Ultrasound-assisted emulsification

According to Beoher, an emulsion is a heterogeneous system oonsisting of two immiscible liquids one of whioh (the dispersed phase) is intimately dispersed in the other (the continuous phase) in the form of small droplets whose diameters generally exoeed 0.1 pm [28], Although emulsions are heterogeneous systems, — they oonsist of two immiscible phases — emulsification is intended to produce a homogeneous system in terms of chemical structure. 

The type of emulsion formed (normally water-in-oil or oil-in-water, commonly expressed as wlo or olw, w denoting the aqueous phase and o the organic phase) is determined by the volume ratio of the two liquids and also by the phase addition sequence and the nature of any additives used to promote emulsification [29] the affinity of emulsifiers for oil and water is measured on the hydrophile-lipophile balance (HLB) scale [30]. Oil-in-water emulsions are most common in all application fields. 

mayonnaise, butter, margarine, skin lotions, formulated cosmetics, pharmaceutical ointments, varnishes, paints and lubricants are among the most typical examples of emulsions. Also, emulsions are commonplace in a wide range of technologies and play a key role in materials processing, from metal working to textile finishing [31]. 

The other ma]or olass of emulsion additives is that of stabilizers. In contrast to emulsifiers, stabilizers are non-surface active macromolecules which are added to emulsions in order to inorease the visoosity of the continuous phase, thus reducing the mobility of the droplets in order to prevent emulsification from ooalescence [32]. 

Ultrasound Assistance to Heterogeneous Liquid-Liquid Systems 

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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)... 

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. 

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. 

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. 

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