Liquid emulsion membrane discussion

Many discussions of life on Titan have considered the possibility that water, normally frozen at the ambient temperature, might remain liquid following heating by impacts.22 Life in this aqueous environment would be subject to the same constraints and opportunities as life in water. Water droplets in hydrocarbon solvents are, in addition, convenient cellular compartments for evolution, as Tawfik and Griffiths have shown in the laboratory.23 An emulsion of water droplets in oil is obtainable by simple shaking. This could easily be a model for how life on Titan achieves the isolation necessary for Darwinian evolution, and it provides an interesting alternative for membranes, discussed in earlier chapters as a common feature of terran life. 

In this section, we will discuss the major characteristics of coupled transport systems. The discussion will be illustrated with results obtained with supported liquid membranes. However, the same principles apply to emulsion membranes. We use supported liquid membrane results because the geometry of supported membranes is well-defined and it is possible to maintain the conditions of the feed and product solutions constant. This allows parametric studies to be per-... 

Commercial eind laboratory applications of liquid membrane technology are discussed including gas transport, sensor development, metal ion recovery, waste treatment, biotechnology and biomedical engineering. Immobilized liquid membranes, emulsion or liquid surfactant membranes, and membrane reactors are discussed. Economic data from the literature for liquid membrane processes are presented and compared with existing processes such as solvent extraction and cryogenic distillation of air. 

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. 

As discussed by Frankemfeld and Li(28) and del Cerro and Boey(29), liquid membrane extraction 28,29) involves the transport of solutes across thin layers of liquid interposed between two otherwise miscible liquid phases. There are two types of liquid membranes, emulsion liquid membranes (ELM) and supported liquid membranes (SLM). They are conceptually similar, but substantially different in their engineering. 

All the novel separation techniques discussed in this chapter offer some advantages over conventional solvent extraction for particular types of feed, such as dilute solutions and the separation of biomolecules. Some of them, such as the emulsion liquid membrane and nondispersive solvent extraction, have been investigated at pilot plant scale and have shown good potential for industrial application. However, despite their advantages, many industries are slow to take up novel approaches to solvent extraction unless substantial economic advantages can be gained. Nevertheless, in the future it is probable that some of these techniques will be taken up at full scale in industry. 

In this paper an overview of the developments in liquid membrane extraction of cephalosporin antibiotics has been presented. The principle of reactive extraction via the so-called liquid-liquid ion exchange extraction mechanism can be exploited to develop liquid membrane processes for extraction of cephalosporin antibiotics. The mathematical models that have been used to simulate experimental data have been discussed. Emulsion liquid membrane and supported liquid membrane could provide high extraction flux for cephalosporins, but stability problems need to be fully resolved for process application. Non-dispersive extraction in hollow fib er membrane is likely to offer an attractive alternative in this respect. The applicability of the liquid membrane process has been discussed from process engineering and design considerations. 

Some safety engineers have become obsessed with fears of safety problems with hot asphalt, although very few injuries or accidents have been traceable to it over the several centuries that it has been used industrially. Asa result, there have been many efforts to substitute cold asphalt putties for hot asphalt as a membrane material. These materials and their limitations have been discussed above. As a membrane for floor installations, such putties are often usable providing they are never subject to a standing liquid head, but less frequently for trenches and pits. Remember, if asphalt emulsions are put in service before all the water has dried out of them, they can reemulsify and may be washed out. 

According to configuration definition, three groups of hquid membranes are usually considered (see Fig. 1.1) bulk (BLM), supported or immobilized (SLM or ILM), and emulsion (ELM) liquid membrane transport. Some authors add to these definitions polymeric inclusion membranes, gel membranes, dual module hollow-fiber membranes, but, to my opinion, the first two types are the modifications of the SLM and the third is the modification of BLM. It will be discussed in detail in the respective chapters. 

In addition to the traditional dermal delivery formulations discussed above, several other pharmaceutical semi-solid and liquid formulation types have been the subject of a considerable amount of R D. These include sprays, foams, multiple emulsions, microemulsions, liposomal formulations, niosomes, cyclodextrins, glycospheres, dermal membrane structures and microsponges. Although some of these formulations form part of the pharmaceutical armamentarium, they are yet to achieve widespread application and are not within the scope of this chapter. The interested reader is referred to the excellent coverage by Osborne and Amann (1990), Kreuter (1994) and Liu and Wisniewski (1997). 

Carrier Chemistry. The use of structurally modified macrocycllc polyethers (crown ethers) as CcU rlers In bulk, emulsion, and Immobilized liquid membranes Is the subject of the chapter by Bartsch et al. (111). They discuss the use of lonlzable crown ethers for the coupled transport of alkali metal cations. The lonlzable carboxylic and phosphonlc acid groups on the macrocycles eliminate the need for an anion to accompany the catlon-macrocycle complex across the liquid membrcuie or for an auxiliary complexlng agent In the receiving phase. The influence of carrier structure on the selectivity and performance of competitive alkali metal transport across several kinds of liquid membranes Is presented. 

