High Temperature Behavior of a Gel-Derived SiOC Glass: Elasticity and Viscosity

The high temperature behavior of a sol-gel derived silicon oxycarbide glass containing 12 at.% carbon has been characterized by means of creep and in-situ ultrasonic echography measurements. Temperature induced changes include structural relaxation and densification from 1000 to 1200°C, and crystallization to form a fine and homogeneous -SiC/glass-matrix nanocomposite with 2.5 nm large crystals above 1200°C. Young's modulus measurements clearly reveal a consolidation of the material upon annealing below 1200°C. Crystallization is almost complete after few hours at 1300°C and results in a significant increase in Young's modulus. The viscosity of the oxycarbide glass is much higher than that of fused silica, with two orders of magnitude difference at 1200°C, and the glass transition temperature ranges from 1320 to 1370°C.     

Investigation of the diffusion coefficient of polymers and micelles in aqueous solutions using the Soret effect in cw-laser thermal lens spectrometry

The concentration gradient (Soret effect) induced in cw-laser thermal lens spectrometry subsequently to the formation of the thermal gradient (thermal lens effect) has been investigated in aqueous solutions of various macromolecular species including micelles, mixed micelles and polymers. It is shown that the build-up of the concentration gradient is much shorter than that in classical Soret experiments, reaching steady-state values in less than 1 min. The time evolution of the Soret signal has been used to derive mass-diffusion times from which mass-diffusion coefficients were calculated. Our data are in agreement with previous results obtained from quasi-elastic light scattering studies for the micellar solutions and calculated from a known molecular weight-dependent power law for polymer solutions.     

Band splitting in overloaded isocratic elution chromatography I. The experimental evidence

A series of samples of increasing volume (from 0.001 to 4.0 cm3), containing the same constant concentration (40 g/l) of two simple compounds, ethylbenzoate and 4-tert.-butylphenol were injected on a Kromasil-C18 column with methanol-water (62:38. v/v) as the mobile phase. Complex band profiles were observed when the volume of the sample became large enough and strong band interference took place. The analysis of the fractions collected during the elution of the mixed band demonstrates that, for samples larger than 2 cm3, the band of 4-tert.-butylphenol is split into two separate bands, one eluted before and the other eluted after the band of ethylbenzoate. Such a phenomenon has never been observed yet in RPLC, under isocratic elution conditions.     

ESR. Spectra and Structures of Radical Anions in the Dibenzo[a,e]cyclooctene Series

ESR. studies are reported for the radical anions of 5,6-didehydro- and 5,6,11,12-tetradehydro-dibenzo[a,e]cyclooctene (III and IV, resp.), in addition to that of dibenzo[a,e]cyclooctene (II) itself, the spectrum of which has been reexamined. Comparison of the proton and ¹³C coupling constants for II·?, III·? and IV·? indicates that the three radical anions do not differ greatly in their electronic and molecular structures. This statement implies that II·? should also be substantially planar, i.e., the tub-shaped eight-membered ring in II is expected to flatten on passing from the neutral molecule to its radical anion. Support for postulating such a change in geometry, analogous to that encountered with the parent cyclooctatetraene (I), is provided by INDO calculations.     

Preparative liquid chromatography

The status of the theory and the main methods of implementation of preparative liquid chromatography are reviewed. On the theory front, the focus has recently shifted. The theory of non-linear, non-ideal chromatography has given rise to numerous models whose advantages, disadvantages and ranges of application are now well understood. Interest now resides in investigating the equilibrium thermodynamics of complex new systems, in the study of the kinetics of mass transfers in conventional chromatographic systems, and in the application of the various models of chromatography to optimize the experimental conditions. Progress in computer technology allows the use of sophisticated models, provided their parameters can be measured. This allows the detailed investigation of separations for which the mass transfer kinetics is slow such as chiral separations, the purification of basic compounds, and the extraction of recombinant proteins. On the applied front, in addition to numerous incremental improvements in reliability and economic performance, a few essential new features should be noted, i.e. the availability of instruments for simulated moving bed separations at the scale needed for preparative chiral separations, the use of expanded beds for the extraction of recombinant proteins from fermentation broths, and the attention given to improvements in the performance of packed beds. A survey of the literature dealing with practical applications and recent meetings shows that preparative chromatography is becoming a well established separation and purification method in the pharmaceutical industry.     

