Kinetic Rate Constants (kinetic + rate_constant)

Distribution by Scientific Domains

Selected Abstracts

Kinetics and Molecular Weight Development of Dithiolactone-Mediated Radical Polymerization of Styrene

Jesús Guillermo Soriano-Moro
Abstract Calculations of polymerization kinetics and molecular weight development in the dithiolactone-mediated polymerization of styrene at 60,°C, using 2,2,-azobisisobutyronitrile (AIBN) as initiator and , -phenyl- , -butirodithiolactone (DTL1) as controller, are presented. The calculations were based on a polymerization mechanism based on the persistent radical effect, considering reverse addition only, implemented in the PREDICI® commercial software. Kinetic rate constants for the reverse addition step were estimated. The equilibrium constant (K,=,kadd/k -add) fell into the range of 105,106 L,·,mol,1. Fairly good agreement between model calculations and experimental data was obtained. [source]

Study of Electrochemical Processes with Coupled Homogeneous Chemical Reaction in Differential Pulse Voltammetry at Spherical Electrodes and Microhemispheres

Eduardo Laborda
Abstract Homogeneous chemical reactions coupled to oxidation-reduction processes at electrode surfaces are very common in electrochemistry. In this paper we cope with the application of Differential Pulse Voltammetry at spherical electrodes and microhemispheres for the study of this kind of systems. Analytical expressions are deduced from which the influence of the different experimental variables is examined. Several diagnostic criteria for elucidation of the reaction mechanism from DPV peak parameters are given, as well as working curves for extraction of the kinetic rate constants of the chemical reaction. [source]

Kinetic modeling of aqueous phenol degradation by UV/H2O2 process

Maryam Edalatmanesh
A dynamic kinetic model for the oxidation of phenol in water by an UV/H2O2 process is developed. The model is based on the elementary chemical and photochemical reactions, initiated by the photolysis of hydrogen peroxide into hydroxyl radicals. The model is validated by using experimental data obtained from the open literature for an actual UV/H2O2 process. Using those data and the developed kinetic model, kinetic rate constants for phenol intermediates, catechol and hydroquinone, are estimated. Moreover, the optimum initial hydrogen peroxide concentration is estimated by means of the validated model. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 40: 34,43, 2008 [source]

Influence of Storage Temperature on the Kinetics of the Changes in Anthocyanins, Vitamin C, and Antioxidant Capacity in Fresh-Cut Strawberries Stored under High-Oxygen Atmospheres

I. Odriozola-Serrano
ABSTRACT:, Changes in the main antioxidant properties of fresh-cut strawberries stored under high-oxygen atmospheres (80 kPa O2) were studied at selected temperatures (5 to 20 °C). The suitability of zero- and 1st-order kinetics as well as a model based on Weibull distribution function to describe changes in experimental data is discussed. A non-Arrhenius approach was used to determine the temperature dependence of the estimated rate constants. A Weibull kinetic model most accurately (R2adj, 0.800) estimated changes in anthocyanins and antioxidant capacity of fresh-cut strawberries throughout the storage period, whereas a 1st-order model adequately fitted (R2adj, 0.982) the variation of vitamin C. The temperature dependency of the kinetic rate constants for each antioxidant property was successfully modeled through the non-Arrhenius approach (R2adj, 0.709). The Tc obtained for anthocyanins, vitamin C, and antioxidant capacity degradation were 290, 284, and 289 K, respectively, indicating the temperature at which a marked acceleration of the losses in the antioxidant potential of strawberry wedges occurs. These findings will help to describe the variation of the antioxidant potential of fresh-cut strawberries upon storage time and temperature. [source]

Reduction of Acrylamide and Its Kinetics by Addition of Antioxidant of Bamboo Leaves (AOB) and Extract of Green Tea (EGT) in Asparagine,Glucose Microwave Heating System

