Ultrasonic emulsification

The top-down approach involves size reduction by the application of three main types of force — compression, impact and shear. In the case of colloids, the small entities produced are subsequently kinetically stabilized against coalescence with the assistance of ingredients such as emulsifiers and stabilizers (Dickinson, 2003a). In this approach the ultimate particle size is dependent on factors such as the number of passes through the device (microfluidization), the time of emulsification (ultrasonics), the energy dissipation rate (homogenization pressure or shear-rate), the type and pore size of any membranes, the concentrations of emulsifiers and stabilizers, the dispersed phase volume fraction, the charge on the particles, and so on. To date, the top-down approach is the one that has been mainly involved in commercial scale production of nanomaterials. For example, the approach has been used to produce submicron liposomes for the delivery of ferrous sulfate, ascorbic acid, and other poorly absorbed hydrophilic compounds (Vuillemard, 1991 ... 

Barium carbonate also reacts with titania to form barium titanate [12047-27-7] BaTiO, a ferroelectric material with a very high dielectric constant (see Ferroelectrics). Barium titanate is best manufactured as a single-phase composition by a soHd-state sintering technique. The asymmetrical perovskite stmcture of the titanate develops a potential difference when compressed in specific crystallographic directions, and vice versa. This material is most widely used for its strong piezoelectric characteristics in transducers for ultrasonic technical appHcations such as the emulsification of Hquids, mixing of powders and paints, and homogenization of milk, or in sonar devices (see Piezoelectrics Ultrasonics). 

Abismad B, Canselier JP, Wilhelm AM, Delmas H, Gourdon C (1999) Emulsification by ultrasound Drop size distribution and stability. Ultrason Sonochem 6 75-83... 

Mahdi C, Oualid H, Fatiha A, Christian P (2010) Study on ultrasonically assisted emulsification and recovery of copper(II) from wastewater using an emulsion liquid membrane process. Ultrason Sonochem 17(2) 318-325... 

The effect of ultrasound on liquid-liquid interfaces between immiscible fluids is emulsification. This is one of the major industrial uses of ultrasound (74-76) and a variety of apparatus have been devised which will generate micrometer-sized emulsions (9). The mechanism of ultrasonic emulsification lies in the shearing stresses and deformations created by the sound field of larger droplets. When these stresses become greater than the interfacial surface tension, the droplet will burst (77,78). The chemical effects of emulsification lie principally in the greatly increased surface area of contact between the two immiscible liquids. Results not unlike phase transfer catalysis may be expected. 

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. 

As noted above this type of mechanical transducer is predominantly used for homo-genisation/emulsification. These devices differ markedly from the more usual bath and probe types in that they derive their power from the medium (by mechanical flow across the blade) rather than by the transfer of energy from an external source to the medium. The majority of the chemical effects observed on using whistle type transducers for the sonication of homogeneous reactions can be attributed mainly to the generation of very fine emulsions rather than the ultrasonic irradiation itself. 

Abismail, B., Canselier, J.P., Wilhelm, A.M., Delmas, H., Gourdon, C. (1999). Emulsification by ultrasound drop size distribution and stability. Ultrasonics Sonochemistry, 6, 75-83. 

Behrend, O., Ax, K., Schubert, H. (2000). Influence of continuous phase viscosity on emulsification of ultrasound. Ultrasonics Sonochemistry, 7, 77-85. 

High-frequency or diagnostic ultrasound in clinical imaging (3-10 MHz) Medium-frequency or therapeutic ultrasound in physical therapy (0.7-3.0 MHz) Low-frequency or power ultrasound for lithotripsy, cataract emulsification, liposuction, tissue ablation, cancer therapy, dental descaling, and ultrasonic scalpels (18-100 kHz)... 

The dispersions were obtained by emulsification via ultrasonication of a toluene solution of the unsaturated homopolymer in an aqueous surfactant solution. This was followed by exhaustive hydrogenation with Wilkinson s catalyst at 60°C and 80 bar H2 to produce a dispersion with an average particle size of 35 nm (dynamic light scattering and transmission electron microscopy analyses). The same a,co-diene was used as comonomer in the ADMET polymerization of a phosphorus-based monomer, also containing two 10-undecenoic acid moieties... 

Figure 6 Oil droplet size distribution of on olive oil emulsion, stabilized with hydroxy-propyl mcthylceUkiloae, after different emulsification procedures blender (triangles), ultrasonic probe (squares), and ultrasonic homogenize (stars). Theoretical distributions were calculated from Coulter Counter measurements using a software program, assuming spherical particles.

The extension of the interfacial area by emulsification explains Miyagawa s [95] observation that the nitration rate can be considerably increased by the action of ultrasonics on a reacting system. For example, nitration of m- xylene to trinitro-m- xylene, which generally takes 2 hr, takes only 30 min when ultrasonics are used. There is no evidence as yet whether and how ultrasonic waves effect group orientations. 

In high force dispersion devices, ultrasonication is used today especially for the homogenization of small quantities, whereas rotor-stator dispersers with special rotor geometries, microfluidizers, or high-pressure homogenizers are best for the emulsification of larger quantities. 

The mechanism for ultrasonic emulsification is primarily that of cavitation. A typical sonicator for emulsification consists of a velocity transformer coupled to a transducer, capable of oscillating in a longitudinal mode, where the velocity transformer is immersed in the liquid. Figure 4 illustrates the basic parts of a sonicator with a continuous flow attachment, like the one used in this work. In this case, the flow cell is secured to the velocity transformer by a flange and a Teflon 0-ring. The intensity of cavitation depends on the power delivered to the velocity transformer, which is relayed to the transducer from a variable transformer or some other control device not shown in Fig. 4. 

Ultrasound-assisted emulsification was initially developed by Wood and Loomis [38]. The first patent of an ultrasonic emulsifier was granted in 1944 in Switzerland. Since then, research on US-assisted emulsification and underlying mechanisms has grown in parallel due to interest in the process [32]. 

One mechanism similar to that of capillary waves is based on the oscillation and subsequent disruption of droplets under US action. The corresponding resonance radius at a frequency of 20 kHz (common for ultrasonic sources) is about 10 xm. This mechanism must be considered as one source of US-assisted emulsification, but can only be applied to immiscible liquid-liquid systems with a diameter within the established range for most of the droplets. In fact, most immiscible liquid-liquid systems are formed by droplets with... 

Although discrete emulsification can be accomplished with ultrasonic baths, probes are more frequently used for this purpose because they can directly transmit US energy to a liquid-liquid system. Figure 6.7A illustrates a straightfonward procedure for obtaining an ultrasonic emulsion. The sonotrode is immersed either into the continuous phase and the phase to be dispersed is gradually added or into the two-phase system while ultrasonic energy is applied. In the latter case, the tip of the probe can be positioned at the interface [44] between the two immiscible liquids or in the continuous (or dispersed) phase, irrespective of their... 

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