Ultrasonic irradiation, dispersal

Large-scale ultrasonic irradiation is extant technology. Liquid processing rates of 200 liters/minute are routinely accessible from a variety of modular, in-line designs with acoustic power of several kW per unit (83). The industrial uses of these units include (1) degassing of liquids, (2) dispersion of solids into liquids, (3) emulsification of immiscible liquids, and (4) large-scale cell disruption (74). While these units are of limited use for most laboratory research, they are of potential importance in eventual industrial application of sonochemical reactions. 

The possible mechanisms which one might invoke for the activation of these transition metal slurries include (1) creation of extremely reactive dispersions, (2) improved mass transport between solution and surface, (3) generation of surface hot-spots due to cavitational micro-jets, and (4) direct trapping with CO of reactive metallic species formed during the reduction of the metal halide. The first three mechanisms can be eliminated, since complete reduction of transition metal halides by Na with ultrasonic irradiation under Ar, followed by exposure to CO in the absence or presence of ultrasound, yielded no metal carbonyl. In the case of the reduction of WClfc, sonication under CO showed the initial formation of tungsten carbonyl halides, followed by conversion of W(C0) , and finally its further reduction to W2(CO)io Thus, the reduction process appears to be sequential reactive species formed upon partial reduction are trapped by CO. 

At the instant of bubble collapse, this pressure will be released into the liquid. It is these very high liquid pressures which give rise to certain of the well-known effects of ultrasonic irradiation, such as erosion, dispersion and molecular degradation. 

Another important consideration when using baths to perform sonochemical reactions is that it may be necessary to stir the mixture mechanically to achieve the maximum effect from the ultrasonic irradiation. This is particularly important when using solid-liquid mixtures where the solid is neither dispersed nor agitated throughout the reaction by sonication alone and simply sits on the base of the vessel where it is only partially available for reaction. The reason that additional stirring is so important in such cases is that it ensures the reactant powder is exposed as fully as possible to the reaction medium during sonication. 

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. 

Thus, macroradicals have been obtained by stretching fibers (20), deforming plastics by compression (37), ball mill grinding (11), freezing and grinding of polymer solutions (10), ultrasonic irradiation (I), mastication (19), dispersion in a microblender (25), and other mechanical techniques (36). Many reviews on the formation of macroradicals by degradative processes have also been published (5, 12,13,16, 33). 

Dispersal of DODAC or DHP In water by ultrasonic Irradiation results In the formation of fairly uniform single compartment vesicles (Figure 1). DODAC and DHP vesicles are stable for weeks... 

Furthermore, the ultrasonic copolymerization was studied in terms of pH [30], It was found that with a decrease in pH, both the copolymer yield and AN content in the copolymer were increased. In a later study, the mixture ultrasonically irradiated was a stable water-based dispersion and the particle size of the copolymer increased with decreasing pH. No acrylonitrile homopolymer was found in the reaction product. 

The possibility that ultrasound can be involved chemically in an intended electrochemical system is not without precedent at Wesleyan University. In earlier work involving the electroreduction of a,a -dibromo ketones at a mercury cathode, ultrasound was employed just for stirring [201,202]. It was then realized that although the dihaloketone was stable to mercury (without electrolysis) over weeks in silent conditions, the ultrasonic irradiation, which tended to produce a range of finely divided mercury droplets above the pool, induced sonochemical reduction on the metal [203]. The authors later deduced experimental conditions using either electrochemistry or chemical reaction on ultrasonically dispersed mercury that could select from the range of possible products [204]. 

The effect of ultrasonic irradiation on the mechanical property, morphology, and crystal structure of PP/EPDM blends were examined by Chen and Li (16). Appropriate ultrasonic intensity can increase the toughness of PP/EPDM blends noticeably. SEM showed that with ultrasonic irradiation, the morphology of a well-dispersed EPDM phase is formed in the PP/EPDM blend. Ultrasonic irradiation interestingly... 

For dilute spins, such as and 31p, the dipolar broadening can be removed by sufficiently intense rf irradiation at the proton resonance frequency.2-4 Nevertheless, a substantial broadening due to the anisotropy of the chemical shift remains. In order to obtain "high resolution" NMR spectra, it has become customary to subject multllamellar dispersions to prolonged ultrasonic irradiation.5 This process, which results in particles of reduced size with reduced reorientational correlation times, does indeed improve the resolution of the NMR spectya however, its exact physical and chemical consequences are a subject of much debate.6 We describe below a method whereby high resolution spectra can be obtained without resorting to sonication. 

The foregoing latex mixtures were subjected to ultrasonic irradiation after ion exchange to determine if this treatment affected the degree of dispersion no significant differences were found, and the appearance of the electron micrographs and the number ratios of small/large particles were similar to those reported above for the nonirradiated samples. 

For incompatible blends, the slope of log G versus log G" plots in the terminal region was less than 2. The slopes of the sonicated and compatibilized samples were higher than that of untreated samples in the linear viscoelastic region (see Figure 8.28), which meant that the compatibility could be enhanced by ultrasonic irradiation as well as by a compatibilizer. Palierne [97] has developed a model that can predict the linear viscoelastic behavior of a polymer emulsion, by considering the droplet size in a matrix and the interfacial tension between the components. From this model the unknown interfacial tension between the matrix and the dispersed phase could be estimated, and the predicted values decreased by ultrasonic irradiation due to ultrasonic compatibilization. 

Alkenes with perfluoroalkyl substituents have been prepared by the reaction of perfluoroalkyl iodides with terminal alkynes in the presence of ultrasonically dispersed zinc and Cul. A general procedure for the formation of organocopper reagents from alkyl and aryl halides and lithium metal in the presence of Cul or l-pentynylcopper(I) under ultrasonic irradiation has also been described. ... 

Some surfactants possessing two hydrocarbon chains attached to a single head group, e.g. the dialkyldimethylammonium chlorides, form lamellar structures when dispersed in aqueous solution. When such turbid solutions are subjected to ultrasonic irradiation, optically clear solutions are formed in which the surfactant is dispersed in the form of closed vesicles [104] similar in structure to the liposomes formed by phospholipids. The use of these totally synthetic bilayer vesicles as model membranes and as potential drug delivery systems is currently under investigation. 

Figure 4.42 Effect of time of ultrasonic irradiation on the weight-average aggregate weight of lecithin dispersed in (a) 0.01 M sodium chloride solution and (b) deionized water. From Attwood and Saunders [285].

These studies will help to answer many questions, such as What is the role of coordination modulators and surfactants in nanoMOFs How they can be used to control the size, shape, stability, and dispersibility of nanoMOFsl Can microwave irradiation, ultrasonic irradiation, and use of emulsions be general methodologies for nanoMOFs synthesis Can the exfoliation method be used to prepare large single MOF sheets Can the assembly of nanoMOFs be controlled to create MOF superstructures with desired shapes The answers to these and many other questions will provide the basis for better controlling the synthesis of nanoMOFs and facilitate their exploitation for diverse applications. 

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