Applications multiple emulsions

Multiple-Emulsion Stability 300 Multiple-Emulsion Applications 301... 

Microencapsulation can be used to provide a temporary barrier between a chemical species and its surrounding environment see also Section 14.3). This permits controlled (slow) release of the active agents following application. Depending on the product and the situation, an active ingredient such as a pesticide may need to be released slowly at low concentration, or slowly at high concentrations. Such controlled release can both reduce the number of crop applications that are required and also help prevent over use and subsequent run-off. The barrier can be provided by a polymer film, in the case of suspensions [867], or a liquid membrane, in the case of single or multiple emulsions [865], Microemulsions have also been used [234,865],... 

Double emulsions are also very useful for food application. Sensitive food materials and flavors can be encapsulated in w/o/w emulsions. Sensory tests have indicated that there is a significant taste difference between w/o/w emulsions and o/w emulsions containing the same ingredients, and that there is a delayed release of flavor in double emulsions [61]. W/o/w or o/w/o multiple emulsions having a concentrated aqueous-soluble flavor or a concentrated oil-soluble flavor encapsulated in the internal phase can be prepared. Food products obtained with these particulates exhibit enhanced flavor perception and extended shelf-life [62]. 

The exact mechanism of inversion remains unclear, although obviously some processes of coalescence and dispersion are involved. In the region of the inversion point multiple emulsions may be encountered. The process is also not always exactly reversible. That is, hysteresis may occur if the inversion point is approached from different sides of the composition scale. Figure 18 shows the irreversible inversion of a diluted bitumen-in-water emulsion brought about by the application of shear (60). 

In some disciplines, certain multiple emulsions have been termed liquid membrane systems, as the liquid film which separates the other liquid phases acts as a thin semi-permeable film through which solute must diffuse moving from one phase to another. There are, therefore many potential practical applications of multiple emulsions. 

Silva Chunha, A. Grossiord, J.L. Seiller, M. The formulations and industrial applications of multiple emulsions an area of fast development. In New Products and Applications in Surfactant Technology, Karsa, D.R., Ed. CRC Press LLC Boca Raton, FL, 1998 Vol. 1, 205-226. 

Several classes of formulations of disperse systems are encountered in the chemical industry, including suspensions, emulsions, suspoemulsions (mixtures of suspensions and emulsions), nanoemulsions, multiple emulsions, microemulsions, latexes, pigment formulations, and ceramics. For the rational preparation of these multiphase systems it is necessary to understand the interaction forces that occur between the particles or droplets. Control of the long-term physical stability of these formulations requires the application of various surfactants and dispersants. It is also necessary to assess and predict the stability of these systems, and this requires the application of various physical techniques. 

All of the above processes are influenced by the nature of the two emulsifiers used to prepare the multiple emulsion. Most reports on multiple emulsions are based on conventional nonionic surfactants, but unfortunately most of these surfactant systems produce multiple emulsions with Hmited shelf-Uves, particularly if the system is subjected to large temperature variations. During the past few years, multiple emulsions have been formulated using polymeric surfactants for both the primary and multiple emulsion preparation. These polymeric surfactants proved to be superior over conventional nonionic surfactants in maintaining the physical stability of the multiple emulsion, such that today they may be applied successfully to the formulation of agrochemical multiple emulsions. The results obtained using these polymeric surfactants offer several potential applications in formulations. The key in the latter cases is to use polymeric surfactants that are approved by the FDA for pharmacy and food, by the CTA for cosmetics, and by the EPA for agrochemicals. 

Another important use of the PHS-PEO-PHS block copolymer is the formation of a viscoelastic film around water droplets [11, 12] this results from the dense packing of the molecule at the W/O interface, which leads to an appreciable interfacial viscosity. The viscoelastic film prevents transport of water from the internal water droplets in the multiple emulsion drop to the external aqueous medium, and this ensures the long-term physical stability of the multiple emulsion when using polymeric surfactants. The viscoelastic film can also reduce the transport of any a.i. in the internal water droplets to the external phase. This is desirable in many cases when protection of the ingredient in the internal aqueous droplets is required and release is provided on application of the multiple emulsion. 

