Emulsification by ultrasound

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

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. 

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

Behrend O, Ax K, Schubert H Influence of continuous phase viscosity on emulsification by ultrasound, Ultrason Sonochem 7 77—85, 2000. http //www.sciencedirect.com/science/ article/pii/S1350417799000292. 

The polymerisation of styrene in miniemnlsions stabilised with anionic sodium dodecyl sulphate or nonionic Lntensol AT50 results in stable polymer dispersions with particle diameters between 30 and 480 nm and narrow particle size distributions. Steady-state mini-emulsification results in a system with critical stability , i.e. the droplet size is the prodnct of a rate equation of fission by ultrasound and fusion by collisions, and the mini-droplets are as small as possible for the timescales involved. The droplet growth by monomer exchange, or the T1 mechanism, is effectively suppressed by addition of a very hydrophobic material, whereas droplet growth by collisions, or the T2 mechanism, is subject to the critical conditions. The growth of the critically stabilised miniemulsion droplets is usually slower than the polymerisation time therefore, in ideal cases, a 1 1 copy of droplets to particles is obtained, and the critically stabilised state is frozen. 6 refs. 

Emulsion Polymerization. The generation of radicals by ultrasound can also be applied in emulsion polymerization, which comprises a free-radical polymerization in a heterogeneous reaction system, yielding submicrometer polymer particles in a continuous aqueous phase. In literature a number of publications have appeared over the years on the application of ultrasound in emulsion polymerization (36-46). Ultrasoimd can be applied both for emulsification purposes as well as at higher conversions in the emulsion polymerization process (see Heterophase Polymerization). 

Similar results are found in literature for the emulsification of particle loaded droplets by ultrasound [22, 66, 68]. It is questionable if the bulk viscosity inaease is the only reason for the hindered droplet breakup. The complex rheological behavior of suspensions and the possible interactions of the nanoparlicles at the liquid-liquid-interface have to be taken into consideration in understanding these effects. Numerical simulation of the influence of nanoparticles on droplet deformation yielded a decreased deformation for an increased particle load due to viscous and interfacial effects [73]. Depending on particle-particle- and particle-fluid-interactions, suspensions show an increase in shear thinning or viscoelastic behavior with increasing particle load [65]. Investigations with non-Newtonian fluids without particles showed that droplet deformation and droplet disruption is reduced by... 

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. 

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). 

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. 

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]. 

More productive chemical results, which stiU harness the destructive action of ultrasound on certain bonds, can be attained when sonication is applied to biological fluids (e.g. protein solutions) en route to bionanomaterials [15], A conspicuous example can be found in sonochemically-prepared protein microspheres, in which the interplay of mechanical effects (emulsification) and chemical effects (formatiOT of transient species) is noticeable. A protein emulsion is readily created at the interface between two immiscible liquid phases, while radicals generated by water sonolysis promote disulfide bond cross-linking between cysteine residues. Surface modifications, via conjugation with monoclonal antibodies or RGD-containing peptides, can also be carried out [102, 103]. The sonochemical preparation of chitosan microspheres also exploits the intermolecular cross-linking of imine bonds from the sugar precursor [104]. 

Optimization of ultrasound assisted-emulsification-dispersive liquid-liquid microextraction by experimental design methodologies for the determination of... 

Fontana, R, Patd, S., Banerjee, K., Altamirano, J. (2010). Ultrasound-Assisted Emulsification Microextraction for Determination of 2,4,6-Trichloroanisole in Wine Samples by Gas Chromatography Tandem Mass Spectrometry. /. Agric. Food Chem., Vol.58, N°8, pp. 4576-4578, ISSN 15205118. 

Fundamental work by Luche resulted in the hypothesis that ultrasound can influence and change reaction pathways in reaction types with single electron transfer [186, 187]. Ultrasound is also believed to influence reaction systems by mechanical effects [187]. An empirical classification of sonochemical reactions is divided into three types of effects purely chemical effects induced by sonochemical cavitation, hydrodynamic effects (mechanically induced cavitation), and by-passing mass-transport limitation. The latter effects are based on physical rather than chemical phenomena and judged to be false sonochemistry [188]. Nevertheless, these false effects (e.g. emulsification) are often important. The three types of effect are ... 

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