Preparation of multiple emulsions

The most convenient method of preparation is a two stage process as is illustrated 

The yield of the multiple emulsion can be determined using dialysis for W/O/W multiple emulsions. A water-soluble marker is used and its concentration in the outside phase is determined. 

It has been suggested that if a yield of more than 90% is required, the lipophilic (low HLB) surfactant used to prepare the primary emulsion must be 10 x higher in concentration than the hydrophilic (high HLB) surfactant. 

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Kim, H.-J., Decker, E.A., McClements, D.J. (2006). Preparation of multiple emulsions based on thermodynamic incompatibility of heat-denatured whey protein and pectin solutions. Food Hydrocolloids, 20, 586-595. 

The principles discussed in this section can be appliedtothe preparation of multiple emulsions. An O/W/O multiple emulsion can be created by, for example, first making a stable primary O/W emulsion using a high-HLB surfactant and... 

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. 

Choice of surfactant(s) for the preparation of multiple emulsions can, in principle, be made from any of the four classes of surfactants discussed in Chapter 3, although nonionics tend to be materials of choice because they are more easily tailored to meet the needs of the system. The choice will be determined by the characteristics of the final emulsion type desired, such as the natures of the various phases, additives, and solubilities. In many applications (e.g., foods, drugs, cosmetics), the choice may be further influenced by such questions as toxicity, interaction with other addenda, and biological degradation. In a given system, several different surfactants may perform adequately in terms of stability, but produce different types of multiple emulsions (A, B, or C in Figure 11.15), so that the choice will depend on application as well as function. 

The second stage of the process disperses the primary emulsion in an aqueous phase. A high-HLB (9-11) surfactant is dissolved in the external phase. The primary emulsion is then added to the surfactant solution and mixed. The method and intensity of mixing are chosen to achieve maximum dispersion with minimal rupture of multiple-emulsion droplets. The resultant multiple-emulsion preparation usually contains two types of droplets (Fig. 1). Although preparations of multiple emulsions usually contain both types of droplets, the relative amount of each type is dependent on the method of preparation and the surfactant at the OAV interface (Florence and Whitehill, 1981). 

Oils to be used for the preparation of multiple emulsions must be pharmaceutically acceptable (no toxicity). The most convenient oils are vegetable oils such as soybean or safflower oil. Paraffinic oils with no toxic effect may be used. Also, some polar oils such as isopropyl myristate can be applied. 

Preparation of Multiple Emulsions 31 Structure of Stable Multiple Emulsions 32... 

The abihty to capture direct images of double-emulsion globules enables verification of the actual existence of the complex, three-phase system. The importance of visualization of double emulsions is seen in the work of Matsumoto and coworkers (1976) and their early success in developing a technique for preparation of stable double-emulsion systems. Similarly Kavaliunas and Frank (1978) used photomicrography to detect the structure of emulsions prepared with a water, p-xylene, and nonylphenol diethylene glycol ether system. As a result the authors were able to determine conditions favorable for the preparation of multiple emulsions, as shown in Figure 3.1. With polarized microscopy, the authors detected diffuse birefringence in multiple emul-... 

Much has been done in developing new and advanced methods of preparation of multiple emulsions. One of the most impressive technologies is the use of specific separation membranes. 

The next chapter by G. T. Vladisavljevic and R. A. Williams is a comprehensive and systematic review of new techniques of preparation of multiple emulsions, emulsions, and microparticles. The authors envisage the ways to form multiple droplets by using membrane emulsification processes and microchan-nel and microcapUlary devices. They also pay special attention to the preparation of solid microparticles via a double emulsion emulsification method using membrane emulsification and microfiuidic devices. 

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