Ev Equation Research Articles

Thermochemical study of theophylline and its hydrate.

Anhydrous theophylline exhibited polymorphism and two modifications, which were named form I and form II, were isolated. The melting point and the enthalpy of fusion of each polymorph determined by differential scanning calorimetry (DSC) were 273.4±1.0°C and 26.4±0.3 kJ/mol for form I and 269.1±0.4°C and 28.2±1.1 kJ/mol for form II. The higher melting form I had a smaller density. In contrast to caffeine, theophylline formed a monohydrate. The dissociation vapor pressure curves of theophylline hydrate and caffeine hydrate were obtained and the difference in their stabilities was discussed. The enthalpy of dehydration was determined by DSC under closed conditions. The dehydration under isothermal conditions appeared to proceed acoording to the mechanism of random uncleation followed by two-dimensional growth of nuclei as represented by the Avrami-Erofe'ev equation. The solubilities of theophylline and its hydrate were determined as a function of temperature. From the van't Hoff type plot, the transition temperature between the hydrate and the anhydrous form was determined.

A kinetic and mechanistic study of the thermal decomposition of nickel malonate

The kinetics of evolution of the main gaseous products (CO 2, CO and CH 4) of the isothermal decomposition (543–613 K) of nickel malonate, after preliminary dehydration, were studied in greater resolution than was possible previously, through use of plots of rate (dα/d t) against extent of reaction, α. An initial deceleratory process (α < 0.12) was followed by a brief acceleration to constant-rate (0.30 < α < 0.60). The subsequent deceleratory region (α > 0.60) contains a subsidiary small maximum at α = 0.92. Activation energies of 176 ± 10 kJ mol −1 were determined from rate measurements at fixed α values. Condensation of the principal gaseous product, CO 2, altered the kinetics by removing the initial deceleratory reaction and producing sigmoid α-time curves which were fitted by the Avrami-Erofe'ev equation with n = 4. Appreciable amounts (up to 20 mol-%) of acetate ion were shown by NMR to participate in the decomposition mechanism. The initial reaction is ascribed to reaction of residual water from the hydrated salt with the malonate ion to form acetate and CO 2. The sigmoid α-time curve, subsequently established, is the result of carboxylate breakdown promoted by the solid nickel carbide product, the reactivities of the acetate and malonate ions being similar. Scanning electron microscopic studies did not provide significant additional information concerning the mechanism.

Crystallization kinetics of some Ge-Sb-Se glasses

The crystallization kinetics of alloys in the Ge-Sb-Se system was investigated by means of differential thermal analysis, differential scanning calorimetry and X-ray diffraction. The calorimetric results obtained both for isothermal and continuous heating experiments were successfully interpreted with the same set of kinetic parameters. The Johnson-Mehl-Avrami-Erofe'ev equation with a rate constant following the Arrhenius behaviour gives the best fit to the experimental data. The values obtained for the effective activation energy increase from 75 to 229 kJ/mol with Sb content, except for pure selenium (81 kJ/mol). The pre-exponentlal factor of the rate constant increases at the same time from 1×10 6 to 1×10 16 s −1, except for pure selenium (4×10 6). The kinetic exponent ranges between 1.2. and 2.5 without any clear trend against composition. Some kind of compensation effect occurs that may be attributed to the primary crystallization of Sb 2Se 3 in most of the glasses.

Kinetics and mechanism of the solid phase dehydration of 11-aminoundecanoic acid dihydrate

The dehydration of 11-aminoundecanoic acid dihydrate was studied on both powdered material and single crystals by means of thermogravimetric measurements, optical observations and diffractometric procedures. The reaction can be described empirically by the Avrami-Erofe'ev equation, with parameters K A and n that change with water vapour pressure and temperature. No Smith-Topley effect is observed, and n ranges between 1 and 2.2, with maximum values at intermediate water vapour pressures. Optical observations show that the reaction starts through surface nucleation and proceeds inwards and laterally from exposed (100) and (010) faces, which correspond to water movement in planes perpendicular to the direction of the molecule chains. Diffractometric observations show that dehydration of single crystals gives rise topotactically to a coherent array of anhydrate microcrystals. These observations are rationalized through a qualitative model that involves surface nucleation, growth and coalescence of nuclei and ingestion of undeveloped nuclei; growth rate has to be described by two parameters representing growth normal to (100) or (010) planes and growth perpendicular to these faces. Under limiting conditions, such a model may give rise to boundary advance or even diffusional control of the dehydration process.

