators, also the SCVP of both inimers and comonomers. The analytical expressions of various molecular parameters for the resulting hyperbranched polymers, such as the molecular size distribution function, the average molecular weight, the polydispersity index and the degree of branching, are reviewed systematically.     

Kinetic investigations on some analytical catalytic processes

Summary Kinetic investigations have been carried out on the rate of oxidation ofp-phenetidine with potassium chlorate in the presence of vanadium as a catalyst and phenol as activator. The activation energy of the reaction was found to be 11400 cal/mol. It has been established that citric acid has an activating effect on the process. It is assumed that this is due to the formation of stable complexes with vanadium with a stronger catalytic effect. The important difference between the action of phenols and citric acid is that while phenols take part directly in the formation of reaction-products, citric acid only increases the activity of the catalyst.

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Zusammenfassung Reaktionskinetische Untersuchungen der Oxydation von p-Phenetidin mit Kaliumchlorat bei Gegenwart von Vanadium als Katalysator und Phenol als Aktivator wurden ausgeführt. Die Aktivierungsenergie der Reaktion beträgt 11400 kal/Mol. Ein aktivierender Effekt von Zitronensäure wurde festgestellt. Es wird angenommen, daß dieser auf der Bildung eines stabilen Vanadiumkomplexes mit verstärkter katalytischer Wirkung beruht. Der wesentliche Unterschied in der Wirkung des Phenols und der Zitronensäure besteht darin, daß sich Phenol unmittelbar an der Bildung von Reaktions-produkten beteiligt, während die Zitronensäure nur die Aktivität des Katalysators verstärkt.

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Résumé On a effectué l'étude cinétique de la vitesse d'oxydation de lap-phénétidine par le chlorate de potassium en présence de vanadium comme catalyseur et du phénol comme activateur. On a trouvé 11400 cal/mol pour valeur de l'énergie d'activation de la réaction. On a établi que l'acide citrique présente un effet d'activation sur le processus. On suppose que ce phénomène est dû à la formation de complexes stables avec le vanadium, présentant un effet catalytique plus marqué. La différence importante entre l'action des phénols et celle de l'acide citrique vient du fait que, tandis que les phénols prennent part directement à la formation des produits de réaction, l'acide citrique augmente seulement l'activité du catalyseur.

     

Kinetic approach to partially overlapped thermal decomposition processes

Practical usefulness of the kinetic deconvolution for partially overlapped thermal decomposition processes of solids was examined by applying to the co-precipitated basic zinc carbonate and zinc carbonate. Comparing with the experimental deconvolutions by thermoanalytical techniques and mathematical deconvolutions using different statistical fitting functions, performance of the kinetic deconvolution based on an accumulative kinetic equation for the independent processes overlapped partially was evaluated in views of the peak deconvolution and kinetic evaluation. Two-independent kinetic processes of thermal decompositions of basic zinc carbonate and zinc carbonate were successfully deconvoluted by means of the thermoanalytical measurements in flowing CO₂ and by applying sample controlled thermal analysis (SCTA). The deconvolutions by the mathematical curve fittings using different fitting functions and subsequent formal kinetic analysis provide acceptable values of the mass-loss fractions and apparent activation energies of the respective reaction processes, but the estimated kinetic model function changes depending on the fitting functions employed for the peak deconvolution. The mass-loss fractions and apparent kinetic parameters of the respective reaction processes can be optimized simultaneously by the kinetic deconvolution based on the kinetic equation through nonlinear least square analysis, where all the parameters indicated acceptable correspondences to those estimated through the experimental and mathematical deconvolutions. As long as the reaction processes overlapped are independent kinetically, the simple and rapid procedure of kinetic deconvolution is useful as a tool for characterizing the partially overlapped kinetic processes of the thermal decomposition of solids.     

Kinetic modeling of dynamic processes in the cholinergic synapse

A kinetic model describing the dynamics of synaptic “discharge”, taking into account the kinetics of the neurotransmitter injection into the synaptic cleft, pH dependence of the enzyme catalytic activity, and proton removal by diffusion was proposed and studied. In the framework of the kinetic model, functioning of the cholinergic synapse was considered. The results of mathematical modeling of the change in the acetylcholine level, induced pH impulse, and the effect of the impulse transmission frequency and acetylcholinesterase inhibition are presented. A physicochemical interpretation was given for a number of key important physiological phenomena, such as neuromuscular paralysis, the mechanism of information recording and storage in the neurological memory, the action of nerve poisons and toxins, and Alzheimer’s disease.

