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Pure Substances (pure + substance)

Distribution by Scientific Domains
Chemistry42%

Selected Abstracts

Immobilized Cytochrome c Sensor in Organic/Aqueous Media for the Characterization of Hydrophilic and Hydrophobic Antioxidants

ELECTROANALYSIS, Issue 18 2003
Moritz Beissenhirtz
Abstract A method for the characterization of antioxidants is introduced, which allows the measurement of pure hydrophilic and hydrophobic substances as well as complex cosmetic creams. The sensor is based on cytochrome c covalently immobilized on a gold wire electrode working in mixtures of phosphate buffer and organic solvents. It is combined with a superoxide generating enzyme system. The decrease of the superoxide concentration in the test solution by the added antioxidants is detected and used for the quantification of their antioxidative efficiency. Electrochemical properties of immobilized cytochrome c, such as formal potential and heterogeneous electron transfer rate constant, have been investigated in mixtures of aqueous buffer and DMSO, methanol, butanediol, and THF. The maximum organic solvent content for quasi-reversible electrode behavior was correlated to spectroscopic measurements. The activity of the radical producing enzyme in such media was determined and the radical generation characterized. The antioxidative properties of pure substance such as ascorbic acid and Biochanin A as well as of five anti-ageing cosmetic creams were studied. This showed also the influence of matrix composition on the efficiency of antioxidative supplements. [source]

Dendritic solidification of binary alloys with free and forced convection

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2005
P. Zhao
Abstract Dendritic solidification with forced convection and free convection driven by contraction and thermo- solutal buoyancy is simulated in two-dimensional space using a sharp-interface model. Both pure substances and alloys are considered. The model is formulated using the finite element method and works directly with primitive variables. The coupled energy- and solutal concentration-equations, along with the Navier,Stokes equations for incompressible flow, are solved using different meshes. Temperature is solved in a fixed mesh that covers the whole domain (solid + liquid) where the solid,liquid interface is explicitly tracked using marker points. The concentration and momentum equations are solved in the liquid region using an adaptive mesh of triangular elements that conforms to the interface. The velocity boundary conditions are applied directly on the interface. The model is validated using a series of problems that have analytical, experimental and numerical results. Four simulations are presented: (1) crystal growth of succinonitrile with thermal convection under two small undercoolings; (2) dendritic growth into an undercooled pure melt with a uniform forced flow; (3) equiaxial dendritic growth of a pure substance and an alloy with contraction-induced convection; and (4) directional solidification of Pb,0.2 wt% Sb alloy with convection driven by the combined action of contraction, thermal and solutal buoyancy. Some of the simulation results are compared to those reported using other methods including the phase-field method; others are new. In each case, the effects of convection on dendritic solidification are analysed. Copyright © 2005 John Wiley & Sons, Ltd. [source]

High-Density Energetic Material Hosted in Pure Silica MFI-Type Zeolite Nanocrystals,

ADVANCED MATERIALS, Issue 18 2006
G. Majano
Desensitization of the "green" high-density energetic material Fox-7 inside the pores of pure silica MFI-type nanozeolites (see figure) leads to nonexplosive decomposition about 100,°C above the explosive temperature for the pure substance. [source]

Contact allergy caused by air oxidation of common materials , diagnosis and prevention

CONTACT DERMATITIS, Issue 3 2004
Ann-Therese Karlberg
When considering the allergenic activity of a compound not only the possibility of bioactivation by skin metabolism but also air activation by autoxidation must be taken into account. Natural compounds (terpenes) easily oxidize at air exposure. They are found in products that are common causes of allergic contact dermatitis (ACD) i.e. colophony and fragrances. The introduction of oxygen enables the molecules to form antigens with skin proteins via a nucleophilic- electrophilic interaction or via a radical reaction. The latter mechanism seems to be important since the primary oxidation products, the hydroperoxides, are the most potent sensitizers formed. Oxidative decomposition at air exposure resulting in allergenic oxidation products is observed also for other common compounds e.g. ethoxylated fatty alcohols used as surfactants. It is important to test the patient with the offending compounds for diagnosis of ACD. A negative diagnosis can be due to failure in testing with the correct substances. In the case of air activated compounds, testing should not be performed with the pure substances but rather with the oxidation mixture or the most sensitizing oxidation products (the hydroperoxides). We have in multicenter-studies shown that the common fragrance terpenes, limonene and linalool, are frequent sensitizers when oxidized. This is a challenge in clinical practice since such patch test materials are not easily standardized. Compounds, easily activated at air exposure, should be prevented from oxidative decomposition by addition of antioxidants and proper handling and storage. More research is needed in this area. [source]

