Influence of cooling rate on crystallisation kinetics on microstructure of cast zinc alloys

In this study, the change of the cooling rate in the range of about 0.1–1 °C s^?1 and the addition of Sr on the crystallization kinetics of the cast zinc alloys of the ZnAlCu type, as well as its relation to the microstructure were also investigated. Therefore, the aim of the rapid crystallisation is the achievement of materials with better properties, which can be obtained by refinement of the dendritic or eutectic microstructure, elimination of segregation, or creation of metastable phases and their morphology changes. In the investigated alloys, the change of cooling rate of 1 °C s^?1 has caused microstructure’s refinement as well as increase in hardness. Increase in the cooling rate causes also morphology changes of the η + α eutectic, and makes generally a global overcooling of the alloy as well as change in the temperatures at the beginning of crystallization T _DN and of the alloy crystallization T _S. The presented investigations concerning the electron microscopy methods, including transmission electron microscopy, allow revealing the crystallographic structure, based on the d-spacing changes, as well as the diffraction method used for phase determination, which is a helpful tool for the explanation of the important points in the thermo-derivative analysis curve, where the relation between the amount of phase and the occurrence of new phases can be determined.     

Chemical Kinetics of Methane Pyrolysis in Microwave Plasma at Atmospheric Pressure

Results of chemical kinetics modeling in methane subjected to the microwave plasma at atmospheric pressure are presented in this paper. The reaction mechanism is based on the methane oxidation model without reactions involving nitrogen and oxygen. For the numerical calculations 0D and 1D models were created. 0D model uses Calorimetric Bomb Reactor whereas 1D model is constructed either as Plug Flow Reactor or as a chain of Plug Flow Reactor and Calorimetric Bomb Reactor. Both models explain experimental results and show the most important reactions responsible for the methane conversion and production of H₂, C₂H₂, C₂H₄ and C₂H₆ detected in the experiment. Main conclusion is that the chemical reactions in our experiment proceed by a thermal process and the products can be defined by considering thermodynamic equilibrium. Temperature characterizing the methane pyrolysis is 1,500–2,000 K, but plasma temperature is in the range of 4,000–5,700 K, which means that methane pyrolysis process is occurring outside the plasma region in the swirl gas flowing around the plasma.     

Current air quality analytics and monitoring: A review

This review summarizes the different tools and concepts that are commonly applied in air quality monitoring. The monitoring of atmosphere is extremely important as the air quality is an important problem for large communities. Main requirements for analytical devices used for monitoring include a long period of autonomic operation and portability. These instruments, however, are often characterized by poor analytical performance. Monitoring networks are the most common tools used for monitoring, so large-scale monitoring programmes are summarized here. Biomonitoring, as a cheap and convenient alternative to traditional sample collection, is becoming more and more popular, although its main drawback is the lack of standard procedures. Telemonitoring is another approach to air monitoring, which offers some interesting opportunities, such as ease of coverage of large or remote areas, constituting a complementary approach to traditional strategies; however, it requires huge costs.     

Nature of the water/aromatic parallel alignment interactions

The water/aromatic parallel alignment interactions are interactions where the water molecule or one of its O? H bonds is parallel to the aromatic ring plane. The calculated energies of the interactions are significant, up to ΔE_{CCSD(T)(limit)} = ?2.45 kcal mol^?1 at large horizontal displacement, out of benzene ring and CH bond region. These interactions are stronger than CH···O water/benzene interactions, but weaker than OH···π interactions. To investigate the nature of water/aromatic parallel alignment interactions, energy decomposition methods, symmetry‐adapted perturbation theory, and extended transition state‐natural orbitals for chemical valence (NOCV), were used. The calculations have shown that, for the complexes at large horizontal displacements, major contribution to interaction energy comes from electrostatic interactions between monomers, and for the complexes at small horizontal displacements, dispersion interactions are dominant binding force. The NOCV‐based analysis has shown that in structures with strong interaction energies charge transfer of the type π → σ*(O? H) between the monomers also exists. © 2014 Wiley Periodicals, Inc.     