Glass-to-crystalline transformation in rapidly quenched Fe 78B 9Si 13 ferromagnetic alloy

Calorimetric studies of the ferromagnetic Curie temperature T c, and the crystallization kinetics of the metallic glass Fe 78B 9Si 13 have been performed in an attempt to elucidate the possibility of reversible relaxation processes near T c and the crystallization mechanisms taking place. From the change of T c with heating rate and on annealing it appears that ageing irreversibly increases the Curie temperature. Crystallization is thermally activated following an Arrhenius behaviour and proceeds in two stages, the best fit to the experimental data for each stage of crystallization has been obtained by use of a Johnson-Mehl-Avrami-Erofe'ev equation. The effective activation energy and the kinetic exponent are respectively E = (4.7 ± 0.1) eV, n = 2.0 ± 0.2 for the first and E = (4.5 ± 0.1) eV, n = 4.0 ± 0.2 for the second stage of crystallization. From these results it appears that the mechanisms of crystallization are quite different in both stages.

Kinetic study of isothermal and continuous heating crystallization in GeSe 2GeTeSb 2Te 3 alloy glasses

Calorimetric studies of chalogenide alloy glasses, (GeSe 2) 60(GeTe) 40 − x (Sb 2Te 3) x obtained by water quenching at room temperature, have been performed by isothermal and continuous heating in a Perkin-Elmer DSC-2 calorimeter. The samples show a simple peaked crystallization exotherm when x = 0, 10 and two stage crystallization for x = 20. The results obtained in both iso- and non-iso-thermal experiments have been successfully interpreted with the same set of kinetic parameters. Among the several kinetic models which we analysed, the best fit to our experimental data has been obtained in the case of the Johnson-Mehl-Avrami-Erofe'ev equation, with values of the kinetic exponent dependent only on composition. The effective activation energy decreases slowly from E = (229 ± 6)kJ/mol at x = 0 to E = (184 ± 5)kJ/mol at x = 20 (first crystallization peak).

Rigorous determination of kinetic parameters from DTA measurements

A rigorous analysis of the application of the DTA technique to the solid-state reaction formally described by the Avrarni-Erofe'ev equation leads to a quantitative relationship among the experimental parameters obtainable from thermograms. A non-linear regression procedure allows the simultaneous evaluation in a very simple way of the activation energy,Eact, of the reaction ordern and of the pre-exponential Arrhenius factor,A. The knowledge of these quantities leads to considerations about the reaction mechanism and to the complete prevision of the behaviour of the transformation. Isothermal literature data totally agree with our experimental results.

Solid state decomposition studies on fluoroperoxo species of transition metals. Part XII. Kinetics of isothermal decomposition of K 2VO(O 2) 2 F and K 3Nb(O 2) 2F 4

The kinetics of the isothermal decomposition of K 2VO(O 2) 2F and K 3Nb(O 2) 2F 4 have been investigated in the temperature range 432–463 K using a constant volume apparatus. The α-time plots for both these solids are sigmoidal in nature. The kinetic obedience to the Avrami-Erofe'ev equation ( n = 2) is observed for the initial stage of the decomposition. The contracting volume equation fits well for the deceleratory region. The activation energies are 108 and 53 kJ mole −1 for K 2VO(O 2) 2F, and 163 and 120 kJ mole −1 for K 3Nb(O 2) 2F 4. The effect of grinding on the latter solid shows deceleratory α-time curves. The decomposition of the ground sample is best described by the unimolecular decay and contracting volume equations. The change in reactivity also confirms the topochemical nature of solid state decompositions. In addition to this, the absence of lattice water in these two solids may also be responsible for the appearance of induction and acceleratory regions in pristine solids, in contrast to hydrated fluoroperoxozirconates, wherein the dehydration step precedes peroxide decomposition leading to facile nucleation and the totally deceleratory nature of isothermal decomposition (α-time) curves.

New phases in equimolar PbOV 2O 5 system

The quenching of the equimolar PbOV 2O 5 melt gave rise to an amorphous material or two polycrystalline phases mixed with an amorphous matrix, depending on the cooling rate. These materials were submitted to heat treatment and all generated pure lead metavanadate crystals. All phases were characterized by means of powder X-ray diffraction, differential thermal analysis, scanning electron microscopy, and infrared absorption spectroscopy. Furthermore, the interdependence of all the amorphous and crystalline phases is explained. It was also possible to evaluate the kinetic parameters (activation energy and reaction order) of the glass → α-LPV phase transformation. It was confirmed that this reaction follows the Avrami-Erofe'ev equation.