Subject liquid membranes

Liquid membranes containing carriers to facilitate selective transport of gases or ions were the subject of a considerable research effort in the 1970s and 1980s. 

As described above, the ion transfer through a membrane is controlled practically by the complementary ion transfer reactions at two W/M interfaces when the M contained sufficient electrolytes. This idea was successfully applied to explanations of the following subjects concerning with membrane phenomena [20,21,26]. (1) Influence of ion transfer reaction at one W/M interface on that at another W/M interface under an applied membrane potential (2) Ion transfers through an M in the presence of the objective ion in Wl, M and/or W2 (3) Ion separation by electrolysis under an applied membrane potential (4) Ion transfer through a thin supported liquid membrane. The idea was also demonstrated to be very useful for the elucidation of ion or electron transport process through a bilayer lipid membrane (BLM), which is much thinner than a liquid membrane [21,26]. 

General properties of liquid membrane systems have been a subject of extensive theoretical studies. Six basic mechanisms of transport are schematically shown in Figure 13.2. In a simple transport (Figure 13.2a), solute passes through due to its solubihty... 

The other important subject discussed in this chapter is ionic equilibria at membranes. The focus of this discussion was the membranes involved in ion-selective electrodes. A variety of systems were considered, including both solid-state and liquid membranes. The study of ionic equilibria at membranes is very important for biological systems. Although biological membranes are often more complex structurally, the basic phenomena described here apply there as well. 

Liquid membrane electrodes are subject to interferences from ions other than that of prime selectivity. For example, the Ca-ISE is also responsive to Mg2+ and Ba2+, the selectivity coefficients being approximately 0.01 for each ion. This indicates that the electrode is only 100 times more sensitive to Ca than to these ions, and this is normally much more important with respect to Mg than to Ba where food analysis is concerned. There are techniques which can be used to minimise the interference of Mg2+. 

Liquid membrane technology Is Introduced and Is Identified as a subset of membrane science. A tutorial section discusses configurations, transport mechanisms, experimental techniques, and a survey of basic theoretical approaches. The concepts of reactive liquid membranes which combine traditional unit operations such as extraction or absorption with stripping are discussed. The chapters to follow in this volume are summarized and the subject of each la placed In perspective to the field of liquid membrane technology. 

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. 

This overview chapter has the objective of introducing the SyiQ>oslum Series volume and the subject of liquid membrane technology. If membranes are viewed as semi-permeable phase separators, then the traditional concept of membranes as polymer films can be extended to Include liquids and liquid-swollen polymers. The addition of a mobile complexatlon agent to the membrane Is known as facilitated liquid membrane separation. Often, In liquid phase facilitated transport systems, the solute flux Is coupled to the opposite flux of another species. This process, common in metal ion recovery schemes, is known as coupled transport. 

Facilitated transport of gases has been the subject of numerous Investigations which are summarized in recent review articles (2 2). Immobilized liquid membranes (ILMs) were prepared for the majority of these studies by Impregnating the pore structure of very thin, microporous polymeric substrates with a solution of a solvent and a complexatlon agent (). Such ILMs have two primary experimental problems loss of solvent phase and loss or deactivation of the com-... 

The different uses of liquid-liquid extraction, liquid membranes, and solvent impregnated materials make the subject important for university students of chemistry, metallurgy, hydrometallurgy, and chemical and mineral processing technology. Some universities offer special courses on separation processes in which those techniques are minor topics in more comprehensive courses. Laboratory experiments on liquid-liquid and liquid membranes are common in chemical and mineral processing engineering curricula. Because of the breadth of the subject, the treatment in such courses is often scarce, and more comprehensive text is difficult to find in a form suitable for use directly with students. 

The basic liquid membrane consists of some liquid ion-exchange resin which is restrained by an inert support. It suffers from several faults. The liquid-liquid interface is poorly defined and is subject to stirring effects and pressure differentials. It is mechanically fragile which can lead to mutual contamination of the two liquid phases. 

There are a few points to be kept in mind. The liquid membrane must be nonvolatile since it is subjected to vacuum on the permeate side and can evaporate on the feed side and be lost to the feed solution (Figure 8.1.49(d)). Further, wetting of the porous membrane structure spontaneously by the membrane liquid present outside requires that the surface tension of the membrane liquid should be equal to or lower than the critical surface tension of the polymeric or ceramic substrate being employed. For example, y,. for polypropylene (PP) is -33 dyne/cm. Many organic liquids will spontaneously wet it. However, y,. values for various fluoropolymers (FPs) are usually <25... 

Cagnoni et al. synthesized cyclopolymers of crown ethers containing of 5 and 7 oxygen atoms (Fig. 9) and used them as ion carriers in transport of alkali metal cations across liquid membranes [7]. The monomers were subjected to free radical polymerization, either thermally or photochemically induced. 

Specific-Ion Electrodes In addition to the pH glass electrode specific for hydrogen ions, a number of electrodes that are selective for the measurement of other ions have been developed. This selectivity is obtained through the composition of the electrode membrane (glass, polymer, or liquid-liquid) and the composition of the elec trode. Tbese electrodes are subject to interference from other ions, and the response is a function of the total ionic strength of the solution. However, electrodes have been designed to be highly selective for specific ions, and when properly used, these provide valuable process measurements. 

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