Nanodispersion formation, extracting emulsions

In Chapter 6, characteristic features of emulsion liquid membrane systems are examined by Yurtov and Koroleva. The effects of surfactant and carrier concentrations and external and internal phase compositions upon the properties of the extracting emulsions are discussed. Several mathematical models for the rheological curves are considered, and regions of applicability for the models are evaluated. An influence of nanodispersion formation on mass transfer through the interface and on the properties of extracting emulsions for cholesterol is demonstrated. 

One of the most important phenomena which may occur at the interface in an extracting emulsion is formation of small particles on the order of 10-50 nm (i.e, a nanodispersion). Formation of a nanodispersion is promoted by the presence of a surfactant and a co-surfactant in the system (5) and by disruption of the equilibrium between the phases of the extracting emulsion (Le. by mass transfer of components of the phases and the solvents through the interface). When microemulsifiers are included in the emulsion to decrease interfacial tension, nanodispersion formation result. The microemulsifiers can diffuse through the interface of the emulsion which results in interface instability and nanodispersion formation. The microemulsifiers can be co-surfactants, such as ethanol and (Uethyl ether (6,7). 

Photomicrographs of extracting emulsions were conducted with Pt/C replicas, which were obtained by the freeze-cleavage method. The data indicate an absence of the nanodispersion in the initial emulsion before extraction vsdth formation of 17-25 nm droplets during the course of extraction by the emulsion (7). 

Appropriate concentrations of microemulsifiers are necessary in an extracting emulsion to lower the interfacial tension. Figure 9 compares the dependence of the coefficient for cholesterol extraction by an emulsion (curve 1) and the interfacial tension in a two-phase, non-emulsion system (curves 2-4) on the ethanol to diethyl ether molar ratio. There is a maximum on the cholesterol extraction curve which corresponds to minima in the interfacial tension curves. At this composition the greatest interfacial instability due to diffusion of ethanol and diethyl ether takes place. Consequently the nanodispersion formation is the greatest at the same composition for which the cholesterol extraction coefficient has a maximal value. 

Special conditions for pre-treatment of the initial emulsion to enhance nanodispersion formation have been developed. An additional amount of ethanol (10-25 %) is first incorporated into the internal phase of the extracting emulsion. The resulting emulsion is contacted with an aqueous buffer solution of pH equal to that of the internal aqueous phase and then with doubly distilled water or an aqueous buffer at pH 7.4. During this process some of the ethanol diffused from the internal phase of the emulsion into the external aqueous solution which stimulates nanodispersion formation. 

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