Effect of surfactant on interfacial gas transfer studied by axisymmetric drop shape analysis-captive bubble (ADSA-CB)

A new method combining axisymmetric drop shape analysis (ADSA) and a captive bubble (CB) is proposed to study the effect of surfactant on interfacial gas transfer. In this method, gas transfer from a static CB to the surrounding quiescent liquid is continuously recorded for a short period (i.e., 5 min). By photographical analysis, ADSA-CB is capable of yielding detailed information pertinent to the surface tension and geometry of the CB, e.g., bubble area, volume, curvature at the apex, and the contact radius and height of the bubble. A steady-state mass transfer model is established to evaluate the mass transfer coefficient on the basis of the output of ADSA-CB. In this way, we are able to develop a working prototype capable of simultaneously measuring dynamic surface tension and interfacial gas transfer. Other advantages of this method are that it allows for the study of very low surface tensions (<5 mJ/m2) and does not require equilibrium of gas transfer. Consequently, reproducible experimental results can be obtained in a relatively short time. As a demonstration, this method was used to study the effect of lung surfactant on oxygen transfer. It was found that the adsorbed lung surfactant film shows a retardation effect on oxygen transfer, similar to the behavior of a pure DPPC film. However, this retardation effect at low surface tensions is less than that of a pure DPPC film.     

Analysis of 5-hydroxy-7-methoxyflavones by normal-phase high-performance liquid chromatography

The separation and identification of flavones present in a chloroform extract of Baccharis trinervis leaves was investigated. The chromatographic system consisted of a amino-bonded column, gradient elution from hexane-chloroform (85:15) to chloroform-acetonitrile (40:60) and detection at 346 nm. Four flavones were found. From NMR and MS data they were identified as 5-hydroxy-7,4′-dimethoxyflavone (I), 5-hydroxy-7,3′,4′-trimethoxyflavone (II), 5,3′-dihydroxy-7,4′-dimethoxyflavone (III) and 5,4′-dihydroxy-7-methoxyflavone (IV). Flavone II and III have not been found in Baccharis trinervis before. The chromatographic system showed good selectivity for the separation of the flavones. The relation between t_R and the structure is discussed.     

Proton mobility in hydrated sulfonated polystyrene: NMR and impedance studies

Different ionomers were obtained by sulfonation of a commercial polystyrene (PS), using acetyl sulfate as reagent. The sulfonation was assessed by FTIR and XPS spectroscopies and the thermal and mechanical properties were deduced from DMA measurements. The acid form of the ionomers was characterized by means of ¹H and ¹³C MAS-NMR spectroscopies. Polymer hydration under controlled humidity atmosphere was analyzed by gravimetric, NMR and complex impedance techniques. In the hydrated state, two stages associated with formation of hydronium (H₃O⁺) species and proton hopping between adsorbed water molecules were deduced from NMR data. Both processes are responsible for important changes detected in mechanical properties and conductivity of sulfonated polymers during hydration processes.     

Similar molecular constitutions but different conformations and different supramolecular assemblies in two related fused tetracyclic benzo[b]pyrimido[5,4‐f]azepine derivatives

A simple and effective two‐step approach to tricyclic pyrimidine‐fused benzazepines has been adapted to give the tetracyclic analogues. In (RS)‐8‐chloro‐6‐methyl‐1,2,6,7‐tetrahydropyrimido[5′,4′:6,7]azepino[3,2,1‐hi]indole, C₁₅H₁₄ClN₃, (I), the five‐membered ring adopts an envelope conformation, as does the reduced pyridine ring in (RS)‐9‐chloro‐7‐methyl‐2,3,7,8‐tetrahydro‐1H‐pyrimido[5′,4′:6,7]azepino[3,2,1‐ij]quinoline, C₁₆H₁₆ClN₃, (II). However, the seven‐membered rings in (I) and (II) adopt very different conformations, with the result that the methyl substituent occupies a quasi‐axial site in (I) but a quasi‐equatorial site in (II). The molecules of (I) are linked by C—H...N hydrogen bonds to form C(5) chains and inversion‐related pairs of chains are linked by a π–π stacking interaction. A combination of a C—H...π hydrogen bond and two C—Cl...π interactions links the molecules of (II) into complex sheets. Comparisons are made with some similar fused heterocyclic compounds.     

