Comparative kinetic study of decomposition of some diazepine derivatives under isothermal and non-isothermal conditions

Thermal analysis is one of the most widely used methods for studying the solid state of pharmaceutical substances. TG/DTG and DSC curves provide important information regarding the physical properties of the pharmaceutical compounds (stability, compatibility, polymorphism, kinetic analysis, phase transitions etc.). The purpose of a kinetic investigation is to calculate the kinetic parameters and the kinetic model for the studied process. The results are further used to predict the system’s behaviour in various circumstances. A kinetic study regarding the diazepam, nitrazepam and oxazepam thermal decomposition was performed, under non-isothermal and isothermal conditions and in a nitrogen atmosphere, for the temperature steps: 483, 498, 523, 538 and 553 K. The TG/DTG data were processed by three methods: isothermal model-fitting, Friedman’s isothermal-isoconversional and Nomen-Sempere non-parametric kinetics. In the model-fitting methods the kinetic triplets (f(α), A and E _a) that defines a single reaction step resulted in being at variance with the multi-step nature of diazepines decomposition. The model-free approach represented by isothermal and non-isothermal isoconversional methods, gave dependences of the activation energies on the extent of conversion. It is very difficult to obtain an accord with the similar data which resulted under non-isothermal conditions from a previous work. The careful treatment of the kinetic parameters obtained in different thermal conditions was confirmed to be necessary, as well as a different strategy of experimental data processing.     

Thermal behavior study and decomposition kinetics of salbutamol under isothermal and non-isothermal conditions

The thermal decomposition of salbutamol (β₂ — selective adrenoreceptor) was studied using differential scanning calorimetry (DSC) and thermogravimetry/derivative thermogravimetry (TG/DTG). It was observed that the commercial sample showed a different thermal profile than the standard sample caused by the presence of excipients. These compounds increase the thermal stability of the drug. Moreover, higher activation energy was calculated for the pharmaceutical sample, which was estimated by isothermal and non-isothermal methods for the first stage of the thermal decomposition process. For isothermal experiments the average values were E _act=130 kJ mol⁻¹ (for standard sample) and E _act=252 kJ mol⁻¹ (for pharmaceutical sample) in a dynamic nitrogen atmosphere (50 mL min⁻¹). For non-isothermal method, activation energy was obtained from the plot of log heating rates vs. 1/T in dynamic air atmosphere (50 mL min⁻¹). The calculated values were E _act=134 kJ mol⁻¹ (for standard sample) and E _act=139 kJ mol⁻¹ (for pharmaceutical sample).     

Thermal behaviour, compatibility study and decomposition kinetics of glimepiride under isothermal and non-isothermal conditions

In the present work, the thermal decomposition of glimepiride (sulfonylurea hypoglycemic agent) was studied using differential scanning calorimetry (DSC) and thermogravimetry/derivative thermogravimetry (TG/DTG). Isothermal and non-isothermal methods were employed to determine kinetic data of decomposition process. The physical chemical properties and compatibilities of several commonly used pharmaceutical excipients (glycolate starch, microcrystalline cellulose, stearate, lactose and Plasdone®) with glimepiride were evaluated using thermoanalytical methods. The 1:1 physical mixtures of these excipients with glimepiride showed physical interaction of the drug with Mg stearate, lactose and Plasdone®. On the other hand, IR results did not evidence any chemical modifications. From isothermal experiments, activation energy (E _a) can be obtained from slope of lnt vs. 1/T at a constant conversion level. The average value of this energy was 123 kJ mol^–1. For non-isothermal method E _a can be obtained from plot of logarithms of heating rates, as a function of inverse of temperature, resulting a value of 157 and 150 kJ mol^–1, respectively, in air and N₂ atmosphere, from the first stage of thermal decomposition.     

Temperature and crystallinity profiles in polyolefines isothermal and non-isothermal solidification processes

Temperature and crystallinity profiles developed during the isothermal and dynamic solidification of polypropylene (PP) and high density polyethylene (HDPE), which were measured using different experimental techniques, are analyzed. Appropriate models are developed and used to correlate those experimental results. Profiles obtained by simulation showed to be very similar to those obtained experimentally. This means that those models can predict the degree of crystallinity developed during solidification stages, and values for properties directly related to it, that are relevant technical specifications for the PP and HPDE possible applications. This technique could also make possible the optimization of processing parameters.     

