Showing papers on "Standard molar entropy published in 1995"

Kinetic Study of the Irreversible Thermal Deactivation of Palmito (Acanthophoenix rubra) Polyphenol Oxidase and Effect of pH

Abstract: The optimal temperature of palmito (Acanthophoenix rubra) polyphenol oxidase (PPO) is 30 degrees C. The Arrhenius activation energy was calculated to be 5.41 kJ mol(-1). Standard enthalpy of the reaction is -60.99 kJ mol(-1). At 25 degrees C, standard free energy and standard entropy were, respectively, 16.75 kJ mol(-1) and -260.87 J mol(-1) K(-1). The enzyme heated at temperatures above 30 degrees C loses its activity. Fifty percent inhibition is reached in 18 min at 70 degrees C, in 8 min at 75 degrees C, and in 2.5 min at 80 degrees C. The kinetics of the thermal irreversible denaturation of this enzyme is characterized by two steps: N leads to X(Td) leads to D, where N represents the native form, X represents an intermediate form, the structure of which depends on the deactivation temperature Td, and D is the completely denatured form of the enzyme. Our experimental results rule out a two-isoenzyme (with varying heat sensitivity) model. The thermodynamic parameters of the irreversible denaturation of the intermediate form were 102.70 and 97.10 kJ mol(-1) for activation enthalpy and activation energy, respectively, and 16.85 J mol(-1) K(-1) for activation entropy at 60 degrees C. Furthermore, this paper describes the effect of pH on the activity of the PPO. Studies were carried out with 4-methylcatechol and pyrogallol as substrates. The pH profile was not a function of the nature of the substrate assayed. The pH optimum was 5.2. The plot of logVmax app vs pH indicates that the oxidation of the substrates depended of the ionization of two groups in the enzyme-substrate complex with apparent pK values of 3.06 and 7.29 and 3.44 and 7.12, respectively, for 4-methylcatechol and pyrogallol. The very slight differences between the values suggest the existence of only one site on the molecule for both substrates.

Micellization of Polystyrene-b-Poly(ethylene/butylene)-b-Polystyrene Triblock Copolymers in 4-Methyl-2-pentanone

Abstract: The micellization thermodynamics, micelle structural parameters, and micelle size distribution for polystyrene-b-poly(ethylene/butylene)-b-polystyrene copolymer solutions in 4-methyl-2-pentanone were studied. The solvent used is selective for the outer PS blocks of the copolymer. Standard Gibbs energy, standard enthalpy, and standard entropy of micellization were determined from light scattering measurements. The influence of the copolymer molar mass on these magnitudes was analyzed. The structural parameters of micelles formed by different triblock copolymers were determined by static light scattering. Micelle molar mass and size increase with the length of the copolymer chain for a constant copolymer composition ; however the association number decreases. Micelle size distribution functions were obtained from dynamic light scattering and size exclusion chromatography measurements. The results obtained by means of both techniques were compared.

Phase transition and entropy of amorphous ices

Abstract: Mishima’s recent data [J. Chem. Phys. 100, 5910 (1994)], showing that the isothermal interconversion of low‐ and high‐density forms of pressure amorphized ice is of first order thermodynamically, have been analyzed in terms of the configurational entropy. At the equilibrium pressure of 2 kbar and 135 K, the entropy of transformation is zero or negligible so that the configurational entropy of the two forms is about the same. This distinguishes the low‐density form (whose entropy is about 3J/mol/K higher than that of hexagonal ice) from the hyperquenched glassy water. Comparison of calorimetric and relaxation behaviors of the pressure‐amorphized low density and hyperquenched glassy water also indicates that their structures differ. Reasons for this difference are given and the low magnitude of the residual entropy of glassy water is discussed. It is argued that model‐based calculations of residual entropy do not yield a plausible value for liquids.

The enols of acetic acid and methyl acetate

Abstract: We have determined the heat of formation of 1,1-dimethoxyethene, ΔHf(l) = −75.56 ± 0.87 kcal mol−1, by measuring the heat of hydrolysis. The heat of vaporization, 8.47 ± 0.10 kcal mol−1, was estimated from the boiling points at various pressures. The standard entropy of gaseous 1,1-dimethoxyethene, 82.02 ± 2. cal deg−1 mol−1, was calculated using frequencies obtained by MO calculations at the 6-31G** level. The free energy of transfer, 1.02 ± 1, was estimated by additivity with allowance for a distant polar interaction. Thus the free energy of formation in aqueous solution, ΔGf(aq) = −37.07 ± 1.48 kcal mol−1, could be calculated. The free energies of hydrolysis for enol ethers appear to be insensitive to structure; we used an average value and so calculated the values of the free energies of formation of the enols of acetic acid and methyl acetate, −67.90 ± 1.67 and −52.48 ± 1.53 kcal mol−1, respectively. These values are in good agreement with other recent evaluations. This method should be applicable to...

Entropy of adsorption of mixed surfactants from solutions onto the air/water interface

Abstract: The partial molar entropy change for mixed surfactant molecules adsorbed from solution at the air/water interface has been investigated by surface thermodynamics based upon the experimental surface tension isotherms at various temperatures. Results for different surfactant mixtures of sodium dodecyl sulfate and sodium tetradecyl sulfate, decylpyridinium chloride and sodium alkylsulfonates have shown that the partial molar entropy changes for adsorption of the mixed surfactants were generally negative and decreased with increasing adsorption to a minimum near the maximum adsorption and then increased abruptly. The entropy decrease can be explained by the adsorption-orientation of surfactant molecules in the adsorbed monolayer and the abrupt entropy increase at the maximum adsorption is possible due to the strong repulsion between the adsorbed molecules.

