Computer simulation of the kinetic behaviour of surface reactions driven by a linear potential sweep: Part I. Model 1-electron reaction with a single adsorbed species

The kinetic behaviour of a simple 1 e surface reaction is treated theoretically, especially with regard to (a) characteristic aspects of its behaviour when significant attractive or repulsive interactions arise in the electrodeposited film and (b) formulation of characteristic features of its kinetic and equilibrium behaviour which can provide a reference case for distinguishing, by comparison, the behaviour of other more complex sequential reactions treated in Part II.     

On the application of open circuit relaxation spectroelectrochemistry to the diagnosis and evaluation of the kinetics of succeeding second order chemical reactions

The spectroelectrochemical technique of open circuit relaxation (OCR) has found limited application in the kinetic characterization of systems wherein the electrogenated absorbing species is consumed in a second or higher order reaction. This limited usage arises from the inapplicability of conventional homogeneous kinetic data analysis techniques to the absorbance-time transients observed in the OCR experiment. Treatment of such transients by conventional kinetic expressions (e.g., 1/A vs. t for second order reactions) results in non-linear plots which neither serve as diagnostic criteria for the assignment of kinetic order nor provide meaningful rate constants. This paper presents an empirical method for the treatment of spectroelectrochemical OCR data arising from post-electron transfer kinetic processes which exhibit second order dependence on the concentration of the electrogenerated absorbing species. This procedure, which takes into account the inhomogeneous distribution of the reactant species, affords reduced data plots which not only are linear, hence diagnostic of kinetic order, but also provide a valid measure of the bimolecular rate parameters characteristic of this type of dynamic system. The procedure has been applied to the treatment of OCR data for the reaction of 9,10-diphenylanthracene cation radical with 4-cyanopyridine in acetonitrile. Excellent agreement between the resulting kinetic parameters and those obtained using stopped-flow techniques has been demonstrated.     

Determinations cinetiques par microcalorimetrie differentielle programmation de temperature. IV. Parametres cinetiques del l'homolyse de l'azo-bis-isobutyronitrile en solution

Free-radical decomposition rate constants of A.I.B.N. in different solvents, obtained from differential scanning micro-calorimetry are analyzed; calculated kinetic parameters are discussed. Free energy of activation (ΔG^≠) is proposed as a characteristic of the thermal stability of free-radical initiators.     

A new kinetic model for bulk polymerization of vinyl chloride based on two-phase hypothesis

A kinetic model has been developed for the bulk polymerization of vinyl chloride using Talamini's hypothesis of two-phase polymerization and a new concept of kinetic solubility which assumes that rapidly growing polymer chains have considerably greater solubility than the thermodynamic solubility of preformed polymer molecules of the same size and so can remain in solution even under thermodynamically unfavourable conditions. It is further assumed that this kinetic solubility is a function of chain length. The model yields a rate expression consistent with the experimental data for vinyl chloride bulk polymerization and moreover is able to explain several characteristic kinetic features of this system. Application of the model rate expression to the available rate data has yielded 2.36 × 10⁸l mol^?1 sec^?1 for the termination rate constant in the polymer-rich phase; as expected, this value is smaller than that reported for homogenous polymerization by a factor of 10–30.     

Determinations cinetiques par microcalorimetrie differentielle en programmation de temperature. VIII. Thermolyses de perbenzoates de t-butyle p-substitues. Discussions sur la correlation de hammett

The study of the thermolysis of some p-substituted t-butyl peroxybenzoates in diphenyl ether confirms that (i) electron-withdrawing groups stabilize the peroxydic bond, and (ii) electron-repelling groups make it weaker. However, as the influence of these substituents is not considerable, the differences between the kinetic or activation parameters being not much greater than experimental errors, one must observe some caution before drawing conclusions on the correlations between electronic effects and stability characteristic parameters.     

Determinations cinetiques par microcalorimetrie differentielle en programmation de temperature. VI. Thermolyse d'azo-1,1′-(cyano-1 cyclanes) dans differents solvants

Bis-azo-cyanocyclo-pentane, -hexane and -heptane are prepared and the kinetics of their thermolysis in several solvents are studied by differential scanning microcalorimetry. Unlike activation enthalpy and entropy, activation free enthalpy does not depend on solvent. Moreover its variation with temperature is quite small in each kinetic study temperature range. Thus ΔG^# is given as an intrinsic stability characteristic of azo-nitriles. “Kinetic compensation effect” between ΔS^# and ΔH^# is discussed.     