A kinetics,isotherms, and thermodynamic study of diclofenac adsorption using activated carbon prepared from olive stones

The objective of this work was to study the efficiency of activated carbon prepared from agricultural waste (olive stones; OS) on diclofenac (DCF) adsorption in aqueous solutions. The prepared charcoals were characterized by Fourier transform infrared (FTIR), scanning electron microscopy (SEM)/Energy Dispersive X-Ray Analyser (EDX), and Brunauer–Emmett–Teller (BET) analysis techniques. The results show that DCF adsorption is favorable in acid medium and low temperature. The adsorption kinetics follows a pseudo-second-order kinetic model. The adsorption was found to be spontaneous (ΔG°?相似文献   

Unified isothermal and non-isothermal modelling of neat PEEK crystallization

A differential generalized Avrami’s law is used to model crystallization kinetic of PEEK in considering that PEEK crystallization results from the contribution of two distinct mechanisms. The form of this equation allows to predict with good accuracy both isothermal and non-isothermal crystallization kinetics. Nevertheless, isothermal model parameters are not entirely satisfactory for predicting non-isothermal crystallization and the identification of kinetic parameters is needed for both isothermal and non-isothermal cases. The results show that the Avrami exponents and Arrhenius activation energies remain constant for both conditions and therefore suggest that these parameters are only material dependent. On the other hand, the other kinetic parameters depend on the crystallization condition and vary with temperature and/or cooling rate.     

The correlation between isothermal and non-isothermal kinetics in thermogravimetry

It is shown that the total differential of the function of the amount of conversion versus temperature and time (=f(T, t)) is equal to zero non-isothermal kinetics at constant heating rate. Hence, the mathematical expression used in the literature for the rate of the non-isothermal transformation, , is not valid.     

Synthesis and non-isothermal kinetic study of the thermal decomposition of gadolinium(III) complexes

Summary Pyrrolidinedithiocarbamate (Pyr), piperidinedithiocarbamate (Pip), morpholinedithiocarbamate (Mor) and diethanolaminedithiocarbamate (DEDC) ammonium salts; pyrrolidinedithiocarbamic acid-pyrrolidineammonium salt (HPyrPyr), piperidinedithiocarbamic acid-piperidineammonium salt (HPipPip), morpholinedithiocarbamic acid-morpholineammonium salt (HMorMor), hexamethylenedithiocarbamic acid-hexamethyleneammonium salt (HHexHex), diethanolaminedithiocarbamic acid-diethanolamineammonium salt (HDEADEDC) were synthesized, characterized by IR and elemental analysis and their thermal behaviours were investigated using thermogravimetry (TG) and differential scanning calorimetry (DSC).     

Comparative study of non-isothermal methods in linear and non-linear temperature variation

A method is proposed for the treatment of data from non-isothermal kinetic experiments with non-linear variation of temperature with time. A comparative study was carried out between this and the linear method on application to a chemical reaction in solution.

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Novel theoretical and computer-assisted modeling of isothermal and non-isothermal depolymerization kinetics

A novel kinetic model accounting for the observed asymptotic approach of the degree of polymerization (DP) to a limiting value significantly greater than unity on prolonged degradation is derived and applied to the solid-state degradation of cellulose (Kraft paper) and poly(acrylic acid) (PAA) under isothermal and non-isothermal conditions. Experimental data were fitted using two iterative computer algorithms: one for isothermal DP data and the other for non-isothermal DP data obtained under a linear temperature ramp. The apparent activation energy for the solid-state recombination of chain radicals was found to be low in each case and was attributed to the proximity of free radicals being facilitated by restrictions imposed by the polymer matrix. The application of the model to non-isothermal DP yielded rate parameters that could be reconciled with those obtained from isothermal analyses, suggesting the novel approach has much merit for the future study of polymer degradation.     

The relationship between isothermal and non-isothermal kinetics for thermoset characterization

General relationships are developed for obtaining non-isothermal (dynamic) rate equation from a knowledge of the isothermal kinetics of the system. Applications to several important specific forms of the isothermal rate expressions are given.     

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.     

Kinetics of crystallization of metallic glasses studied by non-isothermal and isothermal DSC

A method for describing the lengths of induction periods at linear-heating measurements, is employed for the study of induction periods in the crystallisation of metallic glasses. For Fe₇₅Si₁₅B₁₀ glass, close values of the related kinetic parameters were obtained from isothermal and nonisothermal measurements. On the basis of the results obtained, the absence of induction period in the first crystallisation step of Al₉₀Fe₇Nb₃ glass in the isothermal DSC measurement has been elucidated.     

Synthesis and kinetic analysis of isothermal and non-isothermal decomposition of ammonium dinitramide prills

Ammonium dinitramide (ADN) prills were prepared by emulsion crystallization and characterized by optical microscopic, thermogravimetric (TG) and differential scanning calorimetric (DSC) techniques. The isothermal and non-isothermal decomposition kinetics of ADN prills were studied by TG. The differential isoconversional method of Friedman (FR) and integral isoconversional method of Vyazovkin were used to investigate the dependence of activation energy (E _a) with conversion (α) and the results were compared with literature data. The dependence of activation energy was also derived from isothermal data. A strong dependence of E _a with α is observed for the ADN prills. All the methods showed an initial increase in E _a up to α=∼0.2 and later decreases over the rest of conversion. The apparent E _a values of FR method are higher than that of Vyazovkin method up to α=∼0.45. The calculated mean E _a values by FR, Vyazovkin and standard isoconversional method for α between 0.05 and 0.95 were 211.0, 203.9 and 156.9 kJ mol⁻¹, respectively.     

