Extraction, emulsions

While benzene is the most satisfactory solvent for this extraction, emulsions are sometimes produced when it is used for the extraction of other mercaptans. To obviate this difficulty the same amount of ether may be used, provided that the alcohol is first removed by distillation on a steam bath. 

An emulsion is a colloidal suspension of one liquid in another. Minute droplets of an organic solvent are often held in suspension in an aqueous solution when the two are mixed or shaken vigorously these droplets form an emulsion. This is especially true if any gummy or viscous material was present in the solution. Emulsions are often encountered in performing extractions. Emulsions may require a long time to separate into two layers and are a nuisance to the organic chemist. 

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. 

To be effective an extracting emulsion must meet a number of requirements. The emulsion must be stable during the period of application. For such stability... 

Effect of Carrier Concentration. The carrier (extractant) also exerts a significant influence on the stability of an extracting emulsion. As a rule, the carrier will be surface-active which will reduce the stability of the emulsion due to competitive adsorption at the interface. However for ELM systems, a high concentration of carrier in the liquid membrane is usually not necessary. For each carrier, the optimal concentration will be determined by the opposing influences of the carrier on the rate of extraction of the target substance and the stability of the extracting emulsion. 

Effect of External Aqueous Phase Composition. The properties of the extracting emulsion may be varied by altering the composition of the external, continuous phase which influences its viscosity and polarity. An increase in viscosity can provide a certain kinetic stability for the emulsion as a/result of the decrease in rate at which the thickness of a liquid film between two surfaces diminishes (i). This factor can play a significant role in reverse emulsions (ie. oil-in-water-in-oil emulsions). 

Effect of Internal Phase Composition. In addition to the factors mentioned above, the stability of the extracting emulsion is influenced significantly by the composition of the internal phase. Among the factors which play the greatest role in determining the stability are the pH and ionic strength of the internal phase and the presence of organic substances (7). 

Transmembrane transfer of water from the external (continuous) phase into the internal (encapsulated) phase (7c. swelling of the emulsion) is an undesirable process. Some of the primary factors which determine the rate of water transfer are the type and concentration of surfactant in the liquid membrane. The direction of the transmembrane transfer of water in an extracting emulsion is determined by the sign of the water activity gradients. 

Swelling of the emulsion usually does not react equilibrium. Due to the large difference between the osmotic pressures of the external and internal aqueous solutions, the swelling of extracting emulsions may be considerable. Therefore optimization of the residence time of the extracting emulsion in the apparatus to concentrate the target substance to the maximum extent is important. 

Information about the rheological properties of extracting emulsions and their fluidity enables one to optimize the processes of preparation of the extracting emulsion, transportation of the liquid dispersions, and reduction of energy consiimption at the various stages of their handling. 

Study of the flow curves shows that emulsions with a low content of dispersed phase (< )< 0.1) appear to be Newtonian fluids. As the dispersed phase content increases (< ) > 0.1), the extracting emulsion becomes a non-Newtonian, pseudo-plastic liquid. 

For the investigated emulsions, i oo tio Under this condition, the Peeck-Mak-Lean-Williamson equation transforms into the Ferry equation and, therefore, is not suitable for description of the viscosity of extracting emulsions. In the Meter equation, the term (P/Pav) ( oo/T o) approaches 1 at Tjoo i1o- Thus, Equation 8 is transformed into the Ellis equation. Values of P1/2 and the exponential coefficient A for the Ellis model are presented in Figure 8. It should be noted that the value for A is constant and equal to 6 in the equation which describes the rheological curves of the extracting emulsions for the indicated range of dispersed phase content. 

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. 

Since the capacity of the extracting emulsion increases as a consequence of the increase in degree of adsorption of the target substance (cholesterol, in this example)... 

Differing amounts of surfactant can be used for stabilization of the extracting emulsion ... 

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. 

Table II compares the cholesterol extraction properties and stabilities of emulsions prepared with and without the pre-treatment. These extracting emulsions have the same overall chemical compositions, but different structures. For the untreated emulsion, the nanodispersion forms during cholesterol extraction, and in the...

Table II. Comparison of Properties of Extracting Emulsions Prepared With and Without Pre-Treatment ...

A simplified flow sheet for the LEM continuous circuit that was used in the field tests is shown in Figure 1. The technique used four main unit operations (emulsion generation, copper extraction, emulsion breaking, and metal recoveiy). 

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