Mixtures of charged colloid and neutral polymer: influence of electrostatic interactions on demixing and interfacial tension

The equilibrium phase behavior of a binary mixture of charged colloids and neutral, nonadsorbing polymers is studied within free-volume theory. A model mixture of charged hard-sphere macroions and ideal, coarse-grained, effective-sphere polymers is mapped first onto a binary hard-sphere mixture with nonadditive diameters and then onto an effective Asakura-Oosawa model [S. Asakura and F. Oosawa, J. Chem. Phys. 22, 1255 (1954)]. The effective model is defined by a single dimensionless parameter-the ratio of the polymer diameter to the effective colloid diameter. For high salt-to-counterion concentration ratios, a free-volume approximation for the free energy is used to compute the fluid phase diagram, which describes demixing into colloid-rich (liquid) and colloid-poor (vapor) phases. Increasing the range of electrostatic interactions shifts the demixing binodal toward higher polymer concentration, stabilizing the mixture. The enhanced stability is attributed to a weakening of polymer depletion-induced attraction between electrostatically repelling macroions. Comparison with predictions of density-functional theory reveals a corresponding increase in the liquid-vapor interfacial tension. The predicted trends in phase stability are consistent with observed behavior of protein-polysaccharide mixtures in food colloids.     

Trace analysis of rapamycin in human blood by micellar electrokinetic chromatography

A capillary electrophoretic method with UV detection at 278 nm has been developed for analysis of the immunosuppressant rapamycin (sirolimus) in human blood at low microgram per liter levels. Separation has been achieved in an acidic carrier electrolyte containing sodium dodecylsulfate and 30% (v/v) acetonitrile. For sample clean-up and preconcentration, an off-line solid-phase extraction step using a silica-based reversed-phase material and an on-capillary focussing technique were employed. The latter allows the injection of increased sample volumes without excessive band broadening. Although this new method is less sensitive than existing liquid chromatographic procedures combined with mass spectrometry, it is fully suited to routine analysis of rapamycin in blood from patients treated with this drug. Last but not least the low costs make it an attractive alternative to established methods.     

Kinetic versus thermodynamic control during the formation of [2]rotaxanes by a dynamic template-directed clipping process

A template-directed dynamic clipping procedure has generated a library of nine [2]rotaxanes that have been formed from three dialkylammonium salts-acting as the dumbbell-shaped components-and three dynamic, imino bond-containing, [24]crown-8-like macrocycles-acting as the ring-shaped components-which are themselves assembled from three dialdehydes and one diamine. The rates of formation of these [2]rotaxanes differ dramatically, from minutes to days depending on the choice of dialkylammonium ion and dialdehyde, as do their thermodynamic stabilities. Generally, [2]rotaxanes formed by using 2,6-diformylpyridine as the dialdehyde component, or bis(3,5-bis(trifluoromethyl)benzyl)ammonium hexafluorophosphate as the dumbbell-shaped component, assembled the most rapidly. Those rotaxanes containing this particular electron-deficient dumbbell-shaped unit, or 2,5-diformylfuran units in the macroring, were the most stable thermodynamically. The relative thermodynamic stabilities of all nine of the [2]rotaxanes were determined by competition experiments that were monitored by (1)H NMR spectroscopy.     

A novel type of phosphide: synthesis and X-ray crystal structure analysis of (tBu3Si)3P4Li3

The tetraphosphides (tBu3Si)3P4M3 (M = Li, Na) and (tBu2PhSi)3P4Na3 have been synthesized in high yield from the reaction of 3 equivalents of the silanides tBu3SiM (M = Li, Na) and tBu2PhSiNa with P4 in benzene. (tBu3Si)3P4M3 (M = Li, Na) are transformed into the unsaturated triphosphides (tBu3Si)2P3M (M = Li, Na) and tBu3SiPM2 in tetrahydrofuran at ambient temperature.     

Synthese und Molekülstrukturen von N‐substituierten Diethylgallium‐2‐pyridylmethylamiden

Synthesis and Molekular Structures of N‐substituted Diethylgallium‐2‐pyridylmethylamides (2‐Pyridylmethyl)(tert‐butyldimethylsilyl)amine ( 1a ) and (2‐pyridylmethyl)‐di(tert‐butyl)silylamine ( 1b ) form with triethylgallane the corresponding red adducts 2a and 2b via an additional nitrogen‐gallium bond. These oily compounds decompose during distillation. Heating under reflux in toluene leads to the elimination of ethane and the formation of the red oils of [(2‐pyridylmethyl)(tert‐butyldimethylsilyl)amido]diethylgallane ( 3a ) and [(2‐pyridylmethyl)‐di(tert‐butyl)silylamido]diethylgallane ( 3b ). In order to investigate the thermal stability solvent‐free 3a is heated up to 400 °C. The elimination of ethane is observed again and the C‐C coupling product N, N′‐Bis(diethylgallyl)‐1, 2‐dipyridyl‐1, 2‐bis(tert‐butyldimethylsilyl)amido]ethan ( 4 ) is found in the residue. Substitution of the silyl substituents by another 2‐pyridylmethyl group and the reaction of this bis(2‐pyridylmethyl)amine with GaEt₃ yield triethylgallane‐diethylgallium‐bis(2‐pyridylmethyl)amide ( 5 ). The metalation product adds immediately another equivalent of triethylgallane regardless of the stoichiometry. The reaction of GaEt₃ with 2‐pyridylmethanol gives quantitatively colorless 2‐pyridylmethanolato diethylgallane ( 6 ).     

A Novel Environmentally Benign Method for the Selective Oxidation of Alcohols to Aldehydes and Ketones

In the presence of [Ru(terpyridine)(2,6‐pyridinedicarboxylate)], aliphatic and benzylic alcohols are oxidized to the corresponding aldehydes or ketones with high selectivity by using hydrogen peroxide as the oxidant. There is no need for the addition of co‐catalysts or organic solvents. By applying an optimized reaction protocol, high catalyst productivity (turnover number>10 000) and activity (turnover frequency up to 14 800 h^?1) has been achieved.     

Synthesis and Stereochemistry of New Bis(1,3-Oxathian-2-yl) Derivatives: Epimerisation and Chair–Twist Equilibria

The synthesis and stereochemistry of some new bis(1,3-oxathian-2-yl) derivatives are reported. The conformational behavior of these compounds and the orientation of their substituents are deduced from the NMR spectra, from the X-ray crystal structures of two compounds and from the ab-initio level calculations. The epimerization of diastereoisomers and the chair ⇌ 22,5-twist equilibria were studied by variable temperature NMR.