Effects of washing conditions on the thermal decomposition behaviour of gelled UO3 microspheres

DTA, TG and DTG curves obtained in various atmospheres using different heating rates were used together with X-ray examinations to study the thermal decomposition mechanisms of two types of gelled UO₃ microspheres: ammonia-washed (UN) and hot water-washed (UH) microspheres. The kinetics of the thermal decompositions were studied. The specific reaction rate constantk _r for the decomposition of UO₃ to U₃O₈ could be expressed in terms of the activation energy and the pre-exponential factor by the expressions: $$\begin{gathered} K_r (s^{ - 1} ) = 1.277 \times 10^{18} \exp \frac{{ - 295.4}}{{RT}}for the UN spheres, \hfill \\ K_r (s^{ - 1} ) = 8.406 \times 10^{19} \exp \frac{{ - 263.2}}{{RT}}for the UH spheres. \hfill \\ \end{gathered} $$      

Effects of drying conditions on thermal decomposition behaviour of UO3 gel microspheres

The effects of the drying conditions on the thermal behaviour of UO₃ gel microspheres were studied by TG, DTA and X-ray examination. The effects of drying with air, steam or alcohol on the crystal structure and crystallite size were also studied. The results indicate that the thermal decomposition of UO₃ gel microspheres involves five steps: the first two for dehydration, the third for ammonia release, the fourth for ammonia oxidation, and the last one for UO₃ recrystallization. It was also found that the crystal growth varied from 110 Å after air drying to 512 Å and 496 Å after steam and alcohol treatment, respectively.

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Zusammenfassung Die Effekte der Trocknungsbedingungen auf das thermische Verhalten von UO₃-Gelmikrokugeln wurden mittels TG, DTA und Röntgenanalyse untersucht. Ebenfalls wurden die Auswirkungen der Trocknung mit Luft, Dampf und Alkohol auf die Kristallstruktur und die Kristallitgröße untersucht. Die Ergebnisse deuten darauf hin, daß die thermische Zersetzung der UO₃-Gelmikrokugeln in 5 Schritten verläuft: die ersten zwei sind Dehydratisierungsprozessen, der dritte der Freisetzung und der vierte der Oxydation von Ammoniak und der letzte der Rekristallisation des UO₃ zuzuschreiben. Die Kristallitgröße variiert zwischen 110 Å nach Lufttrocknung und 512 bzw. 496 Å nach Trocknung mit Dampf bzw. Alkohol.

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, UO₃ . , UO₃. , UO₃ : — , — , — — UO₃. , 110 Å 512 496 Å , , .

     

The phase behaviour ofl-α-phosphatidylcholine in the presence of chlorpromazine under different experimental conditions

The phase behaviour ofl-α-phosphatidylcholine liposomes has been studied as a function of temperature, pH, ionic strength, etc., in the presence and absence of chlorpromazine by a polarization method using 1,6-diphenyl-1,3,5-hexatriene as a fluorescence probe. The gel crystalline to liquid crystalline (T_c), liquid crystalline to cubic (T_II), and cubic to hexagonal (H_II) transition temperatures in multilayer liposomes have been detected, and are found to be affected by chlorpromazine under different experimental conditions.     

X-ray structural parameters and the thermal stability of 1,2-dioxetanes

The structural parameters and thermal activation data for 1,2-dioxetanes are reported, showing that the degree of puckering of the peroxide ring does not influence the thermal stability of these “high energy” molecules.     

Influence of polymyxins on the structural dynamics of Escherichia coli lipid membranes

