Calculation of phase equilibria and construction of phase diagrams by convex hull method

Recent findings for a new method developed to calculate phase equilibria are reviewed and analyzed. This method is based on the construction of the convex hulls for characteristic functions in heterogeneous systems. The theoretical basis of this method, as well as the possibilities and features of its practical application in scientific research and teaching chemical thermodynamics, is considered. Software packages developed at the Department of Chemistry of Moscow State University are briefly described.     

Calculation of phase diagrams of ternary M-Ga-Sb (M = In,Al) systems by convex hull approach

The ternary semiconductor phase diagrams of M-Ga-Sb (M = In, Al) are calculated by means of the convex hull approach. The software package TernAPI is tested as an effective tool for these purposes. The new facilities and advantages of this package are proven. It enables one to construct ternary phase diagrams based on thermodynamic properties of all phases existing in systems. The changes in phase region shapes and phase transformations with temperature can be studied by means of the TernAPI package. Selection of reliable thermodynamic models of solutions is of crucial importance for correct calculations of phase equilibria. It is shown that an adequate phase diagram can be constructed when a polynomial model of liquid solutions in the Al-Ga-Sb system is used.     

Microanalysis of the surface concentration of sulfate groups at polystyrene particle by isothermal titration calorimetry

 Microanalysis of sulfate groups at polystyrene particle surfaces, which were derived as persulfate initiator fragments, was carried out with isothermal titration calorimetry, and compared with a conventional conductometric titration. The quantitative analysis was possible even with an extremely small number of polystyrene particles have 10 μmol sulfate groups. Received: 15 December 1998 Accepted in revised form: 24 February 1999     

A universal method for calculating isobaric-isothermal sections of ternary system phase diagrams

A method for calculating and constructing isobaric-isothermal sections of ternary system phase diagrams with the use of convex hulls was developed. The method is based on the projection of the singularities of the lower part of the Gibbs energy convex hull onto the plane of component mole fractions followed by a geometric analysis of the properties of this projection. The method is applicable to a wide range of ternary systems, it can be used to find tie-line coordinates and determine the phase compositions of all the diagram regions. The quality of the suggested algorithm was estimated, and examples of the construction of several phase diagrams are given.     

The development of a quick method for amorphicity determination by isothermal microcalorimetry

This study presents a contribution in the development of a quick and accurate testing method for the determination of amorphous content using isothermal microcalorimetry. Examples demonstrated how the choice of the experimental conditions, especially sample load, temperature and humidity, influences the crystallization of the amorphous material. The suitability of this systematic approach was first tested on well-known lactose and afterwards on nifedipine as model compounds. It was shown that by proper method design and careful selection of experimental conditions, it is possible to achieve quick determination of the amorphous content in samples with a quantification limit of less than 1%, what is considerably better than by classical analytical methods such as DSC and XRPD. Our optimized microcalorimetry method gave also better results compared to previously reported literature data for nifedipine.     

Characterization of epoxy prepreg SPX 8800 system by isothermal differential scanning calorimetry

Isothermal Differential Scanning Calorimetry (DSC) was used to study the curing behavior of epoxy prepreg SPX 8800 system, which contains DGEBA/DICY/Diuron (Diglycidyl ether of bisphenol A/Dicyandiamide/Diuron) reinforced by three layers of glass fibre. The rate curves from the DSC study agreed well with those obtained from the isothermal FT Near Infrared (FTNIR) study and similar activation energy was obtained in the range of 92.6 to 87.7 kJ/mol up to 50% total conversion. Modelling of the whole DSC trace with empirical equation dx/dt=kx^m(A-x)ⁿ gave relatively good fitting of the experimental curves (the error is lower than 15%.) in the whole studied cure temperature range (75-110°C) and no significant difference in cure kinetics was observed for both epoxy prepreg and neat resin.     

Remarks on the relationship between isothermal and non-isothermal kinetic results. A new method for the kinetic analysis of isothermal data

Addition of 10mg/l of the haevy metal ions Zn⁺⁺, Cu⁺⁺, Pb⁺⁺, Cd⁺⁺ and Cr⁺⁺ to nongrowing E.coli cells shows a decrease in heat production by aerobic and anaerobic glycolysis up to 80 %. The heat production of cells incubated without glucose was decreased drastically too, after addition of 10mg/l of these heavy metal ions.The pattern of heat released during active transport of α MG was influenced by these haevy metal ions in a way, that there was a decrease in maximum heat flow, whereas the total heat production seemed to be constant.After increasing the metal ion concentration up to 100mg/l no endogenous heat production could be detected. The inhibition of heat production after addition of glucose was higher than 90 % in this case and there was a strong inhibition in α MG uptake.The results of these investigations indicate that the basic action of all tested metal ions is the energy metabolism and not the transport as stated earlier.     

