Primary Emulsion Breakdown

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. 

Finally, the presence of surfactant always raises the possibility of micelle formation in the primary continuous phase and the subsequent solubilization of the primary dispersed phase. Solubilization, therefore, represents a convenient mechanism for the transport of primary emulsion components to the secondary continuous phase. Such a solubilization process also represents a convenient mechanism for the transport of material. In the context of a critical application such as controlled drug delivery, in which the mechanism of delivery is diffusion-controlled, such breakdown mechanisms would be very detrimental to the action of the system since they could result in a rapid release of active solute with possibly dangerous effects. 

These natural or spontaneous mechanisms of emulsion breakdown, as well as others, must be addressed in the formulation stage in order to understand and control a particular multiple emulsion system of interest. In all cases, the final stability of the system will depend on the nature of the oil phase of interest, the characteristics of the primary and secondary emulsifier systems, and the relationship between the internal and continuous phases. 

As with simple emulsions, multiple emulsions will also be sensitive to breaking due to physical abuse. Shear and shock sensitivity must always be considered. In some applications, however, such sensitivity may be advantageous. In a cosmetic skin cream, for example, the shear imposed on the system 

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Storage conditions should accurately reflect the conditions encountered in the actual production/storage process. The primary parameter to control is temperature. If the container is not properly seeded, other parameters such as relative humidity, air pressure, and air composition may affect the rate of emulsion breakdown. 

Figure 9.17. Some possible pathways for the breakdown of multiple emulsions (a) coalescence of secondary emulsion drops (b) coalescence of primary emulsion drops (c) loss of primary emulsion dispersed phase to external phase.

Florence and Whitehill [14] distinguished between three types of multiple emulsions, (W/O/W) that were prepared using isopropyl myristate as the oil phase, 5% Span 80 to prepare the primary W/O emulsion and various surfactants to prepare the secondary emulsion (see Chapter 12 for details). A schematic representation of some breakdown pathways that may occur in W/O/W multiple emulsions is shown in Figure 13.26. 

Florence and Whitehill [38] distinguished between three types of multiple emulsions (W/O/W) that were prepared using isopropyl myristate as the oil phase, 5 % Span 80 to prepare the primary W/0 emulsion and various surfactants to prepare the secondary emulsion (a) Brij 30 (polyoxyethylene 4 Lauryl ether) 2%. (b) Triton X-165 (polyoxyethylene 16.5 nonyl phenyl ether (2%). (c) 3 1 Span 80 Tween 80 mixtures. A schematic picture of the three structures is shown in Fig. 1.34. The most common structure is that represented by (b) whereby the large size multiple emulsion droplets (10-100 pm) contain water droplets 1 pm. A schematic representation of some breakdown pathways that may occur in W/O/W multiple emulsions is shown in Fig. 1.35. 

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