Thermal investigations on γ-radiation processed natural medicinal products (Ashwagandha,Amla and Hartiki)

DSC and TG studies were carried out on -radiation processed Indian natural products of medicinal importance, namely Ashwagandha (Withania Somnifera), Amla (Emblica Officinalis) and Hartiki (Terminalia chebula). DSC thermoanalytical curves were recorded from 35 to 400°C in air and nitrogen atmosphere. Similarly, TG thermoanalytical curves were taken from 35 to 700°C in air and nitrogen atmosphere. Irradiated products gave significantly different thermoanalytical profiles in comparison to non-irradiated samples. The differences were observed above decomposition temperature of 200°C and were non-linear with respect to radiation dose. Partial oxidation of the products during irradiation in air could be responsible for the observed differences.     

Thermoanalytical study of inner and outer residue of coffee harvest

The better use of agricultural residues is expected, when they are mostly disposed of improperly and it is often burned in the natural environment. This study of the thermal decomposition of residues was performed from the coffee crop for energy purposes and in this case was used thermal analysis techniques for such assessment. The TG/DTG and DSC curves showed that the thermal decomposition occurs in four consecutive events and it is predominantly exothermic. The first mass loss evidenced in TG/DTG curves has an endothermic peak in DSC curve, which it can be associated with the water liberation of the material. This first thermal event also can be related to the liberation of volatile compounds present in the sample, which is also corroborated by the endothermic peak. The other events of mass loss are related with the thermal decomposition of the material. This decomposition has an exothermic behavior, which is positively applied to the main aim of this scientific research: the coffee straw use like biomass energy font. The thermoanalytical techniques were satisfactory in the characterization of this material.     

The applicability of the arrhenius model in thermal analysis

It is shown experimentally that dynamic thermoanalytical curves provide insufficient information for the purpose of reaction kinetic calculations. Mathematical considerations prove also that the parameters of the Arrhenius model cannot be calculated correctly from the thermoanalytical curves by curve-fitting and that there is no unique correlation between the estimated parameters and the measured curves.     

A program for the kinetic evaluation of thermoanalyttcal curves obtained by differential scanning calorimetry

The kinetic investigation of thermoanalytical curves of complex chemical reactions requires tedious calculation and large arrays for data processing. A computer program (BASIC) has been developed with which the intricate thermoanalytical curves can be resolved into individual signals and the signals evaluated kinetically according to several mathematical expressions. A computed simulation of the thermoanalytical curves using the parameters of bulk reation is carried out to verify the validity of the parameters. The method is suitable for determining the amount of isothermal conversion in dynamically observed curing reaction of epoxy resins.     

Preparation,thermoanalytical and IR study of mixed ligand complexes formed in water-1,2-ethanediol- nickel(II)-sulfate systems

Summary Mixed ligand nickel(II) complexes of different compositions were prepared with water, sulfate ion and 1,2-ethanediol as ligands. The magnetic susceptibility data, the IR spectra and the thermoanalytical curves of the complexes were recorded. Oxygen atoms bound by one or two coordinate bonds to the metal ion, or by hydrogen-bonds in the crystal state were observed. All complexes are sensitive for moisture. The bis complex proved to be the more stable complex.     

Thermal stability and combustibility of butyl and halogenated butyl rubbers

The effects of flame retardants such as hydrated aluminium oxide, antimony trioxide and chloroparaffin on the thermal properties and flammability of sulphur vulcanizates of butyl and halogenated butyl elastomers were studied. The thermoanalytical curves of the elastomers were interpreted. Greater tendencies to thermal degradation were observed for halogenated butyl elastomers than for the original butyl rubber elastomer. This was confirmed by elastomer combustibility studies. The use of these flame retardants allowed the formation of self-extinguishing vulcanizates of the investigated elastomers.     

Investigation of the cyclodextrin complexes of mandelic acid derivatives

Racemic free mandelic acid and its methyl, ethyl, isoamyl and benzyl esters were found to form inclusion complexes with all the three studied natural cyclodextrins proved by thermoanalytical results. Differences between the solid state stability of guests were detected mainly by evolved gas analysis. Even signs of an eventual optical resolution by molecular inclusion were observed in several cases, but still not sufficiently proven. Due to the rather high volatility and low melting points of the majority of guest substances DSC technique was found to be suitable for studying the cyclodextrin complexes of mandelic acid. Dedicated to Prof. József Szejtli on the occasion of his 60^th birthday     

A simple device for continuous and selective detection of water vapour evolved during thermal decomposition reactions

A technique developed for the continuous and selective detection of water vapour formed during thermal decomposition reactions is described. The device can be connected to different types of thermoanalytical instruments without any difficulties. The detector can closely follow changes in the amount of water released during decomposition reactions, with negligible time delay. The signal curves obtained by the detector can be compared to the simultaneously recorded thermoanalytical curves and used to determine the step in which the water was released. The device as a free standing unit can be used to detect water plugs in different gas flows as well.     