Yu Zhang
ABSTRACT:, This study investigated the effect of antioxidant of bamboo leaves (AOB) and extract of green tea (EGT) on the formation and kinetics of acrylamide in an equimolar asparagine,glucose model system. The substrates spiked with AOB and EGT were microwave-heated at 180 °C and the acrylamide content in final reaction products was quantified by ultra-performance liquid chromatography,tandem mass spectrometry (UPLC-MS/MS). The results showed that both AOB and EGT could effectively reduce the formation of acrylamide in an asparagine,glucose microwave heating model system and achieved a maximum reduction rate when the addition levels of AOB and EGT were both 10,6 mg/mL reaction solution. To describe the kinetic behavior of acrylamide, a simplified kinetic model was optimized and relative kinetic rate constants were evaluated under isothermal conditions. The results indicated that the reduction effect of AOB and EGT on the acrylamide formation may partly be ascribed to the decrease of the formation rate constant (kF) in both AOB and EGT-spiked systems (43.4% and 32.3% of decrease, respectively, P < 0.05). The kinetic parameter kE, which represents the elimination rate of acrylamide in both AOB and EGT-spiked systems, was not significantly different (6.9% of increase and 10.9% of decrease, respectively, P > 0.05). The results of the kinetic study indicated that addition of AOB and EGT could significantly reduce the formation rate constant (kF) of acrylamide, but could not significantly affect the elimination rate constant (kE) of acrylamide. [source]

Structural identifiability analysis of the dynamic gas,liquid film model

AICHE JOURNAL, Issue 8 2006
J. Navarro-Laboulais
Abstract A structural identification analysis of the dynamic gas,liquid film model is applied to identify its theoretically accessible parameters. The analysis was performed considering the different experimental situations found in nonstationary bubble-column reactors. The system has been modeled considering both the liquid and the gas phases in a well-mixed flow regime with a global second-order irreversible chemical reaction. To describe the mass-transfer phenomenon at the gas,liquid interface level, chemical reactions and diffusive mass transport were considered simultaneously in the liquid film. The identifiability analysis shows that the simultaneous measurement of concentrations in the liquid and the gas phases is required to determine the parameters. Furthermore, model parameters such as the diffusion coefficients, the specific interfacial area, or the film thickness cannot be uncoupled from other parameters, although the kinetic rate constants could be identified in the fast or slow chemical regimes. It is demonstrated that the specific interfacial area cannot be measured from chemical data coming from a nonstationary bubble-column reactor. This result is discussed with respect to the classical Danckwerts' method for determination of the interfacial area in gas,liquid equipment. © American Institute of Chemical Engineers AIChE J, 2006 [source]

Reactivity of C4-indolyl substituted 1,4-dihydropyridines toward superoxide anion (O2,) in dimethylsulfoxide

Ricardo Salazar
Abstract Reactivity of two new C4-indolyl substituted 1,4-dihydropyridines (1,4-DHPs) toward superoxide anion (O2,) in dimethylsulfoxide (DMSO) is reported. Reactivity was followed by electrochemical and spectroscopic techniques. Gas chromatography-mass spectrometry (GC,MS) was used to identify the final products of the reaction. C4 indolyl-substituted-1,4-DHPs reacted toward O2, at significant rates, according to the calculated kinetic rate constants. Results are compared with 4-phenyl-DHP and the commercial 1,4-DHPs, nimodipine, nisoldipine, and amlodipine. Indolyl-substituted 1,4-DHPs were more reactive than the commercial derivatives. The direct participation of proton of the 1-position of the secondary amine in the quenching of O2, was demonstrated. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Nitroxide-Mediated Polymerization of Methyl Methacrylate Using an SG1-Based Alkoxyamine: How the Penultimate Effect Could Lead to Uncontrolled and Unliving Polymerization

Yohann Guillaneuf
Abstract Summary: The nitroxide-mediated polymerization (NMP) of MMA initiated with a new crowded SG1-based alkoxyamine was performed. Contrary to the results expected after a kinetic analysis (Fischer's diagram), the polymerization of MMA at 45,°C with SG1 showed only partial control and livingness during the first 15% of conversion. Simulations using PREDICI highlighted that the kinetic rate constants currently in use had not been correctly estimated and that a strong penultimate effect drastically increased the equilibrium constant K (7,×,10,7), preventing a well-controlled polymerization. Experimental determination of the kc value (1.4,×,104 L,·,mol,1,·,s,1) confirmed a strong penultimate effect on the recombination reaction, whereas for the dissociation reaction this effect is lower (kd,=,10,2,·,s,1). Nitroxide-mediated polymerization of MMA at 45,°C initiated with a new crowded SG1-based alkoxyamine. [source]

