Dispersion, ultrasonic

Perfluoroalkylzinc iodides, prepared in situ from iodides and ultrasonically dispersed Zn, are coupled with allylic halides via an allylic rearrangement[271]. The Pd-catalyzed homocoupling of allylic acetate in the presence of Zn to give a mixture of regioisomers 416 and 417 may proceed via in situ formation of allylzinc species[272,273]. 

Sedimentation (qv) techniques, whether based on gravitational forces or centrifugation, derive the particle size from the measured travel rates of particles in a Hquid. Before the particle analysis is carried out, the sample is usually dispersed in a medium to break down granules, agglomerates, and aggregates. The dispersion process might involve a simple stirring of the powder into a Hquid, but the use of an ultrasonic dispersion is preferred. 

Extrusion of S02 from unsaturated cyclic sulphones, promoted by ultrasonically dispersed potassium, is described in Reference 218. 

Ghasemi et al. [24] have obtained nanostructured Pb02 (50-100 nm) using [5-PbO precursor and in the presence of ammonium peroxydisulfate as an oxidant. Here, the ultrasonication dispersed and then cracked the [5-PbO particles, thereby increasing the contribution of their surface area. Such an ultrasonic treatment resulted in an enhancement in the oxidation of PbO to Pb02 has been observed. Ultrasonic waves also have been found to inhibit the formation of Pb02 particles larger than 150 nm. 

The sonochemistry of the other alkali metals is less explored. The use of ultrasound to produce colloidal Na has early origins and was found to greatly facilitate the production of the radical anion salt of 5,6-benzo-quinoline (225) and to give higher yields with greater control in the synthesis of phenylsodium (226). In addition, the use of an ultrasonic cleaning bath to promote the formation of other aromatic radical anions from chunk Na in undried solvents has been reported (227). Luche has recently studied the ultrasonic dispersion of potassium in toluene or xylene and its use for the cyclization of a, o-difunctionalized alkanes and for other reactions (228). 

The two forms of POM can be obtained by using density fractionation in combination with ultrasonic dispersion. The chemical composition of both fractions is quite different, with the occluded POM having lower... 

Suspensions were prepared in 500 mL polycarbonate bottles by ultrasonically dispersing freeze-dried goethite in dilute nitric... 

The reactants were ultrasonically dispersed in ethylendiamine, then transferred to a Teflon-lined autoclave and gradually heated up to 160°C. The products were filtered out, washed with absolute alcohol, normal HC1 solution and water and finally dried in vacuum. In the product nano-fibres and near single-crystal whiskers were obtained. 

Colloidal potassium has recently been proved as a more active reducer than the metal that has been conventionally powdered by shaking it in hot octane (Luche et al. 1984, Chou and You 1987, Wang et al. 1994). To prepare colloidal potassium, a piece of this metal in dry toluene or xylene under an argon atmosphere is submitted to ultrasonic irradiation at ca. 10°C. A silvery blue color rapidly develops, and in a few minutes the metal disappears. A common cleaning bath (e.g., Sono-clean, 35 kHz) filled with water and crushed ice can be used. A very fine suspension of potassium is thus obtained, which settles very slowly on standing. The same method did not work in THF (Luche et al. 1984). Ultrasonic waves interact with the metal by their cavitational effects. These effects are closely related to the physical constants of the medium, such as vapor pressure, viscosity, and surface tension (Sehgal et al. 1982). All of these factors have to be taken into account when one chooses a metal to be ultrasonically dispersed in a given solvent. 

Composite cast multibilayers provided a route to the formation of multilayer, two-dimensional polymer networks [443, 445]. This method utilized the following steps (i) ultrasonic dispersal of 15 mM of the dialkylammonium surfactant, 35, and 15 mM of the bisacrylate monomer. 36 (ii) addition of... 

Closed, spherical, single-bilayer, 300- to 600-A-diameter surfactant and/or phospholipid aggregates dispersed in aqueous solutions. Ultrasonic dispersal of multilamellar vesicles (MLVs) or employing procedures such as French Press filtration result in SUV formation. 

Periodically, these mixtures were centrifuged and an aliquot of the clay-free supemate taken for counting analysis. Two naturally-occurring clays were selected for the experiments one was labeled kaolin (for the mineral kaolinite) while the second was referred to as attapulgite (or polygorshite). Both were obtained from the Source Clay Mineral Repository (3) as standard clays representative of each class of clay and were used as received. Stable, colloidal suspensions of each were prepared by ultrasonically dispersing weighed quantities of each clay in triple-distilled water. 

Carboxylation.2 Ultrasonically dispersed zinc powder in DMF promotes direct carboxylation (COz or dry ice) of perfluoroalkyl iodides. 

Trifluoromethylation.5 Ultrasonically dispersed zinc in DMF or THF reacts with trifluoromethyl iodide to form trifluoromethylzinc iodide, which adds as formed to aldehydes or ketones (equation I). 

For a monatomic gas, where the heat capacity involves only translational energy, V is independent of sound oscillation frequency (except at ultra-high frequencies, where a classical visco-thermal dispersion sets in). For a relaxing polyatomic gas this is no longer so. At sound frequencies, where the period of the oscillation becomes comparable with the relaxation time for one of the forms of internal energy, the internal temperature lags behind the translational temperature throughout the compression-rarefaction cycle, and the effective values of CT and V in equation (3) become frequency dependent. This phenomenon occurs at medium ultrasonic frequencies, and is known as ultrasonic dispersion. It is accompanied by... 

In a long series of papers on the master equation, Pritchard and his coworkers elucidated for the first time the effects of rotational and vibrational disequilibrium on the dissociation and recombination of a dilute diatomic gas. Ultrasonic dispersion in a diatomic gas was analyzed by similar computational experiments, and the first example of the breakdown of the linear mixture rule in chemical kinetics was demonstrated. A major difficulty in these calculations is that the eigenvalue of the reaction matrix (corresponding to the rate constant) differs from the zero eigenvalue (required by species conservation) by less than... 

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