Study of the change in the properties of Mn-bentonite by aging

In this paper the structure and properties of fresh manganese(II)-bentonite was compared with that of an old substance. It was concluded that the oxidation state of Mn changed. This did not cause many changes in the scanning electron microscope (SEM) and in the X-ray diffraction (XRD) studies; caused minor changes in the Mn concentration (determined by XRF) and thermoanalytical and electron spectroscopy analysis (ESCA). The change in the oxidation state of manganese was indicated by the colors of the samples, the difference in the surface sites, titration curves, redox potentials, adsorption, and catalytic activity of the fresh and the old Mn-bentonite. Potentiometric titration data were evaluated by a surface complexation model using the FITEQL3.2 computer program. Stability constants of edge charge reactions and the number of aluminol, silanol, and edge sites were calculated. Potentiometric titration data of commercial and freshly made MnO2 were also evaluated; the calculated constants and site numbers were compared with that of found in literature. Catalytic and adsorption activity of the samples were also investigated. It was found that fresh Mn-bentonite does not adsorb valine, while the old one and MnO2 does. Fresh Mn-bentonite does not catalyze the decomposition of H2O2, while the old one, as well as MnO2 does.     

Mössbauer effect and thermal analysis investigation of a frozen aqueous solution of ferrous sulphate

Aqueous FeSO₄.7H₂O solutions of different concentrations were quickly frozen to liquid nitrogen temperature and the zero-velocity Mössbauer transmission and differential thermoanalytical (DTA) curves were measured simultaneously as the samples warmed up. On the DTA curves an exothermic peak could be observed at about ?60°C. Also changes in the Mössbauer parameters were found to take place at this temperature.     

Influence of the particle size distribution on the CRTA curves for the solid-state reactions of interface shrinkage type

The kinetic curves at infinite temperature for the solid-state reactions of the interface shrinkage type were drawn theoretically by taking account the particle size distribution in the sample mixture. The CRTA curves for the reactions with the particle size distribution can be drawn by utilizing the universal kinetic curves at infinite temperature. The proper kinetic treatment for the CRTA curves with the particle size distribution is discussed in connection with the property of the kinetic equation with respect to the particle size distribution. The present kinetic consideration is taken as a simulation for the reactions with a certain distribution in among the reactant particles, produced preferably by the mass and heat transfer phenomena during the thermoanalytical measurements. The merit of the rate jump method by a single cyclic CRTA curve is also discussed on the basis of the present results.     

Pharmaceutical compatibility of dexamethasone with excipients commonly used in solid oral dosage forms

Dexamethasone is a glucocorticoid drug used for the treatment of acute and chronic inflammatory conditions, autoimmune diseases, some cancers, and several other pathologies. It is widely marketed worldwide especially under solid dosage forms. This study aimed to assess its compatibility with solid pharmaceutical excipients. Compatibility study was conducted through the preparation of binary mixtures (1:1, w/w) of dexamethasone with 12 selected excipients. Binary mixtures were analyzed by thermoanalytical techniques (thermogravimetric and differential thermal analysis), Fourier transform infrared spectroscopy, and X-ray diffraction. TG curves pointed only slight anticipations of dexamethasone decomposition. DTA curves showed interactions signs with microcrystalline cellulose 101 and 102, magnesium stearate, mannitol, and polyvinylpyrrolidone. Drug infrared absorption profile was not affected by the mixture with most excipients. X-ray diffractograms of all binary mixtures did not exhibit signs of interactions with changes of dexamethasone crystalline state. These results pointed that the interactions found by DTA technique were probably heat-induced. Therefore, the above-mentioned excipients should be carefully used in solid dosage forms with heat-based manufacturing processes.

     

On the kinetic analysis of thermoanalytical diagrams obtained with the “quasi-isothermal” heating technique

A comparative study of “n-order”, Avrami—Erofeev and diffusion-controlled reactions has been carried out using thermoanalytical data, calculated by assuming a “quasi-isothermal” heating technique. The results reported support the hypothesis that a single diagram obtained with this technique allows the discernment of Avrami-Erofeev, “n-order” and diffusion-controlled reactions. This does not occur when a linear heating program is used, as was reported in previous papers. In addition, it is shown that a single thermoanalytical curve obtained with a “quasi-isothermal” heating program does not permit the determination of the actual value of n of those reactions following “n-order” kinetics. On the other hand, it is proved that the kinetic analysis of two thermoanalytical curves obtained with a linear heating program and a “quasi-isothermal” heating technique, respectively, would provide an excellent procedure for discerning the proper mechanism of solid-state reactions.     

