Clusters, suspensions

A model for simulating the extinction spectra during the aggregation process was developed in our work [20]. We assume that a cluster suspension is dilute so that the single scattering approximation is valid. This means that the... 

Clusters of metal atoms can form colloidal suspensions. Colloidal clusters of copper, silver, and gold in glass are responsible for some of the vivid colors of stained glass in medieval cathedrals. Even aqueous suspensions of metal clusters are known (Fig. 8.45). 

Sodium borohydride reductions of gold(I) complexes give Au clusters at RT if sodium borohydride in ethanol is dropped slowly into a suspension of the Au(I) complex in the same solvent. The immediate coloring of the reaction mixture (mostly red), even after only a few drops of the borohydride have been added, indicates fast formation of Au clusters. In view of the complicated composition of these compounds the fast formation is surprising. The use of H2 and CO with HjO as reducing agents in the synthesis of gold clusters has been described (see Table 1, Method A, 8.2.2.2). 

In the preparation of Mo/HUSY, the cluster 1 amounting to 2.5 wt% (as molybdenum metal) of HUSY was added to the suspension of HUSY 92% of the molybdenum was loaded onto HUSY. The Cl/Mo ratio of Mo/HUSY was found to be 0.34, suggesting that in ion exchange the cluster 1 acted as a trivalent cation on the average. These findings indicate that the protons in HUSY are less exchangeable by the cluster cation than the Na cations in NaY. 

Solvents such as organic liquids can act as stabilizers [204] for metal colloids, and in case of gold it was even reported that the donor properties of the medium determine the sign and the strength of the induced charge [205]. Also, in case of colloidal metal suspensions even in less polar solvents electrostatic stabilization effects have been assumed to arise from the donor properties of the respective liquid. Most common solvent stabilizations have been achieved with THF or propylenecarbonate. For example, smallsized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THF after [BEt3H ] reduction of the pre-formed THF adducts (Equation (6)) [54,55,59,206]. Table 1 summarizes the results. 

Subsequently 36 strains of aerobic endospore-forming bacteria, consisting of six Bacillus species and one Brevibacillus species could be discriminated using cluster analysis of ESMS spectra acquired in the positive ion mode (m/z 200-2000).57 The analysis was carried out on harvested, washed bacterial cells suspended in aqueous acidic acetonitrile. The cell suspensions were infused directly into the ionization chamber of the mass spectrometer (LCT, Micromass) using a syringe pump. Replicates of the experiment were performed over a period of six months to randomize variations in the measurements due to possible confounding factors such as instrumental drift. Principal components analysis (PCA) was used to reduce the dimensionality of the data, fol-... 

Isolated polynucleotide clusters from Rhodococcus opacus which encode four polypeptides possessing the activities of a NHase (a and /3 subunits), an auxiliary protein P15K that activates the NHase, and a cobalt transporter protein were expressed in Escherichia coli DSM 14459 cells [34]. Methionine nitrile was added continuously to a suspension of the transformant cells (5.6% w/v of wet cells) in phosphate buffer (50 mM, pH 7.5) at 20 °C, at a rate where the nitrile concentration did not exceed 15 g L 1 while maintaining the pH constant at 7.5. After 320 min, the nitrile was completely converted into amide, corresponding to a final product concentration of 176 gL1.4-Methylthio-a-hydroxybutyramide is readily hydrolyzed with calcium hydroxide, where the calcium salt of 4-methylthio-a-hydroxybutyric acid (MHA) can be directly used as a nutritional supplement in animal feed as an alternative to methionine or MHA. 

Milnes and Mostaghaci [5.5] compared the consequences of different drying methods on the density, the sinter rate and micro structures of sublimated TiO-, suspensions. Evaporation of water in a micro-oven and by radiation heating leds to strongly bound agglomerates, while freeze drying resulted in softly bound secondary clusters. The freeze dried powder reached in 2 h of sintering 98 % of the theoretical density, while differently dried powders needed twice as much time and had a less fine microstructure. 

Another trend that has received considerable recent attention is the decomposition of metal clusters under controlled conditions on solid supports or on liquid suspensions, which generates small metallic particles of specific size, struc-... 

Although the sedimentation velocity of particles tends to decrease steadily as the concentration of the suspension is increased, it has been shown by Kaye and Boardman11 that particles in very dilute suspensions may settle at velocities up to 1.5 times the normal terminal falling velocities, due to the formation of clusters of particles which settle in well-defined streams. This effect is important when particle size is determined by a method involving the measurement of the settling velocity of particles in dilute concentration, though is not significant with concentrated suspensions. 

Kaye, B. H. and Boardman, R. P. Third Congress of the European Federation of Chemical Engineering (1962). Symposium on the Interaction between Fluids and Particles 17. Cluster formation in dilute suspensions. 

In Zerrouki s experiments, the preparation of aqueous phases of identical clusters is performed in six steps. First, colloidal particles of silica, 1.2 pm in diameter, are synthesized. Next, the surface of the particles is made hydrophobic by chemical grafting. Then, an oil-in-water premix emulsion is made by adding an octane suspension of the colloids in an aqueous solution. Controlled shear of the premix in a Couette-type apparatus is subsequently performed to obtain a quasi-monodisperse... 

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