     

Kinetic Monte Carlo study of nucleation processes on patterned surfaces

The properties of template-directed nucleation are studied in the transition region where full nucleation control is lost and additional nucleation beyond the prepatterned structure is observed. To get deeper insight into the microscopic mechanisms, Monte Carlo simulations were performed. In this context, the previously used continuous algorithm [F. Kalischewski, J. Zhu, and A. Heuer, Phys. Rev. B 77, 155401, (2008)] was replaced by a discrete one to reduce simulation time and to allow more detailed calculations. The applied method is based on the assumption that the molecules on the surface occupy the sites of a simple fcc lattice. It is shown that a careful mapping of the continuous Monte Carlo technique onto the discrete algorithm leads to a good reproduction of the former results by means of the latter method. Furthermore, the new method facilitates the calculation of the spatial distribution of nuclei on the surface. This provides a detailed comparison with experimental data.     

A general theory of surface seperation processes

A thermodynamic theory of surface separation has been formulated for mixtures of both inert and chemically interacting substances. The behaviour of the surface separation lines in the vicinity of singular points in concentration diagrams has been investigated. As an example, the behaviour of the foam separation lines in dilute three- and four-component solutions has been considered. The possibility of classification of systems by means of a thermodynamic—topological rule has been exhibited.     

Kinetic energy decomposition scheme based on information theory

We proposed a novel kinetic energy decomposition analysis based on information theory. Since the Hirshfeld partitioning for electron densities can be formulated in terms of Kullback–Leibler information deficiency in information theory, a similar partitioning for kinetic energy densities was newly proposed. The numerical assessments confirm that the current kinetic energy decomposition scheme provides reasonable chemical pictures for ionic and covalent molecules, and can also estimate atomic energies using a correction with viral ratios.     

Kinetic study of the crystallization processes of some decorative ceramic glazes

A kinetic study of the crystallization processes was performed for some decorative ceramic glazes in the PbO-SiO₂-Na₂O-K₂O-CaO(BaO)-Al₂O₃-B₂O₃ system with addition of 10% TiO₂ and ZnO. The crystallization kinetics has been studied in non-isothermal conditions using DTA technique. The apparent activation energies of the crystallization processes were calculated using the Kissinger method. The main crystalline phase, which provides the decorative effect, is rutile. This has been identified by X-ray diffraction and it is clearly visible in the optical microscopy images taken in transmitted light, as needle-like or even prismatic crystals arranged in radial-fibrous aggregates.     

Kinetic theory derivation of the transport coefficients of stochastic rotation dynamics

We use a kinetic theory approach to derive the continuum Navier-Stokes and heat conduction equations for stochastic rotation dynamics, a particle based algorithm for simulating a fluid. Hence we obtain expressions for the viscosity and thermal conductivity in two and three dimensions. The predictions are tested numerically and good agreement is found.     

Density-functional theory studies on microscopic processes of gaas growth

Results for the elementary processes of MBE growth of GaAs on the frequently used GaAs(001) substrate are reviewed. We propose a bottom-up approach, where a growth model is constructed from the results of density functional theory (DFT) calculations. The implications of such a model can be tested against the information from STM images. First, the stable surface reconstructions are reviewed. Under the most commonly used conditions for MBE growth, the arsenic-rich β2 (2 × 4) reconstruction, which contains As dimers as basic building blocks, is the most stable. Next, the adsorption and diffusion of Ga atoms and As molecules on this surface is described. The DFT calculations support the picture that adsorbed Ga atoms are quite stable against re-evaporation. Thus, their mobility determines the homogeneity of the growing layer. Incorporation of Ga atoms proceeds by splitting the As dimers. We propose a model where growth proceeds in two stages: filling of trenches in the β2 (2 × 4) reconstruction, followed by nucleation of islands on the surface regions where the trenches are filled. We demonstrate how clusters of incorporated Ga atoms act as nuclei for the process of trench filling. Concerning island formation, the role of step formation energies and attachment probabilities of mobile adatoms at steps is discussed. Knowledge of these is crucial for an understanding of island shapes. Ongoing research is aiming at understanding of the microscopic mechanisms giving rise to the transition between the step-flow mode and the island-nucleation mode of growth.     

