Experimental study of the concentration dependence of the soret coefficient by thermal field-flow fractionation: the case of polystyrene in decalin

Summary Thermal field-flow fractionation separates polymers with high selectivity according to their Soret coefficient,S _τ, hence, according to their molar mass, and therefore consitutes an efficeint physicochemical tool for the determination of the Soret coefficient of a given polymer in the carrier liquid from its retention time. However, the polymer concentration in the sample influences the retention time and, hence, the value ofS _τ derived from it. An experimental study of the influence of sample concentration on retention,S _τ, and peak shape was performed for the polystyrene-decalin system over a relatively large temperature domain and for various molar masses. It is found that the retention time and the value ofS _τ increase with increasing sample concentration, the more so as the cold wall temperature is lower. This appears to be in contradiction with the general non-equilibrium thermodynamic expression derived for polymer-solvent systems with positive second virial coefficients, such as the present system over the temperature range investigated. There seems to be a temperature for which the dependence ofS _τ on sample concentration vanishes. This temperature is about 375 K for the polystyrene-decalin system. As the sample concentration increases, the peak barycentre and the standard deviation increases. As the peaks are fronting, the skewness is negative and becomes more negative as the sample concentration increases. The peak skewness appears to be a good indicator of the onset of sample concentration effects. The threshold concentration, for which these effects begin to become significant, decreases with increasing molar mass.     

Characterisation of River Po particles by sedimentation field-flow fractionation coupled to GFAAS and ICP-MS

A procedure for elemental composition determination of water-borne river particles (Po River) on both size-fractionated and unfractionated submicron particles (0.1–1 μm) by graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma-mass spectrometry (ICP-MS) is reported. Sample fractionation was performed using sedimentation field-flow fractionation (SdFFF). The distribution of relative mass vs. particle size was determined using UV detection. Fractions were collected over a narrow size range for scanning electron microscopy. With this combination of techniques the mass, elemental composition, and shape distributions can be obtained across the size spectrum of the sample.

The size distributions of the major elements (Al, Fe) were determined by coupling both GFAAS and ICP-MS techniques to the SdFFF. The procedure was validated using a reference clay sample. Satisfactory agreement was found between both the GFAAS and ICP-MS aluminium signal and the UV detector signal. Some discrepancies were observed in the Fe/Al ratios when comparing GFAAS and ICP-MS. Thus further investigation is in order to fully assess the role of SdFFF-ICP-MS and SdFFF-GFAAS techniques for elemental characterisation of aquatic colloids. Both GFAAS and ICP-MS signals unambiguously indicate a significantly higher Fe content in the lower size range, which is consistent with previous investigations.

Trace element levels in unfractionated Po River particles, determined by both GFAAS and ICP-MS, show good agreement. The high levels of Cu, Pb, Cr and Cd found associated with the colloidal particles underlines the significance of the environmental role played by the suspended matter in rivers in both highly industrialised and intensively cultivated areas.     

Catalyst-free synthesis of benzodiazepines and benzimidazoles using glycerol as recyclable solvent

We described herein the use of glycerol as solvent in the catalyst-free synthesis of benzodiazepines and benzimidazoles. This simple and efficient method furnishes the corresponding 1-H-1,5-benzodiazepines and 1,2-disubstituted benzimidazoles in good yields by the condensation of o-phenylenediamine with several ketones and aldehydes, respectively. In addition, glycerol can be easily re-utilized for further condensation reactions up to four times without lost of activity.     

One-pot amide synthesis from allyl or benzyl halides and amines by Pd-catalysed carbonylation

Amides can be prepared from allyl or benzyl halides and primary or secondary amines, using Pd(0) catalyst under CO pressure, in a one-pot synthesis. The reaction proceeds through the acyl palladium halide formation which undergoes an acylic nucleophilic substitution from the amine.     

CABRI AS A COGNITIVE TOOL FOR THE EVOLUTION OF A SENSE OF PROVING

Cognitive technologies have been described in the literature as reorganisers of thinking processes, especially where problem solving is concerned. This paper aims to analyse the possible use of Cabri-Géomètre as a cognitive tool in the elaboration of mathematical justifications in the context of problem-based mathematics. Some empirical examples are given to illustrate the significance of the specific learning situation. The complexity of learning environments incorporating computer-based activities is stressed as a condition for them to be effective in the introduction of the idea of mathematical justification and its evolution towards a sense of proving.     

Olefin Cyclopropanation by Radical Carbene Transfer Reactions Promoted by Cobalt(II)/Porphyrinates: Active‐Metal‐Template Synthesis of [2]Rotaxanes

A Co^II/porphyrinate‐based macrocycle in the presence of a 3,5‐diphenylpyridine axial ligand functions as an endotopic ligand to direct the assembly of [2]rotaxanes from diazo and styrene half‐threads, by radical‐carbene‐transfer reactions, in excellent 95 % yield. The method reported herein applies the active‐metal‐template strategy to include radical‐type activation of ligands by the metal‐template ion during the organometallic process which ultimately yields the mechanical bond. A careful quantitative analysis of the product distribution afforded from the rotaxane self‐assembly reaction shows that the Co^II/porphyrinate subunit is still active after formation of the mechanical bond and, upon coordination of an additional diazo half‐thread derivative, promotes a novel intercomponent C?H insertion reaction to yield a new rotaxane‐like species. This unexpected intercomponent C?H insertion illustrates the distinct reactivity brought to the Co^II/porphyrinate catalyst by the mechanical bond.