Stability of multiple emulsions

Y. Sela, Y. Magdassi, and N. Garti Polymeric Surfactants Based on PolysUoxanes-Graft-Poly(Oxyethylene) for Stabilization of Multiple Emulsions. Colloids Surfaces 83, 143 (1993). 

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. 

The formulation and stability of multiple emulsion systems has recently been reviewed by the present authors (17). 

Specific factors controlling the stability of multiple emulsion... 

Factors Afecting Stability of Multiple Emulsions, and Criteria for Their Stabilisation 235... 

A more sensitive rheological techniques for following the stability of multiple emulsions is to use oscillatory techniques. In this case, a sinusoidal strain or stress is applied to the sample, which is placed in the gap of the concentric cylinder or cone-and-plate geometry the resulting stress or strain sine wave is followed at the same time. For a viscoelastic system, as is the case with multiple emulsions, the stress and strain sine waves oscillate with the same frequency, but out of phase. 

Multiple emulsions have a complex morphology and various important parameters for their prepara tion and characterization have been described (39, 47). Examples are the characteristics of the W/0 glo bules in W/O/W systems, such as their size and volume fraction, W/0 ratio inside the W/O globules, and aver age number and size of water droplets inside the W/O globules. The time dependence of Arose parameters are closely related to the stability of multiple emulsions and their morphology. Other important features are transport properties of substances encapsulated into discrete droplets and the permeability of the layer separating Are internal from the external continuous phase. As was shown above, the NMR PFG method is a sensitive tool to study structure and complex dynamic phenomena, and therefore it is a promising technique in the study of multiple emiAsions. 

Magdassi and Carti [82] showed thaf the stability of multiple emulsions is also affected by the migration of electrolytes from the internal to the external phase. This migration is the result of the diffusion rather than of the droplets breakdown. The release... 

In this chapter the effects of pressure balance and interfadal rheological properties on the stability of multiple emulsions are discussed. 

Numerous applications of multiple emulsions in various fields have been reported. More applications need to be realized if multiple emulsions stability is to be fully understood and approaches to stabilize multiple emulsions fully rationalized. The stability of multiple emulsions is influenced by numerous formulation and process variables. As demonstrated in this chapter, long-term multiple emulsion stability is dependent on the osmotic and Laplace pressures of the inner droplets as well as on the pressure balance between them described by the Walstra equation. Stability also equally, in some cases even more, depends on the strength of the interfacial film formed on the interface of droplets of multiple emulsions. This property can be characterized by interfacial rheology. 

Csoka I, Eros 1.1997. Stability of multiple emulsions I. Determination of factors influencing multiple drop breakdown. Int I Pharm 156 119-123. 

Florence AT, Whitehill D. 1982. The formulation and stability of multiple emulsions. Int J Pharm 11 277-308. 

Kavaliunas DR, Frank SG. 1978. Liquid crystal stabilization of multiple emulsions. J Colloid Interface Sd 66 586-588. 

Magdassi S, Frenkel M, Garti N. 1984. On the factors affecting the yield of preparation and stability of multiple emulsions. / Disper Sci Tech 5(1) 49-59. 

From these structural studies we can conclude that when a small molecular surfactant is used to stabilize inverse droplets inside the globule phase, the inverse droplets are submitted to attractive interaction toward the droplets inner interface, and slowly diffuse along the surface of the globules. A single droplet thus spends an increased amount of time close to the globules interface. This phenomenon appears to decrease the stability of multiple emulsions. 

Michaut E, Hebraud P, Perrin P. 2003. Amphiphihc polyelectrolyte for stabilization of multiple emulsions. Polym Int 52 594-601. 

Sela Y, Magdassi S, Garti N. 1994. Polymeric surfactants based on polysiloxanes— Graft-poly (oxyethylene) for stabilization of multiple emulsions. Colloids Surf A 83 ... 