Thermochemistry of coal oxidation

Five types of Polish bituminous coal of different grades were analysed using, simultaneously, thermogravimetric analysis (TGA), differential thermogravimetry (DTG), differential thermal analysis (DTA) and evolving gas analysis (EGA) to investigate the non-isothermal coal oxidation. The TGA, DTG and DTA curves, together with EGA, provided parameters which characterize the tendency of a given coal towards oxidation. The TGA and EGA parameters can also be used to approximate the specific active surface area of coal in reaction with oxygen. Due to the negative effects of coal oxidation, such as self-oxidation, an inhibitor was proposed and tested by analysis of the above specified thermoanalytical curves and EGA.     

Thermoanalytical behaviour of some coating free films

Differential scanning calorimetry (DSC) was used in the investigation of the behaviour of coating free films. The films were produced from two film-forming polymers which are chemically different but equally used for producing controlled-release dosage forms: Eudragit NE 30 D (synthetically produced polymethacrylate copolymer) and LustreClear product (mixture containing natural and semi-synthetic components: microcrystalline cellulose, hydroxy-ethyl cellulose, carrageenan and polyethylene glycol). During their comparative analysis the characteristic parameters of the DSC curves obtained with dynamic measurement method were used and their changes as a function of storage conditions and storage time were observed. It was found that the thermoanalytical behaviour of the examined methacrylate-based Eudragit NE and cellulose-based LustreClear films was different. The specific enthalpy change of Eudragit NE fresh films was very little, but it increased considerably during storage. The specific enthalpy change of LustreClear films was much greater but its value shows only a slight further increase during storage. The results obtained help to choose the proper temperature for coating and drying.     

Kinetic evaluation of non-isothermal thermoanalytical curves in the case of independent thermal reactions

A least squares curve-fitting method was developed for the following thermo-analytical problem: “Find the kinetic parameters and the unknown initial amounts of the reactants from non-isothermal thermoanalytical curves in the case of two or more independent or quasi-independent thermal reactions”. From a numerical point of view this problem differs from the non-linear least squares techniques used in other areas of reaction kinetics. The special difficulties which have arisen in such calculations were eliminated by parameter transformations and by separating the linear and non-linear parts of the problem. The method can be applied at any T(t) functions. Thermo-analytical curves differing in temperature program can be evaluated simultaneously.     

Relation between kinetic parameters for reactions of organic matter degradation in waste environmental matrix

The organic fraction of urban solid residues disposed of in sanitary landfills during the decomposition yields biogas and leachate, which are sources of pollution. Leachate is a resultant liquid from the decomposition of substances contained in solid residues and it contains in its composition organic and inorganic substances. Literature shows an increase in the use of thermoanalytical techniques to study the samples with environmental interest, this way thermogravimetry is used in this research. Thermogravimetric studies (TG curves) carried out on leachate and residues shows similarities in the thermal behavior, although presenting complex composition. Residue samples were collected from landfills, composting plants, sewage treatment stations, leachate, which after treatment, were submitted for thermal analysis. Kinetic parameters were determined using the Flynn–Wall–Ozawa method. In this case they show little divergence between the kinetic parameter that can be attributed to different decomposition reaction and presence of organic compounds in different phases of the decomposition with structures modified during degradation process and also due to experimental conditions of analysis.     

Use of Thermoanalytical Methods in Prediction of Thermal Endurance of Insulating Impregnating Varnishes

The results of testing the thermal properties of insulating impregnated varnishes obtained by thermoanalytical methods and based on the IEC 216 standard are presented. It was found that the temperature index determined by means of thermoanalytical testing is quite close to that derived by a conventional standardized procedure. It was also established that thermoanalytical methods could be used in the recipe selection for the development of a new varnish. The results obtained confirmed the possibility of using thermoanalytical methods for a quick, prognostic estimation of the thermal endurance of insulating impregnating varnishes. This revised version was published online in August 2006 with corrections to the Cover Date.     

The problems of reaction kinetics calculations based on thermoanalytical curves

The work, results and estimations are summarized which are related to reaction kinetic calculations using thermoanalytical curves. Mathematical operations applied to the calculations are studied. Calculation methods are evaluated on the basis of our test results. The effect of test conditions on reaction kinetic parameters is analysed. Calculation of half-period is also investigated in the study.     

Thermal Decomposition of Purine Derivatives Used in Medicine

Thermal decomposition of several purine derivatives used in medicine –theophylline, theobromine, caffeine, diprophylline and aminophylline was investigated. The analyses were performed using a derivatograph. It has been established, that the thermal decomposition of purine derivatives occurs via three stages. The stages of dehydratation of hydrate and evaporation of aminophylline are distinctly marked on the thermoanalytical curves, which may be used for the control of composition of the studied compounds. The ranges of temperature, in which the analyzed compounds can be technologically transformed without change of their physicochemical properties, were also established. Moreover, the influence of heating rate and sample size on the thermal decomposition of the examined compounds was evaluated. This revised version was published online in July 2006 with corrections to the Cover Date.