Polymyxins are a family of nonribosomic cationic peptide antibiotics highly effective against Gram-negative bacteria. Two members of this family, Polymyxins B and E (PxB, PxE), form molecular vesicle-vesicle contacts and promote a selective exchange of phospholipids at very low concentrations in the membrane, a biophysical phenomenon that can be the basis of their antibiotic mode of action. To get more insight into the interaction of these antibiotics with the lipid membrane, their effect on the structural dynamics of bilayers prepared with lipids extracted from the membrane of Escherichia coli was determined using fluorescently labeled phopholipids. Steady-state anisotropy measurements with probes that localize at different positions in the membrane give information on the effects of polymyxins on the mobility of the phospholipids. Results with PxB, PxE, colymycin M and polymyxin B nonapeptide (PxB-NP), a deacylated derivative with no antibiotic properties, are compared. At low peptide concentrations (<2 mol%) PxB and PxE bind to the membranes superficially, affecting very slightly the ordering of the lipids at the outermost part of the bilayer. Above this concentration, PxB and PxE insert more deeply in the bilayer, increasing lipid order both in the gel and liquid-crystal states and modifying phase transitions. Fluorescence experiments with pyrene labeled phospholipids indicate that the increase in lipid packing is accompanied by an enrichment of phospholipids in the bilayers. In contrast, colymycin M and PxB-NP did not modify lipid packing or phase transition, nor did they induce microdomain formation. The possible significance of these results in the antibiotic mode of action of PxB and PxE is discussed. The combination of spectroscopic techniques described here can be useful as part of a general method of screening for new antibiotics that act on the membrane by the same mechanism as polymyxins.     

Simulations of the dynamics of thermal undulations in lipid bilayers in the tensionless state and under stress

The relaxation processes of height undulations and density fluctuations in a membrane have been studied by molecular dynamics simulations of a coarse grained amphiphilic bilayer model. We observe a double exponential decay in their time correlations, with relaxation rates in good quantitative agreement with the theory by Seifert and Langer [Europhys. Lett. 23, 71 (1993)]. Intermonolayer friction due to slippage between the two monolayers is shown to be the dominant dissipative mechanism at the high wave numbers, q>10 mum(-1), typically encountered in computer simulations. We briefly discuss the ramifications of the slow undulatory relaxation process for the calculation of bending rigidities from the static undulation structure factors. The relaxation rates are sensitive to the surface tension, and at high elongations an oscillatory contribution is observed in the time correlation of the undulations.     

Structural identification of a novel degradant of the antibiotic pirlimycin formed under thermal stress conditions

The structure of a novel, thermally-induced degradant of the antibiotic pirlimycin is presented. The degradant was discovered during thermal stress stability testing of pirlimycin. The structure was determined using high resolution mass spectrometry, ms/ms fragmentation, and extensive nmr studies including a combination of homonuclear TOCSY and gradient, inverse-detected 2-D nmr experiments which included GHSQC and GHMBC. All nmr, high resolution ms and ms/ms fragmentation data are consistent with loss of a molecule of HCl during a ring closure involving one of the hydroxyl moieties from the sugar portion of the molecule forming a tetrahydrofurano[3,2-b]perhydropyran ring structure. The X-ray crystal structure is reported for pirlimycin. Using this structure molecular modelling studies are performed which demonstrate the feasibility of the formation of the new structure.     

The dissimilarity between the electron-impact and photochemical or thermal behaviour of some methyl cyclobut-3-ene-1,2-dicarboxylates and muconates

Comparison of the mass spectral behaviour of the dimethyl esters of the epimeric (cis- and trans-) cyclobut-3-ene-1,2-dicarboxylic acids and the isomeric muconates (cis, cis-, cis, trans- and trans, trans-) and some substituted analogues shows that no correlation exists between the electron-impact-induced fragmentation and the predicted photochemical or thermal transformations in these systems. The comparison involved fragmentation patterns (supported by deuterium labelling), metastable transitions, and also kinetic energy release in the field free region.     

Syntheses,structural characterization,and thermal behaviour of metal complexes with 3-aminopyridine as co-ligands

Transition Metal Chemistry - Six mixed metal complexes with 3-aminopyridine (3-ampy) as a co-ligand have been synthesized: catena-{[M(μ2-3-ampy)(H2O)4]SO4·H2O} (M=Ni (1) and Co (2)),...     

Investigation of the thermal and structural behaviour of diclofenac sodium-sugar ester surfactant systems

Sugar esters (SEs) have a wide range of hydrophilic-lipophilic balance (HLB) values (1–16) and hence can be applied as surfactants or as solubility or penetration enhancers. They can be used for hot melt technology and solvent method which are frequently applied techniques to preparation of solid dispersions. In this study drug-SE products were prepared by physical mixing, melt technology and solvent methods. The products were investigated by DSC, X-ray powder diffraction and dissolution tests. Diclofenac sodium (DS) as model drug and two SEs, P1670 (HLB=16) and S970 (HLB=9) were used for the preparation of the products. DSC curves revealed considerable melting range and enthalpy decreases for the DS-SE products. The dissolved drug molecules broke down the structures of the SEs but were not built into the crystalline phase of the carrier. The melt technology led to a solid dispersion while in the case of the solvent methods the DS was in molecularly dispersed form which resulted in faster dissolution. The drug release was influenced by the structures resulting from the various treatments, by the HLB and by the gel-forming behaviour of the SEs.     

