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61.
F López Arbeloa J Ba?uelos Prieto V Martínez Martínez T Arbeloa López I López Arbeloa 《Chemphyschem》2004,5(11):1762-1771
Absorption and fluorescence (steady-state and time-correlated) techniques are used to study the photophysical characteristics of the pyrromethene 650 (PM650) dye. The presence of the cyano group at the 8 position considerably shifts the absorption and fluorescence bands to lower energies with respect to other related pyrromethene dyes; this is attributed to the strong electron-acceptor character of the cyano group, as is theoretically confirmed by quantum mechanical methods. The fluorescence properties of PM650 are intensively solvent-dependent. The fluorescence band is shifted to lower energies in polar/protic solutions, and the evolution of the corresponding wavelength with the solvent is analysed by a multicomponent linear regression. The fluorescence quantum yield and the lifetime strongly decrease in polar/protic solvents, which can be ascribed to an extra nonradiative deactivation, via an intramolecular charge-transfer state (ICT state), favoured in polar media. 相似文献
62.
X. N. Yang 《Journal of solution chemistry》1998,27(3):261-272
A diaphragm cell has been used to measure mutual diffusion coefficients at 25°C for four binary nonelectrolyte mixtures: ethylbenzene + n-hexane, carbon tetrachloride + ethylbenzene, cyclohexane + p-xylene, and 1,2-dichloroethane + cyclohexane. A free-volume predictive approach for binary mutual diffusion coefficients was developed and tested. Only infinite dilution diffusion coefficients, some readily available pure substance data, and UNIFAC group contribution parameters are used in the model. No binary equilibrium thermodynamic information is required. For 73 binary systems with an overall average absolute deviation of 5.2%, it has been shown that the developed method is better than two commonly available reference methods for the prediction of liquid diffusion coefficients. 相似文献
63.
64.
65.
Romero S. Bustamante P. Escalera B. Cirri M. Mura P. 《Journal of Thermal Analysis and Calorimetry》2004,77(2):541-554
Differential scanning calorimetry (DSC), supported by hot stage microscopy, IR spectroscopy and X-ray powder diffractometry,
was used to investigate the characteristics of the solid phases of mefenamic, niflumic, and flufenamic acids and of paracetamol,
before and after equilibration with saturated solutions in different solvents. Mixtures of Lewis base (dioxane and ethyl acetate)
and amphiprotic solvents (ethanol and water) were prepared for evaluating the influence of both nature and polarity of the
solvents. Solid-state analysis performed on the original samples (commercial products) made it possible to establish that
paracetamol, mefenamic acid and flufenamic acid were in their respective Form I. No polymorphic modifications are known for
niflumic acid. Paracetamol, niflumic and mefenamic acids did not show any change after equilibration with the various solvents
or solvent mixtures, regardless of their different chemical nature. In contrast, DSC, IR and X-ray analyses revealed the partial
recrystallization of flufenamic acid into its polymorphic Form III in solid phases at equilibrium with ethanol, ethyl acetate
and their blends, as well as in dioxane-water mixtures containing 30 to 100% dioxane and in ethanol-water mixtures with a
water content less than 50%.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
66.
Elizabeth P.G. Arêas José A.G. Arêas James Hamburger Warner L. Peticolas Paulo S. Santos 《Journal of colloid and interface science》1996,180(2):578
A remarkably high viscosity has been induced in protein aqueous solutions by the addition of certain structurally related organic solvents. The effect has been observed for lysozyme aqueous solutions containing tetramethylurea (TMU), dimethylsulfoxide (DMSO), dimethylformamide, and hexamethylphosphortriamide. The effect has also been induced in ferrocytochrome c aqueous solutions by TMU. Critical concentrations for both the protein and organic solvent were verified for the onset of the viscosity increase. A common feature of the solvents which were able to induce the effect is a dipolar moiety (C=O, S → O and P → O) and a nonpolar region represented by the methyl groups. The resulting fluids show an extremely restricted flow and a typical non-Newtonian, pseudoplastic behavior. Use was made of1H nuclear magnetic resonance (NMR) and Raman spectroscopy to characterize protein structural modifications and of13C NMR to investigate changes in relaxation times and chemical shifts in the solvent/water solutions. A systematic rheological characterization of the systems was undertaken for some of the solvents, and unusual patterns of viscous effects were identified for the solvent/water systems both with and without protein. The process was found to be at least partially reversible, as concluded from the recovered original solution rheological characteristics and the original protein1H NMR spectrum, after eliminating the organic solvent by ultrafiltration. The whole process was characterized as consisting of two mutually independent stages. The first involves an extensive conformational transition of the polypeptide backbone, from a predominantly α-helical to increased random coiled and β-sheet structures, with the occurrence of nonorthodox protein secondary structures at regions above the solvent critical point. The second stage consists of short-lived interchain contacts leading to an entanglement of the macromolecular system as a whole. A microphase reversion in the organic solvent/water mixture, supported by13C NMR and rheological results, is proposed as the driving force causing the observed behavior. 相似文献
67.
M. Nagy 《Colloid and polymer science》1985,263(3):245-265
68.
Grader G.S. Shter G.E. Avnir D. Frenkel H. Sclar D. Dolev A. 《Journal of Sol-Gel Science and Technology》2001,21(3):157-165
The effect of wetting non-hydrolytic derived alumina xerogels with water and organic solvents in the 20–70°C range on the alumina's properties was investigated. Wetting with organic solvents does not affect the alumina. However, contact with water was found to change the sharp crystallization at 800°C to a continuous crystallization starting at 450°C. Water treatment for a day at room temperature (RT) followed by second calcination decreased the surface area by 10%. This decrease in surface area is less pronounced with increasing wetting periods. On the other hand water treatment at 50–70°C followed by a second calcination resulted in a surface area increase of up to 15%. Upon water treatment the total pore volume has decreased from 0.65 (cm3/gr) to 0.48 (cm3/gr) and the average pore size decreased from 6.8 nm to 4.1 nm. The Cl content was found to be uneffected by the water treatment, remaining at 2.5% wt. Wetting with water at elevated temperature (70°C) accelerated the morphological changes, eliminating the crystallization peak at 800°C in one hour. A dissolution-reprecipitation mechanism is suggested to explain the results. In addition, Mass-Spectroscopy of the effluent gas during heat treatment revealed the emission of CO2 and water upon phase transition into -Al2O3, at 1150–1300°C. 相似文献
69.
甲醇—乙醇—水—盐体系的等压气液平衡 总被引:4,自引:1,他引:4
木文采用CPⅡ型沸点仪测定了在93.33kPa下甲醇-乙醇-水-盐(NaCl,NaBr,Nal,LiCl,CaCl_2)体系的气液平衡数据。实验证明,当三元混合溶剂的相对组成固定时,体系的沸点与加入的盐量呈线性关系,用热力学理论导出了该线性方程,用Gibbs相律探讨了该线性关系的起因。 相似文献
70.
Supercritical fluid extraction of flumetralin in tobacco 总被引:2,自引:0,他引:2
Summary This work evaluates the level of Flumetralin residues in real tobacco samples. Conventional extraction methods were compared with SFE methods in both static and dynamic modes using CO2 and CO2 with modifier. Additionally, in the dynamic SFE mode, different collection approaches, such as collection at room temperature without solvent, collection at room temperature in the presence of solvent (hexane) and collection in an ice bath, without solvent, were studied. SFE showed itself to be a promising extraction technique for pesticide residues in tobacco samples. 相似文献