用耦合簇运动方程计算分子的多光子吸收截面

简短评述了多光子吸收过程的原理与应用．由于该过程的高度非线性，对于分子的高激发态性质的描述要求很高，因为从本质上讲，这涉及到从低激发态到高激发态的跃迁过程．我们提出了用耦合簇运动方程方法并结合半经验哈密顿参数来计算大分子体系的多光子吸收截面．以蒽一卟啉一蒽分子为例，从激发态之间的跃迁密度分析发现，分子内电荷转移过程可以极大地增强三光子吸收．     

Kinetics and enthalpy measurements of interaction between beta-amyloid and liposomes by surface plasmon resonance and isothermal titration microcalorimetry

The objective of this research is to understand the interaction mechanism of β-amyloid (Aβ) with cell and were basically divided into two parts. The first part focused on the time-dependent structural changes of Aβ (1-40) by circular dichroism (CD) spectroscopy, thioflavin T (ThT) fluorescence assay, and atomic force microscopy (AFM). The second part emphasized the kinetics and enthalpy of interaction between Aβ (1-40) and liposome by surface plasmon resonance (SPR) and isothermal titration microcalorimetry (ITC). Results obtained from CD, ThT and AFM confirmed the formation of 1 μm fibril after single day incubation. The driving force of kinetic interaction between Aβ and liposomes was revealed by SPR to be electrostatics. Further studies indicated that fresh Aβ has high GM1 affinity. Besides, addition of cholesterol to the liposome could alter membrane fluidity and affect the interactions of fresh Aβ with liposomes especially in the amount of Aβ absorbed and preserving the structure of liposome after adsorbing. Hydrophobicity was found to be the driving force leading to the interaction between Aβ fibrils and liposomes. These reactions are endothermic as supported by ITC measurements. When the composition of liposomes is zwitterionic lipids, the interaction of Aβ with liposomes is predominantly hydrophobic force. In contrast, the driving force of interaction of charged lipids with Aβ is electrostatic.     

Studies of the binding mechanism between aptamers and thrombin by circular dichroism, surface plasmon resonance and isothermal titration calorimetry

Thrombin, a multifunctional serine protease, has both procoagulant and anticoagulant functions in human blood. Thrombin has two electropositive exosites. One is the fibrinogen-binding site and the other is the heparin-binding site. Over the past decade, two thrombin-binding aptamers (15-mer and 29-mer) were reported by SELEX technique. Recently, many studies examined the interactions between the 15-mer aptamer and thrombin extensively, but the data on the difference of these two aptamers binding to thrombin are still lacking and worth investigating for fundamental understanding. In the present study, we combined conformational data from circular dichroism (CD), kinetics and thermodynamics information from surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC) to compare the binding mechanism between the two aptamers with thrombin. Special attentions were paid to the formation of G-quadruplex and the effects of ions on the aptamer conformation on the binding and the kinetics discrimination between specific and nonspecific interactions of the binding. The results indicated reasonably that the 15-mer aptamer bound to fibrinogen-binding site of thrombin using a G-quadruplex structure and was dominated by electrostatic interactions, while the 29-mer aptamer bound to heparin-binding site thrombin using a duplex structure and was driven mainly by hydrophobic effects.     

Potential energy surface, bound states, and rotational inelastic cross sections of the He-CH4 system: a theoretical investigation

We determined two potential energy surfaces (PES) for the He-CH(4) system by means of MP4 and Valence Bond (VB) calculations. The MP4 potential is similar to the one commonly adopted for this system [U. Buck, K. H. Kohl, A. Kolhase, M. Faubel, and U. Staemmler, Mol. Phys. 55, 1255 (1985)], while the VB PES is slightly more attractive. To evaluate the reliability of these potentials, we investigated the scattering properties by performing close coupling calculations, and concluded that: (i) the available experimental data do not permit the ranking among the PES considered; (ii) some theoretical predictions differ considerably from the experimental data, and these discrepancies cannot entirely be ascribed to the inaccuracy of the ab initio calculations; (iii) the scattering properties at low energy might discriminate between the MP4 and VB potentials.     