Study of oxidation properties and decomposition kinetics of three-dimensional (3-D) braided carbon fiber

The XRD, SEM, isothermal oxidation-weight loss and non-isothermal thermogravimetry (TG)-differential thermogravimetry (DTG) were used to study the oxidation properties and oxidation decomposition kinetics of three-dimensional (3-D) braided carbon fiber (abbreviated as fiber). The results showed that the non-isothermal oxidation process of fiber exhibited self-catalytic characteristic. The kinetic parameters and oxidation mechanism of fiber were studied through analyzing the TG and DTG data by differential and integral methods. The oxidation mechanism was random nucleation, the kinetic parameters were: lg A=10.299 min⁻¹; E_a=156.29 kJ mol⁻¹.     

Computer simulation of the kinetic behaviour of surface reactions driven by a linear potential sweep: Part III. Monolayer formation by a nucleation and growth mechanism

The kinetic behaviour of a surface process involving the deposition of a two-dimensional surface film by a nucleation and growth mechanism is treated for the case of linear potential sweep control. Features which distinguish nucleation and growth in monolayer formation from random electrodeposition (Langmuir case) treated in previous papers are emphasized. Two main mechanisms are considered: one where the growth occurs from a fixed surface density of nuclei and the other where growth occurs from a potential-dependent density of nuclei. Computer simulations and some analytical derivations of the kinetic behaviour for these two cases are made and the characteristic kinetic features of the process are deduced, enabling the latter to be distinguished in terms of experimentally accessible criteria. The extent of reversibility of the processes can be usefully expressed in terms of a limiting sweep-rate parameter, s₀, which is related to the rate constant for nucleation or the surface density of nuclei and the rate constant for growth.     

Crystallization kinetics of amorphous Se

Differential scanning calorimetry was used to study crystallization behavior in selenium glass under non-isothermal conditions. The crystallization kinetics were described in terms of the Johnson–Mehl–Avrami nucleation-growth model; activation energies and kinetic parameter m _JMA were determined. The study was performed in dependence with particle size, so that a novel approach to the evaluation of crystallization kinetics—the advanced interpretation of characteristic kinetic functions—could be employed. Extensive discussion of all aspects of a full-scale kinetic study for a complex crystallization process was performed within the framework of the introduced conception. The complexity of the crystallization process was found to be represented by very closely overlapping consecutive competing surface and bulk nucleation-growth mechanisms. Mutual interactions of both mechanisms as well as all other observed effects were explained in terms of thermal gradients, surface crystallization centers arising from the sample preparation treatments and a changing amount of volume nuclei originating from the combination of the pre-nucleation period and the actual glass preparation phase. The main objective of the study is to demonstrate the extent of so-far neglected information hidden in the characteristic kinetic functions and introduce a convenient tool for its acquisition.     

Determinations cinetiques par microcalorimetrie differentielle en programmation de temperature. IX. thermolyses de perbenzoates de t-butyle p-substitues. Parametres cinetiques et d'activation

Kinetic and activation parameters of thermolysis in several solvents are compared. The free activation enthalpy is taken as a specific characteristic of the thermal stability; it is then applied to “evaluate” the influence of aromatic ring substituents. A symbatic variation in the case of the activation energy and the pre-exponential factor of the Arrhenius relation as well as in that of the activation enthalpy and entropy are found. The significance of the corresponding kinetic compensation effect is discussed as well as that of the isokinetic temperature of the thermolysis of p-substituted peroxybenzoates.     

Theoretical analysis of the influence of the adsorption process on the voltammetric curves in the case of a second order adsorption step followed by a first order charge transfer reaction: Part III. Variation of the transfer coefficient and application to semiconductor electrodes in l.p.s.v. and s.s.c.v.

The kinetic behaviour of a two-step sequential reaction including a 2nd order adsorption process is treated theoretically in the case of semiconductor electrodes. It is shown that the electronic structure of the semiconductor can be taken into account in some simplified cases by introducing a formal transfer coefficient comprised between 0 and 1 and potential independent. In both linear potential sweep voltammetry and steady state cyclic voltammetry, the usual characteristic features (E_M, _M, i_M) of the voltammetric curves are calculated as a function of the sweep rate for different values of the transfer coefficient.     