Some observations on the thermal behaviour of curcumin under air and argon atmospheres

The TG, DTG and DTA curves of curcumin(I) have been recorded in static air and inert dynamic argon atmosphere over the range between ambient temperature and 600°–700°C using a Netzsch thermal analyser STA 429. Careful examination of these curves reveals appreciable differences in the behaviour of I under either atmospheres, which are easily recognized by comparing the profiles of their thermal curves, particularly in the melting point, thermalL stability of intermediates, percent weight loss and exothermicity of the chemical processes. Gaschromatographic analysis of volatile pyrolysates trapped during thermal analysis indicates the formation of (CH₃)₂CO, CH₃COH and C₆H₅OCH₂COOH (phenyl oxyacetic acid). However, in static air CO₂ and H₂O were identified as well. X-ray diffractometry reveals the formation of amorphous carbon as a final product in argon and a mixture of amorphous carbon and graphite in air. It seems that the relatively high mass of argon plays an important role in the reactions and stability of intermediates. In either atmospheres curcumin is thermally stable up to 249°C withm.p. of 176.4°–177.5°C. The unique shape of the DTA curve of I could be used forits identification.     

Studies on the Toxic Oil Syndrome: proposal of a mechanism for the thermal conversion of 3-N-phenylamino-1,2-propanediol esters into anilides under deodorisation conditions

A study on the reaction mechanism for the conversion of title esters, the species recognised as toxic biomarkers of the oil batches responsible for the Toxic Oil Syndrome, into the corresponding anilides under the thermal conditions of an oil deodorisation process was performed using experimental and computational techniques. The results obtained suggest a reaction course that includes two basic steps: an intramolecular process involving the reaction of the amine group of the diester derivative with the secondary ester of the same compound, followed by the attack of an aniline molecule to form the (E)-isomer of an imidic acid, which would finally tautomerise to give the final anilide.     

Effect of thermal treatment conditions on the phase composition and structural characteristics of V-Mo-Nb-O catalysts

The formation of an active phase in V-Mo-Nb oxide catalysts for the selective oxidation and ammoxidation of ethane during thermal treatment in air and in helium was studied using high-temperature in situ and ex situ X-ray diffraction analysis, transmission electron microscopy, IR spectroscopy, and the differential dissolution method. It was found that, in thermal treatment below 500°C, the formation occurred through the same irreversible steps with the formation of a unidimensionally ordered layered compound with structure elements like Mo₅O₁₄ regardless of the calcination atmosphere. Above 500°C, the formation of crystalline phases took place; the composition and structure of these phases depended on the atmosphere of thermal treatment. The unidimensionally ordered V-Nb-Mo oxide with structure elements like Mo₅O₁₄ exhibited the best catalytic properties.     

Interaction of E2 and NS3 synthetic peptides of GB virus C/hepatitis G virus with model lipid membranes

The membrane-interacting properties of two potential epitopes of the GB virus C/Hepatitis G virus, located respectively at the regions (99-118) of the E2 structural protein and (440-460) of the NS3 non-structural protein were studied. Changes in the intrinsic fluorescence of Trp and Tyr residues after the addition of DPPC-LUV revealed that the peptide-membrane interaction was optimal above the gel-liquid crystalline transition temperature of the lipid. Differential scanning calorimetry studies showed that the E2 peptide incorporated into lipid bilayers perturbs the packing of lipids and affects their thermotropic properties. Moreover, the 20-mer structural peptide induced a slow leakage of vesicular contents at 55 degrees C.     

Synthesis of polyamides from active diacyl derivatives of 3-hydroxy-1,2-benzisoxazole and diamines under mild conditions

New active diesters and diamides derived from 3-hydroxy-1,2-benzisoxazole were prepared for use in polyamide synthesis. The active esters and amides reacted readily with amines to give quantitative yields of amides. The high reactivity to these active esters and amides was discussed in relation to the electon-withdrawing effect of the leaving group and intramolecular general-base catalysis. Solution polycondensation of new active diesters and diamides with aliphatic and aromatic diamines proceeded slowly under mild conditions to produce polyamides with inherent viscosities up to 1.5.     