Comparative study of acid yellow 119 adsorption onto activated carbon prepared from lemon wood and ZnO nanoparticles loaded on activated carbon

Activated carbon from lemon wood (AC) and ZnO nanoparticles loaded on activated carbon (ZnO‐NP‐AC) were prepared and their efficiency for effective acid yellow 199 (AY 199) removal under various operational conditions was investigated. The dependence of removal efficiency on variables such as AY 199 concentration, amount of adsorbent and contact time was optimized using response surface methodology and Design‐Expert. ZnO nanoparticles and ZnO‐NP‐AC were studied using various techniques such as scanning electron microscopy, X‐ray diffraction and energy‐dispersive X‐ray analysis. The optimum pH was studied using one‐at‐a‐time method to achieve maximum dye removal percentage. Small amounts of the proposed adsorbents (0.025 and 0.025 g) were sufficient for successful removal of AY 199 in short times (4.0 and 4.0 min) with high adsorption capacity (85.51 and 116.29 mg g^?1 for AC and ZnO‐NPs‐AC, respectively). Fitting the empirical equilibrium data to several conventional isotherm models at optimum conditions indicated the appropriateness of the Langmuir model with high correlation coefficient (0.999 and 0.978 for AC and ZnO‐NPs‐AC, respectively) for representation and explanation of experimental data. Kinetics evaluation of experiments at various time intervals revealed that adsorption processes can be well predicted and fitted by pseudo‐second‐order and Elovich models. This study revealed that the combination of ZnO nanoparticles and AC following simple loading led to significant improvement in the removal process in short adsorption time which was enhanced by mixing the media via sonication.     

Comprehensive kinetic studies on isothermal and non-isothermal reactions of some aluminium compounds

Residual differences after model fitting were investigated in both isothermal and non-isothermal kinetics in order to make numerical comparisons between several models and various parameter-estimating methods. Data from two independent experimental series were evaluated. A large data set, collected earlier under isothermal conditions from decompositions and hydrothermal reactions of aluminium hydroxides and oxides, was processed first. It showed that mechanical activation of the starting gibbsite affected reactivity of samples in several subsequent reactions for all model equations tried. The relative residual deviation concept is introduced, and statistics were applied to find a model that fits a certain reaction in most of the cases. In the second study, the sulphate decomposition step of aluminium sulphate octadecahydrate was investigated. TG curves were measured using a constant heating rate. Dynamic models were fitted by three mathematical methods, including a new general purpose one. Fitting ability of the methods with various complexity were compared on the basis of residual deviations obtained after integration of the model equations. As well as evaluating the best fit, this new parameter-estimating method provides a statistical analysis of the reliability of the whole model fitting process.     

Model-free method for isothermal and non-isothermal decomposition kinetics analysis of PET sample

Pyrolysis, one possible alternative to recover valuable products from waste plastics, has recently been the subject of renewed interest. In the present study, the isoconversion methods, i.e., Vyazovkin model-free approach is applied to study non-isothermal decomposition kinetics of waste PET samples using various temperature integral approximations such as Coats and Redfern, Gorbachev, and Agrawal and Sivasubramanian approximation and direct integration (recursive adaptive Simpson quadrature scheme) to analyze the decomposition kinetics.The results show that activation energy (E_α) is a weak but increasing function of conversion (α) in case of non-isothermal decomposition and strong and decreasing function of conversion in case of isothermal decomposition. This indicates possible existence of nucleation, nuclei growth and gas diffusion mechanism during non-isothermal pyrolysis and nucleation and gas diffusion mechanism during isothermal pyrolysis. Optimum E_α dependencies on α obtained for non-isothermal data showed similar nature for all the types of temperature integral approximations.     

Prediction of the isothermal behaviour of solid-gas systems from non-isothermal data

A new procedure for the prediction of the isothermal behaviour of the solid-gas system from non-isothermal data is suggested. It bypasses the use of various approximations of the temperature integral that ground the integral methods of prediction. The procedure was checked for: (1) simulated data corresponding to a first order reaction; (2) experimental data obtained in the isothermal and non-isothermal decompositions of ammonium perchlorate. For the simulated data, a very good agreement between calculated isotherms and those evaluated by means of the suggested procedure was obtained. A satisfactory agreement (errors in time evaluation corresponding to a given degradation lower than 18%, for 0.10￡a￡0.37 and lower than 10% for 0.37￡a￡0.70) was obtained for the experimental data corresponding to the decomposition of ammonium perchlorate. In this last case, the mentioned differences between experimental and calculated data can be due both to the inherent errors in the evaluation of the decomposition isotherms and to the dependence of the activation energy on the conversion degree. This revised version was published online in July 2006 with corrections to the Cover Date.