Ultrasound emulsification

Galindo-Alvarez, J., Boyda, D., Marchal, Ph., Tribet, Ch., Perrin, P., Begue, E.M., Durand, A. and Sadder, V. (2011) Miniemulsion polymerization templates a systematic comparison between low energy emulsification (Near-PIT) and ultrasound emulsification methods. Colloids and Surfaces A Physicochemical and Engineering Aspects, 374 (1—3), 134—141. 

Compared to ultrasound emulsification, the high-pressure homogenizers are able to realize similar local stresses, abrasion is reduced, heat control is easier, and continuous-process solutions at volume streams of up to 50 0001/h exist, which is the basis for industrial apphcations. 

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

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. 

In organometallic chemistry, the use of ultrasound in liquid-liquid heterogeneous systems has been limited to Hg. The emulsification of Hg with various liquids dates to the very first reports on sonochemistry (3,203,204). The use of such emulsions for chemical purposes, however, was delineated by the extensive investigations of Fry and co-workers (205-212), who have reported the sonochemical reaction of various nucleophiles with a,a -dibromoketones and mercury. The versatility of this reagent is summarized in Eqs. (30)-(36). 

The chemical and biological effects of ultrasound were first reported by Loomis more than 50 years ago (4). Within fifteen years of the Loomis papers, widespread industrial applications of ultrasound included welding, soldering, dispersion, emulsification, disinfection, refining, cleaning, extraction, flotation of minerals and the degassing of liquids (5),(6). The use of ultrasound within the chemical community, however, was sporadic. With the recent advent of inexpensive and reliable sources of ultrasound, there has been a resurgence of interest in the chemical applications of ultrasound. 

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. 

First patent on emulsification by ultrasound (Swiss Patent No. 394.390)... 

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)... 

An important problem in this type of analysis is the presence of a matrix, the components of which hamper analysis by falsifying the results or generally making determination impossible. Therefore, in addition to developing appropriate methods of analysis, it is necessary to remove interferents and also to isolate and enrich analytes. For this purpose, various types of extractions are applied, usually LLE and SPE, but also others such as microextraction by packed sorbant (MEPS) and ultrasound-assisted emulsification microextraction (USAEME). 

Magiera, S., Giilmez, S. Ultrasound-assisted emulsification microextraction combined with ultra-high performance liquid chromatography-tandem mass spectrometry for the analysis of ibuprofen and its metabolites in human urine. J. Pharm. Biomed. Anal. 92, 193-202 (2014)... 

Heterogeneous liquid-liquid systems are quite common place in analytical chemistry, which uses them for a variety of purposes, including the following in relation to sample preparation (1) analyte transfer from one phase to another, followed by (a) phase separation in order to feed only the phase enriched with the analyte to the detector or subject it to some other operational step prior to detection, or (b) continuous monitoring of the enriched phase without phase separation (2) the formation of a heterogeneous medium, — small droplets of one phase in another — which is the usual purpose of homogenization and emulsification. Ultrasound (US) has been used to improve the outcome of (1) and (2), albeit with rather disparate results and frequency. 

By contrast, dispersion of a phase as small droplets into another under US assistance until the initial heterogeneous liquid-liquid system is made uniform, which is known as homogenization or emulsification , is a well-documented process in both the analytical and industrial fields. Depending on the operating conditions and the type of ultrasound used, both emulsion formation and destruction can be favoured. 

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]. 

Ultrasound-assisted emulsification in aqueous samples is the basis for the so-called liquid membrane process (LMP). This has been used mostly for the concentration and separation of metallic elements or other species such as weak acids and bases, hydrocarbons, gas mixtures and biologically important compounds such as amino acids [61-64]. LMP has aroused much interest as an alternative to conventional LLE. An LMP involves the previous preparation of the emulsion and its addition to the aqueous liquid sample. In this way, the continuous phase acts as a membrane between both the aqueous phases viz. those constituting the droplets and the sample). The separation principle is the diffusion of the target analytes from the sample to the droplets of the dispersed phase through the continuous phase. In comparison to conventional LLE, the emulsion-based method always affords easier, faster extraction and separation of the extract — which is sometimes mandatory in order to remove interferences from the organic solvents prior to detection. The formation and destruction of o/w or w/o emulsions by sonication have proved an effective method for extracting target species. 

Up to a few years ago the majority of applications and developments involved ultrasonic frequencies in the MHz range for noninvasive analysis. A number of uses have been developed for the measurement of such factors as the degree of emulsification or extent of particle distribution within dispersions. More recently the interest of food technologists has turned to the use of power ultrasound in processing. In this case the mechanical and chemical effects of cavitation are important and applications are very wide ranging (Table 1). 

High-frequency, low-power ultrasound generally within the frequency range 0.5-20 MHz can be used to evaluate foodstuffs in terms of physical characteristics such as the degree of emulsification or the concentration of solids or gas. In... 

The degree of emulsification in such materials can also be estimated by the measurement of ultrasound velocity in conjunction with attenuation [4]. It is possible to determine factors such as the degree of creaming (or settling ) of a sample, i.e. the movement of solid particles/fat droplets to the surface (or to the base) [5], Such information gives details, for example, of the long-term stability of fruit juices and the stability of emulsions such as mayonnaise. The combination of velocity and attenuation measurements shows promise as a method for the analysis of edible fats and oils [6], and for the determination of the extent of crystallization and melting in dispersed emulsion droplets [7]. 

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