Photophysical and structural impact of phosphorylated anions associated to lanthanide complexes in water

A new ligand, LC, bis-[(6'-carboxy-2,2'-bipyridine-6-yl)]phenylphosphine oxide, in which the tridentate 6-carboxy-2,2'-bipyridyl arms are directly linked to a phenylphosphine oxide fragment, has been synthesized. The corresponding [Ln.LC]Cl.xH2O complexes (Ln = Eu, x = 4, and Tb, x = 3) were isolated from solutions containing equimolar amounts of LC and hydrated LnCl3 salts and characterized by elemental analysis, mass spectrometry, and infrared spectroscopy. The interactions of the Eu complex with various anions (AMP(2-), ADP3-, ATP,4- HPO4(2-), and NO3-) were studied by titration experiments, using UV-vis, luminescence spectroscopy, and excited-state lifetime measurements. The results are in keeping with strong interactions with the ADP3-, ATP4-, and phosphate anions in TRIS/HCl buffer (0.01 M, pH = 7.0), as revealed by the determination of the conditional stepwise association constants. These values are higher than the one determined for ligand LB, bis[(6'-carboxy-2,2'-bipyridine-6-methyl-yl)]-n-butylamine (Delta log K approximately 1-2). The interaction of complexes [Ln.LB]+ and [Ln.LC]+ with nitrate, monohydrogenophosphate, methyl phosphate (MeP2-), methyldiphosphate (MeDP3-), and methyltriphosphate (MeTP4-) anions was investigated by means of quantum mechanical (QM) calculations. The results, combined with data on the photophysical impact of the sequential competitive binding of anions to the Eu complexes in water, suggest that LB is too flexible to ensure a good coordination pocket, while the molecular structure of ligand LC stabilizes both the formation of the lanthanide complexes and its adducts with ATP.     

Action des perfluoroalkylcuivre(I) sur les perfluoroalkylethylenes

A series of new polyfluorinated dienes 3, containing the novel -CFCHCHCF-, pattern has been synthesized (50–70% yields) by reacting perfluoroalkyl iodides with perfluoroalkyl-ethylenes in the presence of copper. The monoalkenes R_fCFCHCH₂CF₂R′_f and the saturated compounds R_fCF₂CH₂CH₂CF₂R′_f were obtained by varying the experimental conditions. The ¹H and ¹⁹F NMR spectra are analysed and the reaction mechanism is discussed.     

Liquid-junction potential effects in measurements of sodium ion activity in unbuffered aqueous solutions

Measurements of pNa with the glass-membrane ion-selective electrode in unbuffered aqueous solutions require care in the constitution of the measuring cell, because of varialbe liquid-junction potentials at the salt bridge. Of the bridge electrolytes tested, with calomel or silver/silver chloride reference electrodes, 3 M ammonium chloride in agar-agar gel was most satisfactory. Calibration graphs were of almost theoretical slope in the range 10^?3?5 × 10^?1 M sodium ion.     

Surface organometallic chemistry of main group elements: selective synthesis of silica supported [triple bond Si-OB(C(6)F(5))(3)](-)[HNEt(2)Ph](+)

The reaction of the Lewis acid B(C(6)F(5))(3) with silanol groups of silica surfaces, dehydroxylated at different temperatures (300, 500, 700, and 800 degrees C), has been investigated in presence of the Br?nsted base NEt(2)Ph. The structure of the resulting modified silica supports [triple bond Si-OB(C(6)F(5))(3)](-)[HNEt(2)Ph](+) (1) has been carefully identified by IR and multinuclear solid-state NMR spectroscopies, isotopic (2)H and (18)O labeling, elemental analysis, molecular modeling, and comparison with synthesized molecular models. Highly dehydroxylated silica surfaces were required to transform selectively each silanol group into unique [triple bond Si-OB(C(6)F(5))(3)](-)[HNEt(2)Ph](+) fragments. For lower dehydroxylation temperatures, two sorts of surface sites were coexisting on silica: the free silanol groups [triple bond SiOH] and the ionic species 1.     

Modeling of the adsorption behavior and the chromatographic band profiles of enantiomers : Behavior of methyl mandelate on immobilized cellulose

The adsorption isotherms of (−)- and (+)-methyl mandelate from a hexane-isopropanol (90:10) solution were measured on a chromatographic column packed with 4-methylcellulose tribenzoate coated on silica. These isotherms are accounted for by a bi-Langmuir isotherm model, the two Langmuir terms having widely different initial slopes and saturation capacities, but each term having the same saturation capacity for the two enantiomers. The competitive isotherms were also measured. They are in excellent agreement with the prediction of a competitive bi-Langmuir model based on the single-component isotherms. The individual band profiles are in agreement with the profiles calculated from these isotherms. Thus, a simplified competitive isotherm can be used to model a separation on a chiral stationary phase the recognition mechanism of which is not well identified and the adsorption behavior of which is certainly not ideal.