Entrapped materials, multiple emulsions

The nature of entrapped materials may have a bearing on the stability of the system. Due to the nature of the multiple emulsion, the middle phase may act as an osmotic reservoir, thus virtually all additions to this phase will set up osmotic gradients. This might include high concentrations of surfactant. To this end polymeric microspheres have been used as the internal reservoir when osmotic transfer of water will not compromise stability. 

The ability of multiple emulsions to entrap materials is one of their most useful assets and so the passage of materials from the... 

In most cases multiple emulsions are aimed for slow and sustained release of active matter from an internal liquid reservoir into the continuous phase. In some applications the multiple emulsions can serve also as an internal reservoir to entrap matter from the outer diluted continuous phase into the inner confined space. These applications are aimed to remove toxic matter. In other applications multiple emulsions are reservoirs for improved dissolution or solubilization of insoluble materials. The materials will dissolve in part in the inner phase, in part at the internal interface, and occasionally at the external interface. Applications related to protection of sensitive and active molecules... 

Multiple emulsions are usually not empty. Soluble active materials are entrapped during the emulsification in the inner oily phase. Because of the osmotic pressure gradient, the active matter tends to diffuse and migrate from the internal phase to the external interface mostly through a controlled reverse micellar transport mechanism (Figure 7.10a) (Garti and Bisperink, 1998 Garti and Benichou, 2001). The dilemma that researchers were faced with was how to control the diffusion of oil molecules, as well as the emulsifier molecules... 

Most of release studies are done in W/OAV multiple-emulsion systems where an active water soluble molecule is present in the inner aqueous phase. Several attempts have been made to explain the transport phenomena of entrapped addenda from the inner to the outer phase of multiple-emulsion droplets. It has been demonstrated that for lipid soluble material dissolved in the oil phase, the release obeys first-order kinetics and is diffusion controlled with excellent accordance to Pick s law. Two mechanisms for the permeation through the oil intermediate phase are well accepted, the first being via the reverse micellar transport (Figure 7.10 ) and the second via diffusion across a very thin lamellae of surfactant phase formed in areas where the oil layer is very thin (Figure 10b). 

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