Kinetics of Rate Processes on Uniform Surfaces

1991 ◽  
pp. 191-230
Author(s):  
V. P. Zhdanov
Keyword(s):  
Rate Processes ◽  
Kinetics Of

ANOMALOUS STRESS RELAXATION BEHAVIOR OF POLYCRYSTALLINE ALUMINUM AT LOW TEMPERATURE

2007 ◽  
Vol 21 (10) ◽  
pp. 1745-1754 ◽  
Author(s):  
I. M. GHAURI ◽  
NAVEED AFZAL ◽  
M. ANWAR ◽  
S. A. SIDDIQUE
Keyword(s):  
Stress Relaxation ◽  
Relaxation Rate ◽  
Deformation Process ◽  
Tensile Deformation ◽  
Polycrystalline Aluminum ◽  
Rate Processes ◽  
Basic Equations ◽  
Relaxation Curves ◽  
Kinetics Of ◽  
Barrier Model

The tensile deformation of polycrystalline aluminum (99.999%) was studied between 18 and 300 K. The stress-relaxation at constant strain was determined at strain intervals of about 0.5% with total strain exceeding about 3%. Stress-relaxation curves were logarithmic except at large "t" where they flatten. The relaxation rate "s" was determined by using equation s=d(Δσ)/d ln t, where Δσ(t)=σ0-σ(t) is the amount of stress relaxed at any instant of time "t" from the initial stress level σ0 at which relaxation was allowed to start. The slope ds/dσ0 was observed to attain maxima at about 30 K and minima at about 60 K. The undulation in the temperature dependence of stress relaxation rate in the range 18–60 K is an outcome of changes in the substructures of dislocations which have developed during the deformation process. These changes then require stresses higher than that applied in the basic equations which describe the kinetics of the mode of deformation. The average intrinsic height of potential barrier U0, estimated by means of the single barrier model of stress relaxation, was 1.2±0.3 eV. These values appeared to be compatible with the dislocation-dislocation intersection, controlling the rate processes in polycrystalline aluminum.

The measurement of rapid rate processes by kinetic spectroscopy during a photolysis flash; vibrational relaxation of NO X 2 II from levels v 2

1972 ◽  
Vol 328 (1575) ◽  
pp. 515-527 ◽  
Keyword(s):  
Optical Density ◽  
Vibrational Relaxation ◽  
Energy Exchange ◽  
Vibrational Energy ◽  
Excited Level ◽  
Vibrational Levels ◽  
Order Of Magnitude ◽  
Rate Processes ◽  
Kinetics Of ◽  
Good Agreement

The recording of transient changes in optical density that take place during the period of a photolysis flash can, in principle, allow measurement of the kinetics of photo-processes having half-lives an order of magnitude less than the rise or decay time of the flash itself. The construction and use of a sensitive, ‘split-beam’ kinetic spectrophotometer is described, which permits the detection of transient changes in optical density > 0.01 and the measurement of half-lives >1 (us. The apparatus has been used to study the relaxation of NOX 2 II from its first and second excited vibrational levels in the presence of N 2 0 or CH4 and/or Ar. The efficiency of vibrational energy exchange with N 2 0 decreases with the vibrational quantum number of the excited level and this is shown to be consistent with the reduction in the vibrational spacing caused by anharmonicity. The measured collision numbers are in good agreement with those calculated on the baste of an empirical correlation (Callear 1965).

scholarly journals Tip regeneration and positional information in the slug of Dictyostelium discoideum

Development ◽  
1983 ◽  
Vol 73 (1) ◽  
pp. 151-162
Author(s):  
Balakrishna L. Lokeshwar ◽  
Vidyanand Nanjundiah
Keyword(s):  
Dictyostelium Discoideum ◽  
Sudden Change ◽  
Positional Information ◽  
Anterior Portion ◽  
Regeneration Time ◽  
Cellular Slime ◽  
Rate Processes ◽  
Regeneration Times ◽  
Kinetics Of ◽  
Positional Value

We show in this paper that in the case of the slug of the cellular slime mould Dictyostelium discoideum the time which it takes for a new tip to regenerate at a given level can be used as a measure of positional information at that level. Our basic experiment consists ofamputating slugs at various distances from the existing tip and thereby inducing the regeneration of a fresh tip; the time needed for regeneration is estimated by two independent methods. An identical operation, when performed in the anterior portion of a previously cut slug, tells us how this position-dependent regeneration time adjusts to a sudden change in the size of the slug. The reasons which lead us to conclude that tip regeneration times are in one-to-one correspondence with positional information are as follows, (i) Depending on their positions, the cells in a slug take different times to regenerate a new tip following amputation; (ii) regeneration times are scaled in relation to the total length of the slug and increase monotonically with the length cut off; and (iii) as judged by the regeneration time, cells can assess and remember their positions relative to the length of the slug. Our results highlight the importance of two rate processes. One might think of the slower process as being related to the setting up of a system of positional information in the slug, and the faster process as being a reflection of the kinetics of the positional value changing locally till it reaches the level appropriate to a tip.

