Kolloide Farbstoffe

Ohne Zusammenfassung

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The nature and the cause of edemaFischer, Martin, H., (The Journ. of the Amer. Medic. Association51, 830, 1908.)

     

A Cartesian treecode for screened coulomb interactions

A treecode algorithm is presented for evaluating electrostatic potentials in a charged particle system undergoing screened Coulomb interactions in 3D. The method uses a far-field Taylor expansion in Cartesian coordinates to compute particle–cluster interactions. The Taylor coefficients are evaluated using new recurrence relations which permit efficient computation of high order approximations. Two types of clusters are considered, uniform cubes and adapted rectangular boxes. The treecode error, CPU time and memory usage are reported and compared with direct summation for randomly distributed particles inside a cube, on the surface of a sphere and on an 8-sphere configuration. For a given order of Taylor approximation, the treecode CPU time scales as O(NlogN)$O(N\log N)$ and the memory usage scales as O(N)$O(N)$, where N is the number of particles. Results show that the treecode is well suited for non-homogeneous particle distributions as in the sphere and 8-sphere test cases.     

Highly luminescent silicon nanocrystals with discrete optical transitions.

A new synthetic method was developed to produce robust, highly crystalline, organic-monolayer passivated silicon (Si) nanocrystals in a supercritical fluid. By thermally degrading the Si precursor, diphenylsilane, in the presence of octanol at 500 degrees C and 345 bar, relatively size-monodisperse sterically stabilized Si nanocrystals ranging from 15 to 40 A in diameter could be obtained in significant quantities. Octanol binds to the Si nanocrystal surface through an alkoxide linkage and provides steric stabilization through the hydrocarbon chain. The absorbance and photoluminescence excitation (PLE) spectra of the nanocrystals exhibit a significant blue shift in optical properties from the bulk band gap energy of 1.2 eV due to quantum confinement effects. The stable Si clusters show efficient blue (15 A) or green (25-40 A) band-edge photoemission with luminescence quantum yields up to 23% at room temperature, and electronic structure characteristic of a predominantly indirect transition, despite the extremely small particle size. The smallest nanocrystals, 15 A in diameter, exhibit discrete optical transitions, characteristic of quantum confinement effects for crystalline nanocrystals with a narrow size distribution.     

Mapping the composition of chondritic meteorite Northwest Africa 3118 with micro-Raman spectroscopy

Meteorites provide precious clues about the formation of planets in the solar system. Here, an analytical method to study chondritic meteorites using low- and high-resolution micro-Raman spectroscopy is presented. An approach in mapping the distribution of mineral compositions of a sample is introduced by measuring ～10⁴ Raman spectra along linear, micron-wide paths that traverse the sample to capture detail on small and large spatial scales (from micrometers to millimeters). To refine these analytical procedures, a well-defined chondrule and surrounding matrix of the carbonaceous chondrite Northwest Africa 3118 are analyzed. The morphology and elemental composition of the sample are also studied using scanning electron microscopy with energy-dispersive X-ray spectroscopy and creating composite maps with the images obtained by these techniques along the same Raman linear path. The Raman line scan maps of this sample show clear spatial segregation of constituents including pyroxene and olivine, both within and outside the chondrule. Graphitic carbon is also present and appears clustered in domains of a few hundred microns both in the matrix and in the central core of the chondrule. The results obtained with the scanning electron microscopy and energy-dispersive X-ray spectroscopy techniques show that iron is most abundant in the matrix surrounding the chondrule, while the chondrule is enhanced in silicon, magnesium, calcium, aluminum, and sodium. These findings provide a detailed identification of the elemental and mineralogical, spatial composition of the analyzed regions of Northwest Africa 3118.     

The wrong kind of gravity

The KPZ formula [V.G. Knizhnik, A.M. Polyakov, and A.B. Zamolodchikov, Mod. Phys. Lett. A 3 (1988) 819] shows that coupling central charge c≤1 spin models to 2D quantum gravity dresses the conformal weights to get new critical exponents, where the relation between the original and dressed weights depends only on c. At the discrete level the coupling to 2D gravity is effected by putting the spin models on annealed ensembles of Φ³ planar random graphs or their dual triangulations, where the connectivity fluctuates on the same time-scale as the spins.Since the sole determining factor in the dressing is the central charge, one could contemplate putting a spin model on a quenched ensemble of 2D gravity graphs with the “wrong” c value. We might then expect to see the critical exponents appropriate to the c value used in generating the graphs. In such cases the KPZ formula could be interpreted as giving a continuous line of critical exponents which depend on this central charge. We note that rational exponents other than the KPZ values can be generated using this procedure for the Ising, tricritical Ising and 3-state Potts models.     

Electrostatic stabilization of colloids in carbon dioxide: electrophoresis and dielectrophoresis

Over the past decade, steric stabilization has been achieved for a variety of inorganic and organic colloids in supercritical fluid carbon dioxide (scCO2). Herein we demonstrate that colloids may also be stabilized in CO2 by electrostatic forces, despite the ultralow dielectric constant of 1.5. Zeta potentials of micrometer-sized water droplets, measured in a microelectrophoresis cell, reached -70 mV corresponding to a few elementary charges per square micrometer of droplet surface. This degree of charge was sufficient to stabilize water/CO2 emulsions for an hour, even with water volume fractions of 5%. Hydrogen ions partition preferentially, relative to bicarbonate ions, from the emulsion droplets to the cores of surfactant micelles in the diffuse double layer surrounding the droplets. The micelles, formed with a low molecular weight branched hydrocarbon surfactant, prevent ion pairing of the hydrogen counterions to the negatively charged emulsion droplets. Dielectrophoresis of the water droplets at a frequency of 60 Hz leads to chains containing a dozen droplets with lengths of 50 mum. The ability to form electrostatically stabilized colloids in carbon dioxide is particularly useful in practical applications, because steric stabilization in CO2 is often limited by the poor solvation of the stabilizers.     

Structure elucidation with lanthanide-induced shifts 5. Evaluation of the binding ability of various functional groups

The equilibrium binding constants have been evaluated for the association in CCl₄ of the lanthanide shift reagent. Eu(fod)₃, with a series of adamantane derivatives containing a variety of different functional groups. It is shown that if steric effects are held constant the binding abilities of the adamantane derivatives exhibit a good correlation with the proton affinities of the methyl analogs containing the same functional groups. The results permit the prediction of binding strength of other functional groups with Eu(dpm)₃ as well as with Eu(fod)₃ and thus can be of great assistance in planning and interpreting the results of experiments with lanthanide shift reagents.Part 4:D. J. Raber, M. D. Johnston, jr., C. M. Campbell, C. M. Janks, andP. Sutton, Org. Mag. Res.11, 323 (1978).