Three styryl‐substituted tetrahydro‐1,4‐epoxy‐1‐benzazepines: configurations,conformations and hydrogen‐bonded chains

(2SR,4RS)‐7‐Chloro‐2‐exo‐[(E)‐styryl]‐2,3,4,5‐tetrahydro‐1H‐1,4‐epoxy‐1‐benzazepine, C₁₈H₁₆ClNO, (I), crystallizes as a racemic twin in the space group P2₁ and the molecules are linked into a chain of edge‐fused R₃³(9) rings by a combination of C—H...O and C—H...N hydrogen bonds. The diastereoisomer (2RS,4RS)‐7‐chloro‐2‐endo‐[(E)‐styryl]‐2,3,4,5‐tetrahydro‐1H‐1,4‐epoxy‐1‐benzazepine, (II), also crystallizes as a racemic twin, but in the space group P2₁2₁2₁, and a two‐centre C—H...N hydrogen bond and a three‐centre C—H...(O,N) hydrogen bond combine to link the molecules into a complex chain of rings. In (2R,4R)‐7‐fluoro‐2‐endo‐[(E)‐styryl]‐2,3,4,5‐tetrahydro‐1H‐1,4‐epoxy‐1‐benzazepine, C₁₈H₁₆FNO, (III), which is not isomorphous with (II), the molecules are linked by a single C—H...O hydrogen bond into simple chains, but the molecular arrangements in (II) and (III) are nonetheless very similar. The significance of this study lies in its observation of the variations in molecular configuration and conformation, and in the variation in the supramolecular aggregation, consequent upon modest changes in the peripheral substituents.     

Surfactant-assisted synthesis of defective zirconia mesophases and Pd/ZrO2: Crystalline structure and catalytic properties

Mesoporous zirconia nanophases with structural defects were synthesized by using a surfactant-templated method. Physicochemical properties and crystalline structures of the zirconia nanophases were studied by means of thermogravimetric analysis (TGA), N₂ physosorption isotherm and in situ Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The resultant materials show typical mesoporous features which vary with calcination temperature. The cationic surfactant in the network of the solids induces structural deformation and defect creation. The zirconia consists of monoclinic and tetragonal nanophases which contains many structural defects, and its crystalline structure shows microstrain. Both, concentration of lattice defects and degree of the crystal microstrain, decrease as the calcination temperature is increased. When CO is adsorbed on the surface of Pd/ZrO₂, linear bonds of CO–Pd⁰, CO–Pd^δ+ and CO–Zr⁴⁺ are formed, accompanying with CO₂ production. Catalytic evaluation shows that the Pd/ZrO₂ catalyst is very active for CO oxidation and NO reduction. In the case of oxygen absence from reaction mixture, high selectivity to N₂ is achieved without any NO₂ formation. In the oxygen rich condition, CO conversion is enhanced but less than 19% NO₂ is produced. N₂O is formed only in the reducing condition and its selectivity is sensitive to reaction temperature. The possible mechanisms of NO + CO and NO + CO + O₂ reactions over Pd/ZrO₂ catalyst related to reactant dissociation on the Pd metals and to defective structure of the nanozirconia support are discussed.     

Mechanism of the OH–propene–O2 reaction: An ab initio study

The reaction of OH radicals with olefins is known to be important in atmospheric chemistry. From experimental data a global mechanism has been proposed, but the regioselectivity of the products is uncertain. In this work, the OH–propene–O₂ reaction has been studied with ab initio molecular orbital techniques. Reactants, transition structures, intermediate species and products are optimized at the UMP2/6‐31G** level for the two possible addition paths. In the first step, OH adducts are obtained with the OH radical linked to either the terminal or the central C atoms. Consideration of the second step, the addition of O₂, is required to explain the observed experimental data. The selectivity of the total reaction is found to be temperature and pressure dependent, but independent of the preferred site for the OH attack. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet: 31: 29–36, 1999