Co-anion type and concentration are examined as parameters that can be varied to achieve various metal cation separations in macrocycle-facilitated emulsion liquid membranes. Membrane systems where the metal is present in the source phase as a complex anion or as a neutral complex (cation-anion(s)) are discussed. The experimental separations of Cd(II) from Zn(II) and/or Hg(II), Au(I) from Ag(I), and Au(III) from Pd(II) or Ag(I) are given to illustrate separation design using these membrane systems. The separations are discussed in terms of free energies of hydration, distribution coefficients, and equilibrium constants for the various interactions that occur. 

A recent study with biotechnology applications relates to amino acid extraction. Schugerl and co-workers (71 ) used a quaternary ammonium carrier in an emulsion liquid membrane system for enzyme catalyzed preparation of L-amino acids. Frankenfield et al. (72) discuss a wide variety of biomedical ELM applications including enzyme encapsulation, blood oxygenation, and treatment of chronic uremia. 

Multiple emulsions usually refer to series of complex two-phase systems that result from dispersing an emul sion into its dispersed phase. Such systems are often referred to as water-in-oil-in-water (W/OAV) or oil-in-water-in-oil (O/W/0) emulsions, depending on the type of internal, intermediate, and continuous phase. Multiple emulsions were early recognized as promising systems for many industrial applications, such as in the process of immobilization of proteins in the inner aqu eous phase (37) and as liquid membrane systems in extraction processes (38). W/O/W emulsions have been discussed in a number of technical applications, e.g., as prolonged drug-delivery systems (39-44), in the context of controlled-release formulations (45), and in pharmaceutical, cosmetic, and food (46) applications. 

Figure 24 shows other possibilities for linking up these individual critical fluid-based options into tandem processes. Here the previously discussed option is shown initially as well as the supercritical fluid extraction and chromatographic separation of phospholipids which was noted in Section 3.2.3. Also, our previously-cited example of subcritical water synthesis of fatty acids from natural oil feedstocks is noted, the end product in this case is a mixture of fatty acids contained in an aqueous emulsion. These can be separated from water via a membrane process or counter currently into supercritical or liquid carbon dioxide. Further rectification of the fatty acid mixtures would also be amenable to fractionation via the thermal gradient fractionation column mentioned previously. 

ELM systems are usually prepared by first forming an emulsion between two immiscible phases (2). When this emulsion is dispersed in a third (continuous) phase by agitation, the extraction system is produced. The membrane phase is the liquid phase that separates the encapsulated, internal droplets in the emulsion from the external, continuous phase. Although the internal, encapsulated phase and the external, continuous phase are miscible, the membrane phase cannot be miscible with either in order to be stable. In the discussion which follows, the internal and external phases are aqueous solutions which are separated by an oil phase in water-in-oil-in-water (W/O/W) emulsion systems. 

This review covers recent advances in the theory for emulsion liquid membranes (ELMs) briefly and in ELM applications in more detail. In the theory, the state-of-the-art models for two types of facilitation for ELMs are discussed. In the applications, significant advances have been made recently. Commercial applications include the removal of zinc, phenol, and cyanide fi om wastewaters and in well control fluid. Potential applications include wastewater treatment, biochemical processing, rare earth metal extraction, radioactive material removal, and nickel recovery. The ELM systems for these applications are described. 

This paper reviews the use of emulsions and microemulsions as liquid membranes with sp ial emphasis placed on the separation of mercury, as Hg(N03)2, from water using oleic acid as the extractant Although emulsion (either macro- or micro-) liquid membranes offer advantages in terms of fast rates of separation, new modes of creating a stabilized liquid membrane utilizing hollow fiber contactors offer comparable flux in a more stable format. The paper wiU start with a review of the basic types of liquid membranes as currently used in research. The discussion will then focus on the author s experience with emulsified liquid membrane systems. The last section of the paper will discuss the obvious next step in liquid membrane technology, the use of emulsion liquid membranes in hollow fiber contactors. 

The utility of liquid membranes has been discussed in several reviews and handbooks (1-2). Emulsion liquid membranes remain an actively researched separation technique for the removal of trace contaminants from aqueous streams. Liquid membranes are attractive because they combine extraction and stripping into a single step thus, equilibrium constraints of partitioning are overcome by changing the chemical... 

Carrier-facilitated transport of actinides across bulk, supported, and emulsion liquid membranes, as well as plasticized membranes and recently developed emulsion-free liquid membranes, are reviewed. The discussion includes the effects of important experimental variables upon the solute flux for various types of liquid membranes. Applications of liquid membranes in the recovery and removal of radiotoxic actinides from the nitric acid wastes generated during reprocessing of spent fuel by the PUREX process and wastes produced by other radiochemical operations are surveyed. 

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