General Anesthetics Modify the Kinetics of Nicotinic Acetylcholine Receptor Desensitization at Clinically Relevant Concentrations 

1995 ◽  
Vol 82 (1) ◽  
pp. 276-287 ◽  
Author(s):  
Douglas E. Raines ◽  
Saffron E. Rankin ◽  
Keith W. Miller
Keyword(s):  
Ion Channels ◽  
Acetylcholine Receptor ◽  
Nicotinic Acetylcholine Receptor ◽  
Electric Organ ◽  
General Anesthetics ◽  
Nicotinic Acetylcholine ◽  
Rate Processes ◽  
Kinetics Of ◽  
Gated Ion Channels ◽  
Ligand Gated Ion Channels

Background General anesthetics are thought to induce anesthesia through their actions on ligand-gated ion channels. One such channel, the nicotinic acetylcholine receptor (nAcChoR), can be found in different subtypes in the central nervous system and at the periphery in the neuromuscular junction. The latter subtype of the nAcChoR is a useful model for examining interactions between general anesthetics and ligand-gated ion channels, because it can be isolated and purified in sufficient quantities to allow for biophysical and biochemical studies. This study examines the actions of general anesthetics on agonist-induced conversion of the nAcChoR to inactive desensitized conformational states. Methods Nicotinic acetylcholine receptor membranes were purified from the electric organ of Torpedo nobiliana. Agonist-induced desensitization was characterized from the time-dependent increase in fluorescence intensity that results from the binding of the fluorescent acetylcholine analog, Dns-C6-Cho, to the nAcChoR. Results Mixing Dns-C6-Cho with nAcChoR-rich membranes results in an increase in fluorescence that is characterized by four rate processes. Concentrations of isoflurane and butanol, which range from subclinical to toxic increase the rates of the third and fourth components of fluorescence, corresponding to fast and slow desensitization, respectively. At concentrations that are twice their EC50s for anesthesia, isoflurane, butanol, chloroform, methanol, and cyclopentanemethanol increase the apparent rates of fast and slow desensitization by an average of 92 +/- 22% and 108 +/- 22%, respectively. Conclusions The concentration range over which general anesthetics modify the kinetics of nAcChoR desensitization is similar to those reported for anesthetic actions on the GABAA receptor. Thus, the nAcChoR, like other members of this superfamily, is a sensitive target of general anesthetics.

Mechanism of oxidation of alanine by chloroaurate(III) complexes in acid medium: Kinetics of the rate processes

2009 ◽  
Vol 41 (7) ◽  
pp. 473-482 ◽  
Author(s):  
Pratik K. Sen ◽  
Nasimul Gani ◽  
Jayanta K. Midya ◽  
Biswajit Pal
Keyword(s):  
Acid Medium ◽  
Rate Processes ◽  
Mechanism Of Oxidation ◽  
Kinetics Of

The kinetics of deuterium-neopentane exchange: effects of desorption and diffusion

1968 ◽  
Vol 302 (1469) ◽  
pp. 253-270 ◽  
Keyword(s):  
Product Distribution ◽  
Single Step ◽  
Surface Exchange ◽  
Product Distributions ◽  
Exchange Effects ◽  
Alumina Particles ◽  
Silica Alumina ◽  
Rate Processes ◽  
Kinetics Of ◽  
And Diffusion

The product distribution of isomers from the deuterium exchange of neopentane over palladium supported on silica-alumina particles is influenced by the competition among three rate processes: desorption, diffusion, and surface exchange. The theory based on a single-step surface mechanism is presented and given quantitative treatment, suitable for large conversions. The experimental results show a very wide variety of product distributions over the same catalyst, and can be quantitatively correlated by the theory.

scholarly journals Modelling the kinetics of water loss during deep-fat frying of potato particulates

2014 ◽  
Vol 32 (No. 6) ◽  
pp. 585-594 ◽  
Author(s):  
P.-Y.H. Huang ◽  
Y.-Ch. Fu
Keyword(s):  
Water Loss ◽  
Structural Changes ◽  
Effect Of Temperature ◽  
Order Kinetic ◽  
Deep Fat Frying ◽  
First Order ◽  
Before And After ◽  
Order Kinetic Model ◽  
Rate Processes ◽  
Kinetics Of

We developed an empirical model to describe the water loss during deep-fat frying. Raw potato particulates were sliced to form cylinders and subjected to the deep-fat frying at isothermal temperatures of 160, 190, and 220&deg;C. The microstructure properties were assessed by Field Emission Scanning Electron Microscope (FESEM). The plot of the water content versus the frying time showed two distinct regions. A first-order kinetic model correlated with the two irreversible serial rate processes, rapid process and slow process, was hypothesised to describe the water loss during frying. The results showed the simultaneous two first-order kinetic models adequately predicted the water loss of potato particulates during isothermal frying. The effect of temperature on the rate constants, k<sub>1</sub> and k<sub>2</sub>, for the two processes was adequately modelled by the Arrhenius relationship. The observations of structural changes on the surface and in the inner section of potato particulates are critical. These physical pieces of evidence support our assumption that the mechanisms of the water loss (two-stage rate processes) before and after the transition time are different. &nbsp;

scholarly journals From the Kinetic Theory of Gases to the Kinetics of Rate Processes: On the Verge of the Thermodynamic and Kinetic Limits

