Multiple emulsions coalescence

S. Magdassi and N. Garti Release of Electrolytes in Multiple Emulsions Coalescence and Breakdown or Diffusion Through Oil Phase Collois Surf. 12, 367 (1984). 

Protein-polysaccharide conjugates can also act as the stabilizers of multiple emulsions. Fechner et al. (2007) reported that, under acidic conditions, conjugate-containing water-in-oil-in-water emulsions were more stable to coalescence than the corresponding emulsions made with just sodium caseinate. They also observed that the extent of vitamin B]2 release from the inner aqueous phase of the conjugate-based system was significantly lower. This result could be useful for preparing double emulsions with variable release behaviour. 

Some emulsions are undesirable when they occur. In process industries chemical demulsification is commonly used to separate water from oil in order to produce a fluid suitable for further processing. The specific kind of emulsion treatment required can be highly variable, even within the same industry. The first step in systematic emulsion breaking is to characterize the emulsion in terms of its nature (O/W, W/O, or multiple emulsion), the number and nature of immiscible phases, the presence of a protective interfacial film around the droplets, and the sensitivity of the emulsifiers [295,408,451], Demulsification then involves two steps. First, there must be agglomeration or coagulation of droplets. Then, the agglomerated droplets must coalesce. Only after these two steps can complete phase separation occur. It should be realized that either step can be rate determining for the demulsification process. 

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). 

Mechanisms of drug release from multiple emulsion systems include diffusion of the dmg molecules from the internal droplets (1), from the medium of the external droplets (2), or by mass transfer due to the coalescence of the internal droplets (3), as shown in Fig. 7.16(b)... 

Coalescence of the multiple emulsion drops with the ultimate formation of a W/O emulsion. 

Several industrial systems involve emulsions, of which the following are worthy of mention. Food emulsions include mayonnaise, salad creams, deserts, and beverages, while personal care and cosmetics emulsions include hand creams, lotions, hair sprays, and sunscreens. Agrochemical emulsions include self-emulsifiable oils that produce emulsions on dilution with water, emulsion concentrates with water as the continuous phase, and crop oil sprays. Pharmaceutical emulsions include anaesthetics (O/W emulsions), hpid emulsions, and double and multiple emulsions, while paints may involve emulsions of alkyd resins and latex. Some dry-cleaning formulations may contain water droplets emulsified in the dry cleaning oil that is necessary to remove soils and clays, while bitumen emulsions are prepared stable in their containers but coalesce to form a uniform fihn of bitumen when apphed with road chippings. In the oil industry, many crude oils (e.g.. North sea oil) contain water droplets that must be removed by coalescence followed by separation. In oil slick dispersion, the oil spilled from tankers must be emulsified and then separated, while the emulsification of waste oils is an important process for pollution control. 

Emulsifier I should provide a very effective barrier against coalescence of the water droplets in the multiple emulsion drop. Emulsifier II should also provide an... 

It should be emphasised that polymeric surfactants prevent the coalescence of water droplets in the multiple emulsion drops, as well as coalescence of the latter drops themselves. This is due to the interfacial rheology of the polymeric surfactant films. As a result of the strong lateral repulsion between the stabilising chains at the interface (PHS chains at the W/O interface and PEO chains at the O/W interface), these films resist deformation under shear and hence produce a viscoelastic film. On approach of the two droplets, this film prevents deformation of the interface so as to prevent coalescence. 

It was mentioned earlier that the B and C regions often exhibit multiple emulsions, which is actually the simultaneous occurrence of both emulsion types. There ts some evidence that multiple emulsions start occurring before the catastrophic inversion takes place, and some researchers have proposed a competitive kinetic model in which one of the emulsions could be more stable and thus would prevail (87,89,110). This is consistent with the fact that the variables susceptible to influence the breaking-coalescence mechanisms do influence the location of the inversion tine and hysteresis region. 

Emulsions and microfluidic structures have been used for many purposes, including fusion of reactants present in two droplets, preparation of gel beads, preparation of multiple emulsions, etc. for a comprehensive overview, please consult the review paper of Vladisavljevic and colleagues [1]. Besides this, the microfluidic systems discussed in this entry can be used as analytical tools in various ways. To illustrate this, the use of Y-shaped junctions for dynamic interfacial tension measurement [14] and the use of T-junctions in combination with a coalescence chamber for emulsion stability research [15] are discussed next. 

FIGURE 11.16. Multiple emulsion degradation can take place by several mechanisms. Important pathways include (a) secondary emulsion coalescence with little change in drop size in the PE, (ft) PE drop coalescence with httle change in secondary emulsion characteristics, and (c) loss of PE internal phase to the final external phase due to diffusion or solubilization. 

An emulsion is a heterogeneous system, consisting of at least one immiscible liquid intimately dispersed in another in the form of droplets, whose diameter, in general, exceeds 0.1 micron (italics ours) . He has further opined that such systems possess a minimal stability, which may be accentuated by such additives as surface active agents, finely divided solids etc. [1]. The presence of a surface active agent (see below, and also Chapter 2) obviously makes the system tri-component. More recently, Dickinson [2] accepted the traditional definition of an emulsion as an opaque, heterogeneous system of two immiscible liquid phases ( oil and water ) with one of the phases dispersed in the other as droplets of microscopic or colloidal size . In spite of Becher s contention that the dispersed phase is a liquid, it has been commented that the difference between a liquid-in-liquid emulsion and a solid particle dispersion in a liquid is not entirely distinct [2]. Further, in an emulsion, the dispersed phase itself can be an emulsion, so that this multiple emulsion can be of the types water-in-oil-in water or oil-in-water-in-oil [3,4]. We can also have more than one dispersed phase in a continuous phase, e.g. two kinds of aqueous solution in oil for very short periods before collision and coalescence, which is a very important route for synthetic reactions. Examples of the varieties of emulsions relevant to solid particle preparation will be cited and discussed in later Chapters. 

The last part of the chapter dealt with the preparation of stable water-in-oil-in-water (w/o/w) multiple emulsions that are suitable for application in cosmetics. The main criterion for producing stable w/o/w systems is to use two polymeric surfactants one with a low HLB number for preparation of the primary w/o emulsion and one with a high HLB number for preparation of the final w/o/w multiple emulsion. The primary emulsifier should produce a viscoelastic film fhat prevents leakage from the internal water droplets to the outside continuous phase. It will also ensure high stability (minimum coalescence) of the internal water droplets. The secondary emulsifier should also provide an effective barrier to prevent flocculation and coalescence of the multiple emulsion droplets on storage. It is also essential to balance the osmotic pressure of the internal aqueous droplets and the outside continuous medium. 

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