Basis Of Thermodynamic Quantities Research Articles

Thermodynamic description of the aluminum–rhenium system

Phase relationships in the Al–Re binary system have been thermodynamically reassessed by using the CALPHAD technique to take into account the most recent experimental data in the Al-rich part. A new version of the Al–Re phase diagram considering the congruent melting of the Al11Re4 as well as the modified temperatures and nature of some equilibria has been obtained. The Redlich–Kister polynomials were used to describe the excess Gibbs energy of the liquid phase as well as the fcc_A1 (Al) and hcp_A3 (Re) terminal solid solutions. The l-Al4Re, h-Al4Re and Al3Re phases, which have a homogeneity range, were treated by a two-sublattice model with the respective formulae: Al0.8(Al, Re)0.2, (Al, Re)0.77(Al, Re)0.23 and (Al, Re)0.75(Al, Re)0.25. The five remaining intermediate compounds were considered as stoichiometric. The calculated phase diagram and the generated thermodynamic parameters were critically compared with the experimental data available in the literature. Finally, our results were also confronted to the computed ones by Huang and Chang on the basis of thermodynamic quantities derived from the controversial model of Miedema and coworkers.

Synthesis and characterization of hybrid membranes using poly(vinyl alcohol) and tetraethylorthosilicate for the pervaporation separation of water–isopropanol mixtures

AbstractHybrid membranes were prepared using poly(vinyl alcohol) (PVA) and tetraethylorthosilicate (TEOS) via hydrolysis and cocondensation reaction for the pervaporation separation of water‐isopropanol mixtures. The resulting membranes were characterized by Fourier transform infrared spectroscopy, wide‐angle X‐ray diffraction, and differential scanning calorimetry. The glass transition temperature of these membranes varied from 100 to 120°C with increasing TEOS content. Effects of crosslinking density and feed compositions on the pervaporation performances of the membranes were studied. The membrane containing 1.5:1 mass ratio of TEOS to PVA gave the highest separation selectivity of 900 at 30°C for 10 mass % of water in the feed mixture. It was found that the separation selectivity and permeation flux data are strongly dependent on the water composition of the feed and operating temperature. However, the membrane with the highest crosslinking density showed unusual pervaporation properties. The overall activation energy values were calculated using the Arrhenius‐type equation. The activation energy values for the permeation and diffusion varied from 49.18 to 64.96 and 55.13 to 67.31 kJ/mol, respectively. Pervaporation data have also been explained on the basis of thermodynamic quantities. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1304–1315, 2004

Thermodynamic model of chemical vapour transport and its application to some ternary compounds: I. The model

A new model of chemical vapour transport constructed exclusively on the basis of thermodynamic quantities is presented. The transport process is considered as a series of consecutive cycles consisting of four following stages: (I) volatilization of the solid(s) in the dissolution zone; (II) migration of the vapour solution to the crystallization zone; (III) deposition of the solid(s) in the crystallization zone; (IV) migration of the vapour solution to the dissolution to the dissolution zone. In stage I, equilibrium exists between solid and gas phase. If the solid volatilizes incongruently, another solid has to appear, and therefore equilibrium between the two solids and the gas phase is established. In stage II, because of temperature change, homogeneous reactions (association, dissociation, etc.) proceed in the vapour. Owing to the reactions, the composition of the gas changes and a new equilibrium state is established between gaseous substances. Since no solids participate in these reactions, the vapour can become a supersaturated vapour solution. The solution can be in labile ot metastable state of equilibrium with some solid which can exist in considered system, but which may not necessarily be the solid (or one of two solids) existing in the dissolution zone. In stage III the solid to be deposited is the one remaining in the labile equilibrium state with the gas phase of the composition established after its arrival into the crystallization zone. In such a case, the supersaturation of the gas phase with regard to the solid to be deposited is higher than the so-called critical supersaturation. The deposition of the first solid causes subsequent changes in the gas phase composition. Thety can shift the labile vapour solution into the region in which another solid is deposited as the next one if the supersaturation of the gas phase with regard to the solid is higher than the critical one, and so on. The deposition proceeds until the equilibrium between the deposited solid and the gas phase is established. If the superstation is lower than the critical one, the deposition of the solid cannot proceed and the gas phase remains a supersaturated metastable vapour solution. Considering the gas phase composition changes in the successive stages of the cycle, one can explain the behaviour of the transport system or determine the preconditions for depositing a solid of desirable composition.

Synthesis and Effectively Reversible Oxygenation of Some “Picket Fence” Type Porphyrinatocobalt(II) Complexes

Abstract Some “picket fence” type meso-tetrakis(2-substitu-ted phenyl)porphyrinatocobalt(II) complexes were newly synthesized, which had different peripheral substituents in their numbers and/or geometrical dispositions to form “cavity” on the porphyrin plane. Their O2 affinities were determined with 1-methylimidazole as an axial base at various temperatures in solution, by visible spectral measurements. On the basis of thermodynamic quantities, four neopentylcarbonylamino groups were the most effective as a picket fence, and the thermodynamic values of the complex were nearly comparable to those of cobalt(II)myoglobin. It was found that O2 affinity decreased with an increase in ΔH°, regardless of ΔS°. In the case of using various amines as an axial base, imidazoles were effective to enhance O2 affinity rather than pyridines.