Molecules ◽  
2020 ◽  
Vol 25 (9) ◽  
pp. 2098 ◽  
Author(s):  
Valter H. Carvalho-Silva ◽  
Nayara D. Coutinho ◽  
Vincenzo Aquilanti
Keyword(s):  
Thermodynamic Limit ◽  
Large Volume ◽  
Reaction Rates ◽  
Partition Functions ◽  
Arrhenius Behavior ◽  
Initial State ◽  
Arrhenius Plots ◽  
Rate Processes ◽  
Kinetics Of ◽  
Number Of Particles

A variety of current experiments and molecular dynamics computations are expanding our understanding of rate processes occurring in extreme environments, especially at low temperatures, where deviations from linearity of Arrhenius plots are revealed. The thermodynamic behavior of molecular systems is determined at a specific temperature within conditions on large volume and number of particles at a given density (the thermodynamic limit): on the other side, kinetic features are intuitively perceived as defined in a range between the extreme temperatures, which limit the existence of each specific phase. In this paper, extending the statistical mechanics approach due to Fowler and collaborators, ensembles and partition functions are defined to evaluate initial state averages and activation energies involved in the kinetics of rate processes. A key step is delayed access to the thermodynamic limit when conditions on a large volume and number of particles are not fulfilled: the involved mathematical analysis requires consideration of the role of the succession for the exponential function due to Euler, precursor to the Poisson and Boltzmann classical distributions, recently discussed. Arguments are presented to demonstrate that a universal feature emerges: Convex Arrhenius plots (super-Arrhenius behavior) as temperature decreases are amply documented in progressively wider contexts, such as viscosity and glass transitions, biological processes, enzymatic catalysis, plasma catalysis, geochemical fluidity, and chemical reactions involving collective phenomena. The treatment expands the classical Tolman’s theorem formulated quantally by Fowler and Guggenheim: the activation energy of processes is related to the averages of microscopic energies. We previously introduced the concept of “transitivity”, a function that compactly accounts for the development of heuristic formulas and suggests the search for universal behavior. The velocity distribution function far from the thermodynamic limit is illustrated; the fraction of molecules with energy in excess of a certain threshold for the description of the kinetics of low-temperature transitions and of non-equilibrium reaction rates is derived. Uniform extension beyond the classical case to include quantum tunneling (leading to the concavity of plots, sub-Arrhenius behavior) and to Fermi and Bose statistics has been considered elsewhere. A companion paper presents a computational code permitting applications to a variety of phenomena and provides further examples.

Kinetics of polymer crystallization - I. Discrete and continuum models

1988 ◽  
Vol 416 (1851) ◽  
pp. 443-461 ◽  
Keyword(s):  
Crystal Structure ◽  
One Dimensional ◽  
Ising Antiferromagnet ◽  
Equilibrium Distributions ◽  
Rate Processes ◽  
Lamellar Crystals ◽  
Kinetics Of ◽  
Dynamic Reversibility ◽  
Approximate Equations ◽  
The Continuum

The kinetics of growth of lamellar crystals by chain folding of polymer molecules are described by Markov rate processes whose states are representations of the edge of a lamella. The dynamic reversibility of these processes allows their equilibrium distributions to be found and these describe states of steady crystal growth. For a hexagonal crystal structure the equilibrium distribution is the Gibbs distribution for a constrained, one-dimensional Ising antiferromagnet. For a square crystal structure it is a constrained exponential distribution. These distributions provide a description of the roughness of the edge of a growing crystal and expressions for the growth rate. The continuum limit of these models is shown to coincide with the model of Frank and of Bennett et al . ( J. statist. Phys . 24, 419 (1981)). Frank’s approximate equations (Frank, F. C. J. Cryst. Growth 22, 233 (1974)) are also examined.

Energy Generators Driven by Nonlinear Transport Phenomena

Volume 1 ◽  
2004 ◽  
Author(s):  
Stanislaw Sieniutycz
Keyword(s):  
Chemical Reactions ◽  
Thermal Energy ◽  
Heat Pumps ◽  
Heat Fluxes ◽  
Nonlinear Transport ◽  
Contemporary Theory ◽  
Recent Approach ◽  
Rate Processes ◽  
Unidirectional Fluxes ◽  
Kinetics Of

We describe quantitatively effects of nonlinear transfer phenomena that drive energy generators (thermal engines) and heat pumps. It is shown that these transports can be treated either in a standard way or as peculiar rate processes (chemical reactions governed by appropriate affinities). An approach to nonlinear transports links heat fluxes with differences of temperature in certain power Ta. A more recent approach distinguishes in each elementary transfer step two competitive (unidirectional) fluxes and the resulting flux as their difference. We show how the kinetics of this sort can be implemented into the contemporary theory of thermal energy generators.

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