Studies of dynamic properties of heterogeneous catalytic reactions in a closed isothermal gradientless system over biographically inhomogeneous catalyst surface

The dynamics of a closed isothermal system of constant volume consisting of an ideal gaseous mixture and particles adsorbed over the biographically inhomogeneous catalyst surface, has been studied. The existence, uniqueness and asymptotic stability of the point of detailed equilibrium in each positive reaction simplex has been proved.

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Reaction kinetics of solid-state cyclization of enalapril maleate investigated by isothermal FT-IR microscopic system

To investigate the reaction kinetics of the solid-state degradation process of enalapril maleate, a Fourier transform infrared microspectroscope equipped with thermal analyzer (thermal FT-IR microscopic system) was used. The isothermal stability study was conducted at 120-130 degrees C for 1-2 h and changes in the three-dimensional plots of the IR spectra of enalapril maleate with respect to heating time were observed. The study indicates that the bands at 1649, 1728, and 1751 cm(-1) assigned to intact enalapril maleate gradually reduced in peak intensity with heating time. However, the peak intensities at 1672 and 1738 cm(-1) (due to enalapril diketopiperazine (DKP) formation) and at 3250 cm(-1) (corresponding to water formation) gradually increased with heating time. The solid-state diketopiperazine formation and the degradation process of enalapril maleate via intramolecular cyclization were found to be simultaneous. The isothermal decomposition curves were sigmoidal and were characterized by induction and acceleration periods, indicating the presence of autocatalytic solid-state decompositions. Moreover, the power-law equation (n = 1/4) was found to provide the best fit to the kinetics of decomposition. This isothermal FT-IR microscopic system was easily used to investigate the degradation of enalapril maleate and the concomitant formation of DKP. The solid-state reaction of enalapril maleate required an activation energy of 195+/-12 kJ/mol to undergo the processes of decomposition and intramolecular cyclization.     

Phase equilibria in quasi-binary sections of the Tl-Sn-S system

Phase equilibria in quasi-binary sections of the Tl-Sn-S system: (I) Tl₄SnS₄-SnS, (II) Tl₂SnS₃-SnS, (III) S-Tl₄SnS₄, (IV) S-Tl₂SnS₃, (V) Tl-SnS, and (VI) Tl₂S-SnS were studied by differential thermal analysis and X-ray powder diffraction. It was found that systems I and II are characterized by the eutectic type of interaction, the phase diagrams of systems III and IV are of the eutectic type with a degenerate eutectic at the sulfur melting point, and the phase diagram of system V is of the fourth type according to Roozeboom. Phase equilibria in system VI were specified.     

Implementation of the complex coordinate method for resonances and photoionization cross sections by means of matrix element extrapolation

A new implementation of the complex coordinate method for calculation of molecular resonances and photoionization cross sections is presented. The method originates from the idea of analytic continuation of stabilization graphs, but instead all matrix elements are numerically continued. Exterior scaling is invoked in conjunction with the use of dilationally adapted basis sets. It is operationally equivalent to the bivariational method in the Feshbach-partitioned manifold of complex basis functions, but no complex integrals are calculated. Instead, the complex eigenvalue problem arises from the analytical continuation of the realvalued matrix elements pertinent to any atomic or molecular structure code in its standard (real) form. That includes the possibility of using numerical basis sets. Since the method does not require modification of existing numerical codes, it can serve as a complement to any large scale CI or MCSCF molecular structure calculation.     

On the experimental estimation of surface enhanced raman scattering (SERS) cross sections by vibrational pumping

We present an in-depth analysis of the experimental estimation of cross-sections in surface enhanced raman scattering (SERS) by vibrational pumping. The paper highlights the advantages and disadvantages of the technique, pinpoints the main aspects and limitations, and provides the underlying physical concepts to interpret the experimental results. Examples for several commonly used SERS probes are given, and a discussion on future possible developments is also presented. Obtaining good estimates of SERS cross-sections is, in general, an extremely hard problem and has been a longstanding ambition of the SERS community for reasons that go from the purely applied (quantification of signals) to the more fundamental (comparisons of theoretical electromagnetic enhancement factors with experiment). Any method that can produce a standard protocol for the estimation of cross-sections is, accordingly, of great interest and an effort to understand its principles and limitations is required.