Kinetics of Extraction of Biologically Active Substances from Medicinal Plant Raw Materials using Different Techniques

The problems and peculiarities of obtaining the kinetic characteristics of extraction of biologically active substances (BAS) from a plant matrix are discussed. The establishment of kinetic characteristics of the extraction of BAS under various extraction conditions is studied on the example of samples of Hypericum perforatum and Salvia officinalis. The lowest value of the average rate constant of the extraction of BAS from the materials examined is characteristic of a variant of static extraction with an aqueous alcohol extractant under heating. The methods of static extraction under heating (according to the requirements of pharmacopoeia articles) and ultrasonication, as well as using double ultrasonication differ little from each other in their kinetic efficiency. The method of dynamic extraction of BAS at an elevated temperature and pressure is characterized by the highest values of the rate constant, which leads to an acceleration of the diffusion of the BAS into the extractant’s medium. It is noted that the extraction processes and chemical transformations of some BAS (hyperforin, carnosic acid, etc.) occur simultaneously as confirmed by the decrease in their concentrations in the extractant during the extraction from plant samples.     

Laser photolysis study of the spectral-kinetic characteristics of short-lived triplet exciplexes and the mechanism of atomic hydrogen transfer in the course of 2,6-diphenyl-1,4-benzoquinone triplet quenching with aromatic amines

The spectral-kinetic characteristics of short-lived triplet exciplexes arising in the quenching of 2,6-diphenyl-1,4-benzoquinone triplet with aromatic amines: N,N,N′,N′-tetramethyl-p-phenylenediamine, triphenylamine and diphenylamine have been studied by means of the nanosecond laser photolysis technique. The absorption spectra of triplet exciplexes exhibit distinct maxima characteristic of the absorption spectra of corresponding amine radical cations. The state with complete charge transfer gives the basic contribution to the exciplex structures. A detailed analysis is presented of the kinetic and thermodynamic deactivation characteristics of triplet exciplexes in low-polar solvents.     

Thermal characterization and isothermal kinetic analysis of commercial Creosote decomposition process

Isothermal decomposition process of commercial Creosote was analyzed by thermogravimetric technique in a nitrogen atmosphere, at four different operating temperatures (T = 230, 250, 270, and 290 °C). It was found that the two-parameter autocatalytic ?esták–Berggren kinetic model best describes the investigated process. It was established that the applied logistic function can successfully perform a given kinetic predictions of investigated process, at all operating temperatures. The experimental density distribution function of the apparent activation energy values was evaluated. Based of the characteristic shape of distribution curve, it was concluded that the isothermal pyrolysis of commercial Creosote represent a complex process, which probably includes primary and secondary (autocatalytic) pyrolysis reactions, together with various decomposition reactions and radicals recombination pathways.     

Kinetic compensation effect between the isothermal and non-isothermal decomposition of solids

A “true” kinetic compensation effect was established using the most appropriate kinetic functionF(α) for the non-isothermal decomposition of solids at various heating rates. It is likely that the correct kinetic mechanismF(α) is responsible for the “true” kinetic compensation effect, whereas an inappropriateF(α) would lead to “false” one. An establishment of such a “true” compensation effect between the isothermal and nonisothermal decompositions of a solid implies thatF(α) used is appropriate for both the isothermal and non-isothermal decompositions.     

Classical kinetic energy,quantum fluctuation terms and kinetic-energy functionals

We employ a recently formulated dequantization procedure to obtain an exact expression for the kinetic energy which is applicable to all kinetic-energy functionals. We express the kinetic energy of an N-electron system as the sum of an N-electron classical kinetic energy and an N-electron purely quantum kinetic energy arising from the quantum fluctuations that turn the classical momentum into the quantum momentum. This leads to an interesting analogy with Nelson’s stochastic approach to quantum mechanics, which we use to conceptually clarify the physical nature of part of the kinetic-energy functional in terms of statistical fluctuations and in direct correspondence with Fisher Information Theory. We show that the N-electron purely quantum kinetic energy can be written as the sum of the (one-electron) Weizsäcker term and an (N?1)-electron kinetic correlation term. We further show that the Weizsäcker term results from local fluctuations while the kinetic correlation term results from the nonlocal fluctuations. We then write the N-electron classical kinetic energy as the sum of the (one-electron) classical kinetic energy and another (N?1)-electron kinetic correlation term. For one-electron orbitals (where kinetic correlation is neglected) we obtain an exact (albeit impractical) expression for the noninteracting kinetic energy as the sum of the classical kinetic energy and the Weizsäcker term. The classical kinetic energy is seen to be explicitly dependent on the electron phase, and this has implications for the development of accurate orbital-free kinetic-energy functionals. Also, there is a direct connection between the classical kinetic energy and the angular momentum and, across a row of the periodic table, the classical kinetic energy component of the noninteracting kinetic energy generally increases as Z increases. Finally, we underline that, although our aim in this paper is conceptual rather than practical, our results are potentially useful for the construction of improved kinetic-energy functionals.     