Tadros TF, Taelman MC, Dederen JC. Multiple emulsions with polymeric surfactants. In Grossiord JL, Seiller M, eds. Multiple Emulsions Structure, Properties and Applications. Paris Editions de Sante, 1998 117-137. 

In addition to the traditional dermal delivery formulations discussed above, several other pharmaceutical semi-solid and liquid formulation types have been the subject of a considerable amount of R D. These include sprays, foams, multiple emulsions, microemulsions, liposomal formulations, niosomes, cyclodextrins, glycospheres, dermal membrane structures and microsponges. Although some of these formulations form part of the pharmaceutical armamentarium, they are yet to achieve widespread application and are not within the scope of this chapter. The interested reader is referred to the excellent coverage by Osborne and Amann (1990), Kreuter (1994) and Liu and Wisniewski (1997). 

Another type of emulsion that has gained interest in food applications is the so-called multiple emulsion, which is basically an emulsion contained in a droplet. For example, a water-in-oil-in-water (W/O/W) emulsion means a multiple emulsion of water droplets inside an oil droplet that is dispersed in a continuous water phase. Recently, potential industrial applications in encapsulating active food components were recognized. One of the main difficulties in applying multiple emulsions is their low stability, which limits the applicability when prolonged stability and release are necessary (Muschiolik, 2007). Figure 32.16 shows a schematic representation of a W/O/W emulsion. 

Multiple emulsions may be interesting ways for releasing bioactive compounds in a controlled rate, useful in cosmetic, pharmacy, agricultural, and industrial chemicals nevertheless, their commercial applications have been limited due to their thermodynamic instability and unexpected fast release of encapsulated bioactive molecules (Yoshida et al., 1999 Beer et al., 2013). 

Practical situations are not always so simple, and one may encounter multiple emulsions such as double emulsions, that is, emulsions that are oil-in-water-in-oil (O/W/O) or water-in-oil-in-water (W/O/W). For example, O/W/O denotes a double emulsion containing oil droplets dispersed in aqueous droplets that are in turn dispersed in a continuous oil phase. The double emulsion droplets can be quite large (tens of micrometres) and can contain many tens of droplets of the ultimate internal phase. Developments in and applications of double emulsions have been reviewed by Garti and Bisperink [69]. There can even be more complex emulsion types [33]. Figure 1.2 shows an example of a crude oil W/O/W/O (water-in-oil-in-water-in-oil) emulsion. The type of emulsion that is formed depends on a number of factors. 

The application of multiple emulsion to controlled release or controlled transfer involves nonequilibrium effects in which the most iniemul and most external phases, e.g. W, and W2 in the previous case, have different compositions, so lhat a water transfer would take place under the osmotic pressure gradient. 

Abnormal regions C and B also exhibit low-stability emulsions, a feature that is consistent with the fact that the emulsion type, and thus the interface curvature, is opposite to the one favored by the fomtulation effect. Since multiple emulsions are often made in these regions, a closer look is warranted. For instance, a multiple Wi/O/Wi emulsion i.s found in the C" region. W represents the most internal phase, i.e.. the water dnqtlcts that are located inside the oil drops. It may be considered that the principal" or outside" 0/W2 emulsion has the W,/0 inside" emulsion as internal phase. Since the W,/0 inside emulsion matches the expected type from formulation effects, it is certainly stable, whereas the outside emulsion is not. Thus, such a W,/0/W emulsion would quickly decay in a two-layer system, consisting of a W. phase and an oil layer that would actually be a W /0 emulsion, which is expected to be quite stable if the formulation is sufficiently away from optimum. This means that such unstable" multiple emulsions do not necessarily yield a quick and complete phase separation unless the formulation is q>propriate. e.g.. near-optimum. This feature could be useful for applications dealing with controlled release or capture through mass transfer. 

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