Effects of palmitic acid and cholesterol on proton transport across black lipid membranes

We studied the effect of palmitic acid (PA) and cholesterol (approximately 17 wt.%) on proton translocation across asolectin (charged) and diphytanoylphosphatidylcholine (DPhPC, neutral) black lipid membranes (BLMs). Potential difference (PD), short circuit current (SCC), and conductance (G(total)) were measured with a digital electrometer. Membranes were exposed to pH gradients (0.4-2.0 units), followed by PA addition to bath (symmetrically, 40-65 microM). The membrane conductive pathway was subdivided into an unspecific and a proton-related routes. A computer program estimated the conductances (G(un) and G(H)) of the two pathways from the measured parameters. No significant differences in proton selectivity were found between DPhPC membranes and DPhPC/cholesterol membranes. By contrast, cholesterol incorporation into asolectin increases membranes selectivity to proton. Cholesterol dramatically reduced G(un) reflecting, probably, its ability of inducing order in lipid chains. In asolectin membranes, PA increases proton selectivity, probably by acting as a proton shuttle according to the model proposed by Kamp and et al. [Biochemistry 34 (1995) 11928]. Cholesterol incorporation into asolectin membranes eliminates the PA-induced increase in proton selectivity. In DPhPC and DPhPC/cholesterol membranes, PA does not affect proton selectivity. These results are discussed in terms of the presence of cardiolipin (CL) in asolectin, cholesterol/PA interactions, and cholesterol order-inducing effects on acyl-chains.     

Experimental study on the thermal behaviors of lithium-ion batteries under discharge and overcharge conditions

Iron oxide modified montmorillonite (MMT) as flame retardant was used to polyvinyl chloride (PVC), and the flame retardant and smoke-suppressant properties of the PVC were investigated by the smoke density rating and cone calorimeter tests (CONE), and the thermal degradation behaviors of PVC were studied by thermogravimetric analysis (TG) in air atmosphere. The activation energies for the first stage of thermal degradation were obtained following the equation of Kissinger. The mechanical properties testing resultant data showed that iron oxide modified MMT had little effect on the tensile strength of the sample. The CONE result indicated that iron oxide modified MMT could reduce the heat release rate in flame retardant PVC: a more compact char residue formed on the surface of the sample including iron oxide modified MMT during the combustion process. The TG result showed that the sample with modified iron oxide MMT had higher thermal stability than the pure PVC. Besides, the PVC treated with modified MMT showed high activation energy.     

Evaluation of chlorophyll fluorescence and membrane injury in the leaves of barley cultivars under osmotic stress

Two physiological tests for screening drought tolerance of barley (Hordeum vulgare, L.) plants are compared in this work. Water deficit is induced by treating the plants' roots with polyethylene glycol (PEG 8000). The relative water content (RWC) of the plants is used as a measure of the water status. Conductometrically determined electrolyte leakage from the leaf tissue demonstrates the membrane injury caused by dehydration. It is shown that the injury index increases with the decrease of the RWC of the leaves. The F(v)/F(m) ratio is employed to assess changes in the primary photochemical reactions of the photosynthetic apparatus after dehydration. The results suggest that PSII is weakly affected by the imposed osmotic stress. The fluorescence behaviour of the examined cultivars is related to their RWC.     

Influence of the heating conditions on the thermal and chemical behaviour of solid particles undergoing a fast endothermic decomposition

This paper deals with models describing the thermal and chemical behaviour of solid particles undergoing fast endothermic reactions under the influence of an external heat flux. The heat source temperature is supposed to be constant, to increase with time, or to deliver a simple thermal flash. It is shown that the pyrolysis conditions (reaction temperature, conversion, etc.) depend on the chemical characteristics of the reaction and also to a large extent on the external heating conditions. Relationships are proposed to take into account these parameters. The results are applied to the thermal decomposition of NaHCO₃. The pyrolysis of cellulose is finally chosen in order to show how these operating parameters can also affect the selectivity of a more complex reaction.