Separators ion exchange membranes

Besides separation, ion exchange membranes have other interesting characteristics ion conductivity, hydrophilicity, and fixed carriers. Researchers watch closely to find new applications (sensors and actuators) in addition to separation membranes, by considering these special characteristics. 

Separation of the anode and cathode products in diaphragm cells is achieved by using asbestos [1332-21 -4] or polymer-modified asbestos composite, or Polyramix deposited on a foraminous cathode. In membrane cells, on the other hand, an ion-exchange membrane is used as a separator. Anolyte—catholyte separation is realized in the diaphragm and membrane cells using separators and ion-exchange membranes, respectively. The mercury cells contain no diaphragm the mercury [7439-97-6] itself acts as a separator. 

A newer technology for the manufacture of chromic acid uses ion-exchange (qv) membranes, similar to those used in the production of chlorine and caustic soda from brine (76) (see Alkali and cm ORiNE products Chemicals frombrine Mep rane technology). Sodium dichromate crystals obtained from the carbon dioxide option of Figure 2 are redissolved and sent to the anolyte compartment of the electrolytic ceU. Water is loaded into the catholyte compartment, and the ion-exchange membrane separates the catholyte from the anolyte (see Electrochemical processing). 

Process Description lectrodialysls (ED) is a membrane separation process in which ionic species are separated from water, macrosolutes, and all uncharged solutes. Ions are induced to move by an electrical potential, and separation is facilitated by ion-exchange membranes. Membranes are highly selective, passing either anions or cations andveiy little else. The principle of ED is shown in Fig. 22-56. 

By the time the next overview of electrical properties of polymers was published (Blythe 1979), besides a detailed treatment of dielectric properties it included a chapter on conduction, both ionic and electronic. To take ionic conduction first, ion-exchange membranes as separation tools for electrolytes go back a long way historically, to the beginning of the twentieth century a polymeric membrane semipermeable to ions was first used in 1950 for the desalination of water (Jusa and McRae 1950). This kind of membrane is surveyed in detail by Strathmann (1994). Much more recently, highly developed polymeric membranes began to be used as electrolytes for experimental rechargeable batteries and, with particular success, for fuel cells. This important use is further discussed in Chapter 11. 

The separator is frequently a sintered glass frit, but it may also be any of a wide range of inert, porous materials such as celloton, vycor or porvic or an ion exchange membrane. A number of stable ion exchange membranes suitable for use in aqueous and non-aqueous solvents have become available in recent years. 

Metal nanotube membranes with electrochemically suitable ion-transport selectivity, which can be reversibly switched between cation-permeable and anion-permselective states, have been reported. These membranes can be viewed as universal ion-exchange membranes. Gold nanotube molecular filtration membranes have been made for the separation of small molecules (< 400 Da) on the basis of molecular size, eg. separation of pyridine from quinine (Jirage and Martin, 1999). 

Raistrick ID. 1986. In Van Zee JW, White RE, Kinoshita K, et al., eds. Diaphragms, Separators and Ion Exchange Membranes. Pennington The Electrochemical Society Softbound Proceedings Series. PV 86-13. p. 172. 

The search for models of biological membranes led to the formation of a separate branch of electrochemistry, i.e. membrane electrochemistry. The most important results obtained in this field include the theory and application of ion-exchanger membranes and the discovery of ion-selective electrodes (including glass electrodes) and bilayer lipid membranes. 

Consider the system shown in Fig. 6.3. The ion-exchanger membrane separates solutions of a single, completely dissociated, uni-univalent electrolyte. Two pistons can be employed to form a pressure difference between the two compartments. The two electrodes W and W2 are... 

Electrodialysis is a process for the separation of an electrolyte from the solvent and is used, for example, in desalination. This process occurs in a system with at least three compartments (in practice, a large number is often used). The terminal compartments contain the electrodes and the middle compartment is separated from the terminal compartments by ion-exchanger membranes, of which one membrane (1) is preferentially permeable for the cations and the other one (2) for the anions. Such a situation occurs when the concentration of the electrolyte in the compartments is less than the concentration of bonded ionic groups in the membrane. During current flow in the direction from membrane 1 to membrane 2, cations pass through membrane 1 in the same direction and anions pass through membrane 2 in the opposite direction. In order for the electrolyte to be accumulated in the central compartment, i.e. between membranes 1 and 2 (it is assumed for simplicity that a uni-univalent electrolyte is involved), the relative flux of the cations with respect to the flux of the solvent, /D +, and the relative flux of the anions with respect to... 

Meares, P. (Ed.), Membrane Separation Processes, Elsevier, Amsterdam, 1976. Meares, P., J. F. Thain, and D. G. Dawson, Transport across ion-exchange membranes The frictional model of transport, in Membranes—A Series of Advances (Ed. G. Eisenman), Vol. 1, p. 55, M. Dekker, New York, 1972. Schlogl, R., see page 415. 

Fig. 5. Exploded view of an ion-exchange membrane electrochemical oxygen separator. Oxygen removal characteristics of the flow-through type oxygen removal system are shown. Air cathode area = 100 cm2, water temperature = 40 °C.

Fluorinated polymers, especially polytetrafluoroethylene (PTFE) and copolymers of tetrafluoroethylene (TFE) with hexafluoropropylene (HFP) and perfluorinated alkyl vinyl ethers (PFAVE) as well as other fluorine-containing polymers are well known as materials with unique inertness. However, fluorinated polymers with functional groups are of much more interest because they combine the merits of pefluorinated materials and functional polymers (the terms functional monomer/ polymer will be used in this chapter to mean monomer/polymer containing functional groups, respectively). Such materials can be used, e.g., as ion exchange membranes for chlorine-alkali and fuel cells, gas separation membranes, solid polymeric superacid catalysts and polymeric reagents for various organic reactions, and chemical sensors. Of course, fully fluorinated materials are exceptionally inert, but at the same time are the most complicated to produce. 

Grafting of functional monomers onto fluoropolymers produced a wide variety of permselective membranes. Grafting of styrene (with the following sulfonation), (meth)acrylic acids, 4-vinylpyridine, A-vinylpyrrolidone onto PTFE films gave membranes for reverse omosis,32-34 ion-exchange membrane,35-39 membranes for separating water from organic solvents by pervaporation,49-42 as well as other kinds of valuable membranes. 

Solid Polymer Electrolyte Technology Ion-exchange membranes, often used as cell separators (see Sect. 2.4.3.2),... 

Ion-exchange membranes are interesting as cell separators that are selective for cations or anions. They show a high conductivity, at least in aqueous solutions. But they are not simple separators due to their special characteristics, which have to be discussed here. 

An ion-exchange membrane consists of an ionomer, which contains fixed ions that are covalently bound to the polymer backbone. It is electrically neutral because of included counterions . If water-or probably another sufficiently polar solvent - is absorbed and if the fixed and counterions can be separately solvated to an adequate degree, the counterions become mobile and the ion-exchange membrane can work as an ion conductor. Owing to the electric field of the fixed ions coions with the same charge as the fixed ions are rejected and are typically absent inside the membrane. Thus the membrane is selective for the transfer of counterions ( permselectivity = permeation selectivity, e.g. [70]). 

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