Zum chemischen transport von yttriumtrichlorid mit aluminiumtrichlorid

Abstract: Yttriumtrichlorid ist mit gasformigem Aluminiumtrichlorid im Temperaturbereich von 300°C bis 700°C von hoherer zu tieferer Temperatur chemisch transportierbar. Aus der Abhangigkeit der Transportrate von der Temperatur und vom Gesamtdruck wird ein Gasphasenkomplex YAl4Cl15,g gefolgert und seine Bildungsenthalpie und Standardentropie hergeleitet. Chemical Transport of Yttrium Trichloride by Aluminium Trichloride Yttrium trichloride is transported by gaseous aluminium trichloride in the temperature range of 300°C to 700°C from hot to cold. It is followed a gaseous complex YAl4Cl15,g and the enthalpy of formation as well as the standard entropy of the complex from the dependence of the transport rate on the temperature and the initial pressure of Al2Cl6,g.

Ion exchange of amino acids at a natural organic ion exchanger: Thermodynamics and energetics.

Abstract: The thermodynamics and energetics of the ion exchange of four amino acids at a cellulosic ion exchanger have been studied. Experimental work included determination of ion exchange isotherms and the use of high-sensitivity titration microcalorimetry. A rigorous thermodynamic analysis of the data was developed allowing calculation of the standard free energy, the standard enthalpy, and standard entropy of exchange, and also the differential free energy, incremental enthalpy, and incremental entropy of exchange. The results show that the relative contributions of the enthalpy and entropy to the overall free energy differ markedly for the chosen amino acids. The reasons for these differences are analyzed and discussed. A knowledge of these fundamental thermodynamic properties indicates the solution conditions likely to give enhanced affinity of the ion exchanger for selected amino acids. The experimental techniques and analysis procedures developed are generally applicable to ion exchange separations of biomolecules. (c) 1995 John Wiley & Sons, Inc.

Thermodynamic stability of silicon oxycarbide Si5C6O2 (Nicalon)

Abstract: The standard entropy and heat of formation of amorphous silicon oxycarbide, Si5C6O2, have been assessed to be 161.1 J K−1 mol−1 and — 1282.3 kJ mol−1 respectively, based in part on the literature data of the mass-spectrometric vapour pressure of SiO(g) over a Nicalon fibre. Using the so-obtained standard Gibbs energy of Si5C6O2, new phase relations (predominance diagrams) of the Si-C-O system have been computed and graphically shown for the temperature range 300–2400 K, including such condensed phases as C, Si, SiC, Si2O, SiO, Si2O3, SiO2 and Si5C6O2. The thermal behaviour of Nicalon fibres can be quantified thermodynamically by use of the predominance diagrams.

EMF study on thermodynamic instability and 18 anomaly of (La1−xMx)2CuO4−δ (M = Ba, Sr and Ca) solid solution

Abstract: The standard free energy, the standard enthalpy of formation, and the standard entropy, of a (La1−xMx)2CuO4−δ (M = Ba, Sr and Ca) solid solution, were determined by the EMF (electromotive force) measurement using a solid state cell. The standard free energy change of solid solution formation from simple oxides was determined as a function of x. It was found that in the case of (La1−xBax)2CuO4−δ (LBCO) the solid solution is thermodynamically unstable in the compositional range 0.04 < x < 0.08 and the maximum instability energy is 3 kJ/mol. It was concluded that the thermodynamic instability results in the decreases in carrier concentration and Tc, and causes the so-called ś 18 nomaly’.

Maximum entropy analysis of chemical reaction energy dependence

Abstract: In this article the energy dependence of chemical and ionic reactions is examined and a new model is developed for the calculation of reaction probability and postreaction energy disposal for exchange reactions. The new model is based on the principles of the maximum entropy method. For the exchange reactions the reaction probability is based on either the translational or the vibrational energy of the reagents as appropriate, whereas for dissociation the vibrational energy is used as the controlling factor. The new method is compared with other methods used for direct simulation Monte Carlo calculations. Examples of flowfield solutions are presented and the differences between the results of the new method, those using Bird's method, and continuum solvers are discussed and evaluated.

Thermodynamic properties of water in compacted bentonite under external pressure-free conditions

Abstract: In an attempt to determine the thermodynamic properties of water in bentonite, the vapor pressure of water in compacted bentonite was measured as functions of water content and temperature, under external pressure-free conditions. The relative partial molar Gibbs free energy, enthalpy, and entropy of the water in bentonite were determined at a temperature of 298.15 K. The interlayer distance of montmorillonite in bentonite was also measured by X-ray diffraction. It is probable that one fourth of the total water included in the bentonite at water content of 20.3 wt% and dry density of 1.76 x 10{sup 3} kg/m{sup 3} is nearly free water; the water is not regarded as dilute electrolytic solution but the solution with higher ionic strength. Another one fourth of the water in the bentonite at the water content is bound water; the partial molar entropy of the bound water referred to pure water is from a half to whole of solidification entropy of pure water. The remainder is regarded as intermediately bound water.