The use of proteins as macromolecular amphiphiles was adopted long ago by scientists exploring the stability of multiple emulsions. Gelatin (Zhang et al., 1992), whey proteins (Comec et al, 1998), bovine serum albumin (BSA) (Dickinson et al., 1991 Fredrokumbaradzi and Simov, 1992), human serum albumin (HSA), caseins, and other proteins have been evaluated. In many cases the proteins were used in combination with other monomeric emulsifiers, especially when proteins were used in the internal phase. However, whether the adsorption is competitive or complementary remains a controversial question (Wilde and Clark, 1993 Kerstens et al., 2006). 

Research on the use of protein to improve the stability of multiple emulsions deals with proteins present (or adsorbed) in the inner phase as a surfactant or as encapsulating agent, or at the outer aqueous phase as a surfactant. 

Omotosho et al. (1986) were the first to study the influence of BSA with a nonionic surfactant in the inner aqueous phase to stabilize W/O/W emulsions. They concluded that interfacial complexation between BSA and the non-ionic surfactant occurs at the inner W/O interface. This complex membrane has been found to enhance the stability of multiple emulsions and to slow down release of solute entrapped within the emulsion droplets. BSA has been investigated as a replacement for some of the monomeric surfactants in the inner phase and found to provide good stabilization for W/O/W multiple emulsions (Fredrokumbaradzi and Simov, 1992 Evison et al., 1995). 

Proteins cannot completely replace hydrophobic monomeric emulsifiers in the inner phase of multiple emulsions (Dickinson et al., 1994). However, proteins in combination with other stabilizers do have the capacity to enhance the stability of multiple emulsions, mainly by encapsulating the inner phase and thereby substantially reducing the hydrophobic emulsifier concentration. Protein in the external phase can enhance the multiple emulsion stability without the addition of any emulsifier or stabilizer. 

Vaziri and Warburton (1994) investigated the effects of different combinations of surfactants (Span 80 and Tween 80) and hydrocolloids (acacia and gelatin) on the release rate and stability of multiple emulsions. The combination of surfactants produced stable systems and more prolonged release because a better interfacial film was formed by the emulsifiers compared with that of the multiple emulsions in which each surfactant was employed individually (see Figure 5.15 and Table 5.3). The enhanced stabilization can also be attributed to the protein content in the acacia gum. The protein provided strong adsorption interaction with the oil-water interface during emulsification. 

Hydrocolloids significantly improve the stability of multiple emulsions because they help encapsulate better the inner aqueous phase, preventing uncontrolled release of the entrapped ingredients. Stabilization of the multiple emulsion is achieved mainly by depletion stabilization. However, hydrocolloids significantly affect viscosity and the rheology of a multiple emulsion. 

Knoth et al. (2006) and Fechner et al. (2006) investigated the stabilization of multiple emulsion in two systems. The first system was based on whey protein as the external emulsifier and carboxymethylcellulose (CMC) as the... 

Fechner A, Knoth A, Scherze I, Muschiolik G. 2006. Stabilization of multiple emulsion by milk protein-polysaccharide interaction conjugate. Abstracts of Papers of the Food Colloids Self-assembly and Material Science, Montreux, Switzerland, The University of The Fribourg, Fribourg, Switzerland 23-26 April, B24. 

Garti N. 1998. Influence of the formulation on the characteristics and stability of multiple emulsions. In Multiple Emulsions Structure, Properties and Applications, SeiUer M, Grossiord JL, eds. Paris Editions de Sante, pp 81-116. 

Additional milestones in Prof. Garti s work regarding the stabilization of multiple emulsions should be mentioned ... 

The effects of WPFpolysaccharide conjugates on the stabilization of multiple emulsions are the subject of recent studies. It was observed, for example, that the release of vitamin Bi, entrapped in the core of W/OAV multiple globules, is hampered due to the biopolymer adducts formed at the interface. Similar adducts stabilize OAV/O multiple emulsions and serve as efficient barriers against release of addenda contained in the inner phase. ... 

Part II focuses on the formation and stabilization of multiple emulsions. 

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