Preparation and Stability of Multiple Emulsions

In principle, multiple emulsions can be prepared by any of the many methods for the preparation of conventional emulsion systems, including sonication, agitation, and phase inversion. Great care must be exercised in the preparation of the final system, however, because vigorous treatments normally employed for the preparation of primary emulsions will often break that system if used in secondary emulsion formation, resulting in loss of the identity of the primary phase. 

FIGURE 11.14. The preparation of a multiple emulsion involves two steps (1) the formation of the primary emulsion with a surfactant more soluble in the first continuous phase and a high-energy emulsification process and (2) the use of a second surfactant more soluble in the second continuous phase and a low energy emulsification process. 

Stabilization of such wi/o systems. Generally, that involves the use of an oil soluble surfactant with a low HLB (2-8). The primary emulsion will then be emulsified in a second aqueous solution containing a second surfactant system appropriate for the stabihzation of the secondary 0/W2 emulsion (HLB 6-16). 

As noted above, because of the possible instabihty of the primary emulsion, great care must be taken in the choice of the secondary dispersion method. Excessive mechanical agitation such as in colloid mills, high-speed mixers, and sonication could result in coalescence of the primary emulsion and the production of a simple emulsion. The evaluation of the yield of filled secondary emulsion drops, therefore, is very important in assessing the value of different preparation methods and surfactant combinations. 

The nature of the droplets in a multiple emulsion will depend on the size and stability of the primary emulsion. Three main classes of droplets have been suggested for w/o/w emulsions, based on the nature of the oil-phase 

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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. 

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