Non-ideality in vinyl polymerization—VIII: Retarded polymerization of methyl methacrylate in presence of p-benzoquinone initiated by azobisisobutyronitrile

Detailed kinetic analysis of AIBN-initiated polymerization of methylmethacrylate in presence of p-benzoquinone has been reported. Primary radical transfer, whereby a primary radical transfers its radical reactivity to a transfer agent, has been considered along with macroradical transfer. It is found that the former process is quite appreciable in the system and must be allowed for to arrive at accurate values of transfer constants. Values of transfer constants for both primary radicals and macroradicals towards benzoquinone, and characteristic constants for degradative chain transfer and primary radical transfer have been evaluated applying the mathematical treatment developed previously. The mode of termination of macroradicals by fairly stable microradicals formed as a result of transfer has also been discussed.     

A new evaluation technique for analyzing the thermoluminescence glow curve and calculating the trap parameters

A step-fitting simulation technique was developed for the thermoluminescence (TL) glow-curve analysis and the kinetic trap parameters determination. These parameters include the order of kinetics b, the activation energy E (eV) and the pre-exponential factor S″ (s⁻¹). A general equation was developed to estimate the order of kinetics b. The characteristics point of this equation is that any set of three data points in a TL glow curve can yield the kinetics order. Using this characteristic, an improved procedure was suggested to separate a composite glow curve, which includes several overlapping peaks, into its individual components and to obtain the trap parameters of the glow peaks. The program was used to analyze the TL glow curve of the UV dosimetric material pure zirconium oxide (ZrO₂).     

Synthesis and characterization of poly(methacrylic acid) hydrogels for metoclopramide delivery

Polymeric hydrogels based on biocompatible materials, methacrylic acid (MAA), were designed and synthesized. Synthesis was carried out by free-radical copolymerization using potassium persulfate as initiator and N,N^′-methylenebisacrylamide as crosslinker. Hydrogels were also characterized by Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy (¹H NMR) and differential scanning calorimetry (DSC). DSC was used for the quantitive determination of the amounts of freezing and non-freezing water of the hydrogels with 0.5% of N,N^′-methylenebisacrylamide. Equilibrium swelling of hydrogels was studied in phosphate buffer of physiological pH (1.0, 4.0, 7.4 and 8.5) at 37 °C. The swelling kinetic of the hydrogels were studied and the kinetic characteristic constant of copolymeric systems, k, and the exponent which characterizes the mechanism of water transport at short times, n, were obtained. Metoclopramide hydrochloride was entrapped into the hydrogels by sorption and the “in vitro” release profile of this drug was established in simulated gastric fluid (SGF) and simulated intestinal fluid (SIF). It was observed that the drug release mechanism was non-Fickian.     

Amplification of the enantiomeric excess of a compound in kinetic resolution by a racemic reagent

The possibility of amplifying the small ee of a enantioimpure substance using a racemic reagent in kinetic resolution is discussed. A kinetic treatment of this problem along with some experimental proofs of the concept is presented. Simulation on kinetic resolution of small ee substrate by a racemic reagent showed an important enantioenrichment in the substrate ee, sometimes reaching close to absolute ee. Experiments of kinetic resolution of an amine of a small ee by a racemic acylating reagent gave a substantial amplification in ee of the amine. The possible transformation of an undetectable low ee in substrate to a detectable higher ee by using a suitable racemic reagent is also briefly discussed with help of calculations. The usefulness of asymmetric amplification by a racemic reagent is considered in the context of the biomolecular homochirality on earth.