Sol MWD During Styrene, Vinyl Acetate, Methyl Methacrylate, and Butyl Acrylate Homopolymerization: A Numerical Study Using the NFT Approach

Bibiana Alejandra Yáñez-Martínez
Abstract Complete parameter sensitivity analyses using the numerical fractionation technique are presented for the cases of homopolymerization with chain transfer to polymer and termination by combination. Also, using reported values for the kinetic rate constants associated with the linear and non-linear homopolymerizations of styrene, vinyl acetate, methyl methacrylate and butyl acrylate, overall molecular weight distributions and averages of the MWD were calculated using the NFT. Good agreement with the expected behavior, with MMA and STY not gelling while BA and VAc do, was obtained. It is concluded that the NFT produces coherent and reliable performance for known polymerization systems, whether linear or non-linear. [source]

New Approach in Modeling of Metallocene-Catalyzed Olefin Polymerization Using Artificial Neural Networks

Mostafa Ahmadi
Abstract A new approach for the estimation of kinetic rate constants in olefin polymerization using metallocene catalysts is presented. The polymerization rate has been modeled using the method of moments. An ANN has been used and trained to behave like the mathematical model developed before, so that it gets polymerization rate at different reaction times and predicts reaction rate constants. The network was trained using modeling results in desired operational window. The polymerization rates were normalized to make the network work independent of operational conditions. The model has also been applied to real polymerization rate data and the predictions were satisfactory. This model is specially useful in comparing different new metallocene catalysts. [source]

Simulation of Styrene Polymerization by Monomolecular and Bimolecular Nitroxide-Mediated Radical Processes over a Range of Reaction Conditions

Juliana Belincanta-Ximenes
Abstract Simulations of polymerization rate, molecular weight development and evolution of the concentrations of species participating in the reaction mechanism over a range of operating conditions, and a parameter sensitivity analysis showing the effects of temperature, activation/deactivation equilibrium constant and initial concentrations of controller and initiator (if present) on these variables are presented for the nitroxide-mediated radical polymerization of styrene. The simulations were performed with a computer program based on a detailed reaction mechanism. The simulated profiles of conversion, number average molecular weight (), and polydispersity agree well with experimental data. Previously unknown activation energies for reactions involved in the mechanism are estimated. The temperature dependence of the kinetic rate constants obtained in this study will be useful for future modeling and optimization studies. [source]

Folding kinetics and thermodynamics of Pseudomonas syringae effector protein AvrPto provide insight into translocation via the type III secretion system

Jennifer E. Dawson
Abstract In order to infect their hosts, many Gram-negative bacteria translocate agents of infection, called effector proteins, through the type III secretion system (TTSS) into the host cytoplasm. This process is thought to require at least partial unfolding of these agents, raising the question of how an effector protein might unfold to enable its translocation and then refold once it reaches the host cytoplasm. AvrPto is a well-studied effector protein of Pseudomonas syringae pv tomato. The presence of a readily observed unfolded population of AvrPto in aqueous solution and the lack of a known secretion chaperone make it ideal for studying the kinetic and thermodynamic characteristics that facilitate translocation. Application of Nzz exchange spectroscopy revealed a global, two-state folding equilibrium with 16% unfolded population, a folding rate of 1.8 s,1, and an unfolding rate of 0.33 s,1 at pH 6.1. TrAvrPto stability increases with increasing pH, with only 2% unfolded population observed at pH 7.0. The R1 relaxation of TrAvrPto, which is sensitive to both the global anisotropy of folded TrAvrPto and slow exchange between folded and unfolded conformations, provided independent verification of the global kinetic rate constants. Given the acidic apoplast in which the pathogen resides and the more basic host cytoplasm, these results offer an intriguing mechanism by which the pH dependence of stability and slow folding kinetics of AvrPto would allow efficient translocation of the unfolded form through the TTSS and refolding into its functional folded form once inside the host. [source]

X-ray structure and characterization of carbamate kinase from the human parasite Giardia lamblia