Folding of the Tau Protein on Microtubules

Microtubules are regulated by microtubule‐associated proteins. However, little is known about the structure of microtubule‐associated proteins in complex with microtubules. Herein we show that the microtubule‐associated protein Tau, which is intrinsically disordered in solution, locally folds into a stable structure upon binding to microtubules. While Tau is highly flexible in solution and adopts a β‐sheet structure in amyloid fibrils, in complex with microtubules the conserved hexapeptides at the beginning of the Tau repeats two and three convert into a hairpin conformation. Thus, binding to microtubules stabilizes a unique conformation in Tau.     

pH-Dependent Behavior of Hydrophobically Modified Polyelectrolyte Shells of Polymeric Nanoparticles

Summary: The self-assembled core-shell nanoparticles based on fluorescently double-tagged high-molar-mass polystyrene-block-poly(methacrylic acid), PS-PMA, were prepared in aqueous buffers by dialysis from 1,4-dioxane – water mixtures. The conformations of shell-forming PMA chains were studied using nonradiative excitation energy transfer measurements. The study shows that two populations of distinctly different conformations (collapsed and stretched) coexist in the shell and their ratio depends on pH.     

Properties of polyethylene glycol supported tetraarylporphyrin in aqueous solution and its interaction with liposomal membranes

5,10,15,20-Tetrakis(4-hydroxyphenyl)porphyrin was functionalized by covalent attachment of poly(ethylene glycol) (PEG) chains of various molecular weights, 350, 2000, and 5000 Da. The properties of PEG-functionalized tetraarylporphyrins in aqueous solution and their interactions with liposomes have been studied. Electronic absorption spectroscopy, dynamic light scattering, atomic force microscopy, and fluorescence quenching were used to monitor aggregation of porphyrin chromophores and behavior of the attached PEG chains in the aqueous solution. The tendency for aggregation of porphyrin chromophores in aqueous solution and the efficiency of fluorescence quenching by KI decrease with increasing length of PEG chain linked to the porphyrin ring. The experimental results indicate that polymer clusters are present in aqueous solution of all pegylated porphyrins. The interactions between the pegylated porphyrins and phosphatidylcholine liposomes in the aqueous solution were studied using the fluorescence methods. The apparent binding constants of porphyrin chromophores to liposomes were determined. The degree of binding was found to be dependent on the molecular weight of the attached polymer.     

Blue-Shift Hydrogen Bonds in Silyltriptycene Derivatives: Antibonding σ* Orbitals of the Si−C Bond as Effective Acceptors of Electron Density

Triptycene derivatives are widely utilized in different fields of chemistry and materials sciences. Their physicochemical properties, often of pivotal importance for the rational design of triptycene-based functional materials, are influenced by noncovalent interactions between substituents mounted on the triptycene skeleton. Herein, a unique interaction between electron-rich substituents in the peri position and the silyl group located on the bridgehead sp³-carbon is discussed on the example of 1,4-dichloro-9-(p-methoxyphenyl)-silyltriptycene (TRPCl) which exists in solution in the form of two rotamers differing by dispositions, syn or anti, of the Si−C_Ph (the C_Ph atom is from the p-methoxyphenyl group) bond against the peri-Cl atom. For the first time, substantial differences between the Si−C_Ph bonds in these two dispositions are identified, based on indirect experimental and direct theoretical evidence. For these two orientations, the experimental ¹J(Si,C_Ph) values differ by as much as 10 percent. The differences are explained in terms of effective electron density transfer from the peri-Cl atom to the antibonding σ* orbitals of the Si−X bonds (X=H, C_Ph) oriented anti to that atom. The electronic effects are revealed by an NBO analysis. Connections of these observations with the notion of blue-shifting hydrogen bonds are discussed.