Kinetic parameters of the thermal decomposition processes of certain cobalt and titanium coprecipitates

Journal of Thermal Analysis and Calorimetry - The kinetic parameters of the thermal decomposition reactions of certain coprecipitates were calculated by a thermogravimetric method. The significance...     

Kinetic theory of nucleation based on a first passage time analysis: improvement by the density-functional theory

A recent kinetic theory of nucleation [see, e.g., E. Ruckenstein and B. Nowakowski, J. Colloid Interface Sci. 137, 583 (1990)] is based on molecular interactions and avoids the traditional thermodynamics. The rate of emission of molecules from a cluster is found via a first passage time analysis. This time is calculated by solving the single-molecule master equation for the probability distribution function of a surface molecule located in the potential field created by the cluster. The liquid cluster was assumed to have sharp boundaries and uniform density. In the present paper, this assumption is removed by using the density-functional theory to find the density profiles. Thus, more accurate calculations of the potential field created by the cluster, its emission rate, and nucleation rate are obtained. The modified theory is illustrated by numerical calculations for a molecular pair interaction potential combining the dispersive attraction with the hard-sphere repulsion.     

Simulation of stochastic model processes in the theory of stark-broadening

Calculations following the Method of Model Microfields (MMM) are performed by simulating various model processes. The approach via simulations gives simplifications for the numerical evaluation of the line profile. Furthermore it is shown how to check with its help the validity of certain approximations introduced in the MMM and in general Stark-broadening theory. Results for Lyman-α are shown.     

The general theory of irreversible processes in solutions of macromolecules

The general theory of irreversible processes in solutions of macromolecules, previously formulated by the author, is reviewed. The theory is based upon the Oseen method for determining the perturbation in the hydrodynamic flow pattern produced by the frictional forces exerted by the macromolecule on the solvent, and on a generalized theory of Brownian motion in molecular configuration space. Applications of theory to viscoelastic behavior, flow birefringence, and the Kerr effect, and to dielectric dispersion are presented in outline.     

Kinetic theory of binary nucleation based on a first passage time analysis

The binary classical nucleation theory (BCNT) is based on the Gibbsian thermodynamics and applies the macroscopic concept of surface tension to nanosize clusters. This leads to severe inconsistencies and large discrepancies between theoretical predictions and experimental results regarding the nucleation rate. We present an alternative approach to the kinetics of binary nucleation which avoids the use of classical thermodynamics for clusters. The new approach is an extension to binary mixtures of the kinetic theory previously developed by Narsimhan and Ruckenstein and Ruckenstein and Nowakowski [J. Colloid Interface Sci. 128, 549 (1989); 137, 583 (1990)] for unary nucleation which is based on molecular interactions and in which the rate of emission of molecules from a cluster is determined via a mean first passage time analysis. This time is calculated by solving the single-molecule master equation for the probability distribution of a "surface" molecule moving in a potential field created by the cluster. The starting master equation is a Fokker-Planck equation for the probability distribution of a surface molecule with respect to its phase coordinates. Owing to the hierarchy of characteristic time scales in the evolution of the molecule, this equation can be reduced to the Smoluchowski equation for the distribution function involving only the spatial coordinates. The new theory is combined with density functional theory methods to determine the density profiles. This is essential for nucleation in binary systems particularly when one of the components is surface active. Knowing these profiles, one can determine the potential fields created by the cluster, its rate of emission of molecules, and the nucleation rate more accurately than by using the uniform density approximation. The new theory is illustrated by numerical calculations for a model binary mixture of Lennard-Jones monomers and rigidly bonded dimers of Lennard-Jones atoms. The amphiphilic character of the dimer component (i.e., its surface activity) is induced by the asymmetry in the interaction between a monomer and the two different sites of a dimer. The inconsistencies of the BCNT are avoided in the new theory.