Andrey Galkin
Carbamate kinase catalyzes the reversible conversion of carbamoyl phosphate and ADP to ATP and ammonium carbamate, which is hydrolyzed to ammonia and carbonate. The three-dimensional structure of carbamate kinase from the human parasite Giardia lamblia (glCK) has been determined at 3,Å resolution. The crystals belonged to the monoclinic space group P21, with unit-cell parameters a = 69.77, b = 85.41, c = 102.1,Å, , = 106.8°. The structure was refined to a final R factor of 0.227. The essentiality of glCK together with its absence in humans makes the enzyme an attractive candidate for anti- Giardia drug development. Steady-state kinetic rate constants have been determined. The kcat for ATP formation is 319 ± 9,s,1. The Km values for carbamoyl phosphate and ADP are 85 ± 6 and 70 ± 5,µM, respectively. The structure suggests that three invariant lysine residues (Lys131, Lys216 and Lys278) may be involved in the binding of substrates and phosphoryl transfer. The structure of glCK reveals that a glycerol molecule binds in the likely carbamoyl phosphate-binding site. [source]

Parameters Influencing the Release of Tertiary Alcohols from the Surface of "Spherical" Dendrimers and "Linear" Stylomers by Neighbouring-Group-Assisted Hydrolysis of 2-Carbamoylbenzoates

Alain Trachsel
Abstract Size is not all! Investigation of the controlled release of tertiary alcohols from the surface of dendrimers and "stylomers" as polymer model systems (see scheme) showed that the polarity of the conjugates and structural modifications in close proximity to the release unit have a stronger influence on the rates of hydrolysis than the size (generation) or shape (linear or spherical) of the macromolecules. The influence of structural and physico-chemical parameters on the release of a volatile tertiary alcohol (2-methyl-1-phenyl-2-propanol) by neighbouring-group-assisted cyclisation of 2-carbamoylbenzoates at neutral pH was investigated by comparing the covalent-bond cleavage from the surface of linear, comblike poly(propylene imine) "stylomers" and their corresponding spherical, globular dendrimers. Determination of the kinetic rate constants for the stepwise intramolecular cyclisation of the 2-carbamoylbenzoate moiety by using HPLC showed that the polarity of the conjugates, and thus their solubility in the aqueous reaction medium, has a stronger influence on the rates of hydrolysis than the size (generation) or shape (linear or spherical) of the macromolecules. Furthermore, structural modifications in close proximity to the release unit, such as the presence of functionalities with catalytic activity, have a strong impact on the release efficiency of the active molecules. An understanding of the physico-chemical parameters determining the local environment of the covalent-bond cleavage site is therefore an important prerequisite to transfer the characteristics of small molecules to larger structures such as oligomers and polymers and thus to design efficient macromolecular conjugates for the controlled delivery of bioactive compounds. L'influence des paramètres structurels et physico-chimiques sur le relargage à pH neutre d'un alcool tertiaire volatil (2-méthyl-1-phényl-2-propanol) par cyclisation de 2-carbamoylbenzoates assistée par un groupe voisin a été étudiée. Les ruptures de liaisons covalentes depuis la surface de poly(propylene imines) en forme de peigne ("stylomères") d'une part et de leurs analogues sphériques et globulaires (dendrimères) correspondants d'autre part ont été comparées. La détermination par CLHP des constantes cinétiques pour la cyclisation intramoléculaire des unités 2-carbamoylbenzoate par étapes a montré que la polarité des conjugués, et par conséquent leur solubilité dans le milieu de réaction aqueux, a une influence plus forte sur les vitesses d'hydrolyse que la taille (génération) ou la forme (linéaire ou sphérique) des macromolécules. De plus, des modifications structurelles à proximité immédiate de l'unité de relargage telles que la présence de groupes fonctionnels avec une activité catalytique ont un fort impact sur l'efficacité de relargage des molécules actives. La compréhension des paramètres physico-chimiques qui déterminent l'environnement local du site de rupture de la liaison covalente est par conséquent une condition préalable importante lors du transfert de caractéristiques de petites molécules à des structures plus grandes telles que des oligomères et des polymères, et donc à la conception de conjugués macromoléculaires efficaces pour le relargage contrôlé de composés bioactifs. [source]