Synthesis, Structure, and Reactivity of Novel Intramolecularly Coordinated Organolead(II) Compounds

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 19 2003
Klaus Jurkschat
Abstract The intramolecularly coordinated heteroleptic organolead(II) compounds {4- tBu-2,6-[P(O)(OEt)2]2C6H2}PbX (2, X = Cl; 3, X = SPh) have been synthesized. Single-crystal X-ray analyses reveal that both compounds adopt polymeric chain structures by intermolecular Pb,X···Pb bridges, a structural motif previously unknown for organolead(II) derivatives. Compound 2 reacts with lithium diisopropylamide, iPr2NLi, and lithium bis(trimethylsilyl)methane, (Me3Si)2CHLi, respectively, to provide in situ the corresponding organolead(II) compounds {4- tBu-2,6-[P(O)(OEt)2]2C6H2}PbX (4, X = iPr2N; 5, X = (Me3Si)2CH], which were identified by NMR spectroscopy but could not be obtained as analytically pure substances. Attempts to isolate the intramolecularly coordinated organolead(II) hexafluorophosphate {4- tBu-2,6-[P(O)(OEt)2]2C6H2}Pb+PF6, from the reaction of 2 with TlPF6 also failed. Instead, the unprecedented salt [{5- tBu-1,3-[P(O)(OEt)2]2C6H3}4·(Pb2F)] [PF6]3·4THF (6) was obtained, in which the [(Pb,F,Pb)]3+ cation is stabilized by eight intermolecular P=O,Pb interactions. Compound 6 was characterized by single-crystal X-ray analysis and electrospray mass spectrometry. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

Dendritic solidification of binary alloys with free and forced convection

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2005
P. Zhao
Abstract Dendritic solidification with forced convection and free convection driven by contraction and thermo- solutal buoyancy is simulated in two-dimensional space using a sharp-interface model. Both pure substances and alloys are considered. The model is formulated using the finite element method and works directly with primitive variables. The coupled energy- and solutal concentration-equations, along with the Navier,Stokes equations for incompressible flow, are solved using different meshes. Temperature is solved in a fixed mesh that covers the whole domain (solid + liquid) where the solid,liquid interface is explicitly tracked using marker points. The concentration and momentum equations are solved in the liquid region using an adaptive mesh of triangular elements that conforms to the interface. The velocity boundary conditions are applied directly on the interface. The model is validated using a series of problems that have analytical, experimental and numerical results. Four simulations are presented: (1) crystal growth of succinonitrile with thermal convection under two small undercoolings; (2) dendritic growth into an undercooled pure melt with a uniform forced flow; (3) equiaxial dendritic growth of a pure substance and an alloy with contraction-induced convection; and (4) directional solidification of Pb,0.2 wt% Sb alloy with convection driven by the combined action of contraction, thermal and solutal buoyancy. Some of the simulation results are compared to those reported using other methods including the phase-field method; others are new. In each case, the effects of convection on dendritic solidification are analysed. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Volume-translated equations of state: Empirical approach and physical relevance

AICHE JOURNAL, Issue 6 2003
Leonid V. Yelash
The volume translation technique is widely applied in chemical engineering modeling of phase behavior and thermodynamic properties. This empirical correction of the molar volume improves the correlation of liquid densities when cubic equations of state are employed. Based on a recently proposed method of analyzing equations of state, the influence of the volume translation on the properties obtained from an equation of state, such as virial coefficients, as well as on the phase behavior, critical properties, the isobaric thermal expansion coefficient, and the heat capacities of pure substances is investigated. The influence of different kinds of temperature-dependent volume translations is investigated and compared to approaches that originate from theoretical considerations. The investigation shows that empirical as well as theoretical approaches for the development of equations of state can exhibit nonphysical behavior. However, the understanding of the reasons for such problems can help to develop reliable equations of state based on both kinds of method. [source]