Thermodynamics phases/compounds

Nitridoborates of lanthanum and the lanthanides were obtained from reactions of lanthanide metal or lanthanide metal nitride with layer-like (a-)BN at elevated temperatures (3>1200°C). These reactions require elaborated techniques in the inert gas sample-handling and the use of efficient heating sources, such as induction heating. Only some compounds remain stable in this high-temperature segment, and the yields of such reactions are often limited due to the competing stability of binary phases, allowing only the most (thermodynamically) stable compounds to exist. 

These considerable solvent-dependent shifts demonstrate that for molecules with such pronounced conformational ambiguity, the equilibration of conformations has to be taken into account in the calculation of thermodynamic phase-equilibrium data. In order to enable a consistent treatment, we have implemented an automated conformation equilibration scheme in COSMO therm. A compound X can be represented by a set of COSMO-files for the conformers, and a multiplicity ojx(i) can be assigned to each con-former based on geometrical degeneration aspects. Then the population of a conformer, i, in a solvent S is calculated as... 

In the series of stable five-membered nitrogen-containing heterocycles, tetrazole possesses extreme characteristics that are inferior only to those of pentazole (cf. Section 6.07.4.3). Attention should be paid to the obvious fact that, in contrast to the hypothetical pentazole, tetrazole is a thermodynamically stable compound. This heterocycle surpasses the simple azide and hydrazoic acid by a number of key tests. However, tetrazoles are thermodynamically stable in the condensed phase, relatively weakly sensitive to impact and friction, and not very toxic, unlike HN3 which is notorious for its high sensitivity to impact, friction, thermal and electric impulses, and toxicity <1999THS(3)467>. 

Again, the diffusional theory rests on the assumption of local equilibrium or quasi-equilibrium. However, it is clear that no local equilibrium can exist in any diffusion couple in which the layers of some part of thermodynamically stable compounds are missing. Also, if successive layers of reactants and products are in equilibrium with each other, then all the system is in local equilibrium. Therefore, applying the Gibbs phase rule, it is easy to come to the logical conclusion that under constant temperature and pressure conditions no compound layer can occur at all between two reactants in a binary system since in this case the largest number of co-... 

Solubility is extremely difficult to calculate. Dozens of methods exist, but none is reliable enough to be used in the entire chemical diversity space populated by infinite drug candidates. Experimental solubility errors are relatively high and frequent. Moreover, solubility can change dramatically with the purity of the compounds, stability, and time. Solubility of liquid substances differs from that of solid phase compounds. Solubility is thermodynamically affected by crystal packing, influencing the process of crystal lattice disruption and hence polymorphism, amorphous solid compounds lead to imprecise experimental measures. Finally, publically available databases of solubility values contain a lot of errors. 

Tel. 301-975-2208, fax 301-926-0416, e-mail rdj3 enh.nist.gov Thermodynamic data for almost 5000 gas phase compounds. Estimations of structures drawn into program using Benson s additivity rules. IVTAN-TFIERMO database with enthalpies of formation and other thermodynamic properties for 2300 substances. PCs. 

Thermal analysis methods are widely used in all fields of pharmaceutics. They are unique for the characterization of single compounds. The information correlated with the thermodynamic phase diagrams is extremely helpful for rational preformulation and development of new delivery systems. Very rapid and requiring only very small samples of material, these methods are applicable in development and also in production for quality control. The combination with spectroscopic and crystallographic data will allow better insight in complex phase changes behavior. 

The chemical composition of stratospheric particles at a given altitude is governed by local thermodynamic conditions, and specifically is a complex function of temperature and water vapor concentration. The solubility of gas-phase compounds into liquid aerosol particles... 

Alpha SiC is the high temperature form of SiC. Unlike pSiC which is a single compound, aSiC has a large number of polytypes, approximately 250 having been identified so far.OiHiz] These polytypes have either a rhombohedral or a hexagonal structure. Polytypes, unlike polymorphs, are the same thermodynamic phase, are formed under the same conditions of temperature and pressure, and have similar properties and structure. Their close-packed layers ( 0001 for hexagonal) are identical but have a... 

Reaction 7 involves only gas-phase compounds and the equilibrium constants can be calculated from free energies listed in thermodynamic tables, i.e.,... 

The presence of amorphous oxycarbide (Si-O-C) phase was also revealed in the Nicalon fibers at the interface between SiC core and surface SiOa layer (Pumpuck, 1980 Lipowitz, 1987 Laffon, 1989 Porte, 1989), although any thermodynamically stable compounds have not been found in the Si-O-C system. The formation of such an amorphous silicon oxycarbide phase was also suggested during the pyrolysis of organics-substituted polysiloxane gels to form SiC (Zhang, 1990 Babonneau, 1990 Bums, 1992). 

In liquid phase epitaxial growth of compound semiconductor layers, the growth rate depends on the slope of the liquidus line and on the supercooling AT at the time of the initial contact between substrate and growth solution. This is another example that illustrates that the growth kinetics is determined by thermodynamic phase relations. 

Evaluation of thermophysical properties represents the second main objective of Chemical Engineering Thermodynamics. Pure compounds were considered in the previous three Chapters, a general discussion of mixtures in this one, and phase and chemical reaction equilibria in the ensuing Chapters 12 through IS. 

The calculation of vapor and liquid fugacities in multi-component systems has been implemented by a set of computer programs in the form of FORTRAN IV subroutines. These are applicable to systems of up to twenty components, and operate on a thermodynamic data base including parameters for 92 compounds. The set includes subroutines for evaluation of vapor-phase fugacity... 

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