Membrane chemically modified

Tetraethylene glycol may be used direcdy as a plasticizer or modified by esterification with fatty acids to produce plasticizers (qv). Tetraethylene glycol is used directly to plasticize separation membranes, such as siHcone mbber, poly(vinyl acetate), and ceUulose triacetate. Ceramic materials utilize tetraethylene glycol as plasticizing agents in resistant refractory plastics and molded ceramics. It is also employed to improve the physical properties of cyanoacrylate and polyacrylonitrile adhesives, and is chemically modified to form polyisocyanate, polymethacrylate, and to contain siHcone compounds used for adhesives. 

In most cases, hoUow fibers are used as cylindrical membranes that permit selective exchange of materials across their waUs. However, they can also be used as containers to effect the controUed release of a specific material (2), or as reactors to chemically modify a permeate as it diffuses through a chemically activated hoUow-fiber waU, eg, loaded with immobilized enzyme (see Enzyme applications). 

The new edition of Principles of Electrochemistry has been considerably extended by a number of new sections, particularly dealing with electrochemical material science (ion and electron conducting polymers, chemically modified electrodes), photoelectrochemistry, stochastic processes, new aspects of ion transfer across biological membranes, biosensors, etc. In view of this extension of the book we asked Dr Ladislav Kavan (the author of the section on non-electrochemical methods in the first edition) to contribute as a co-author discussing many of these topics. On the other hand it has been necessary to become less concerned with some of the classical topics the details of which are of limited importance for the reader. 

Song, M. K., Kim, Y. T., Fenton, J. M., Kunz, H. R. and Rhee, H. W. 2003. Chemically modified Nafion (R)/poly(vinylidene fluoride) blend ionomers for proton exchange membrane fuel cells. Journal of Power Sources 117 14-21. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Over 30 commercial formulations have been surveyed in depth. Compressive strength measurements permit the exclusion of materials obviously prone to fail under pressure. FTIR (MX-1, Nicolet Instrument Corp.) analysis has identified formulations with volatile diluents capable of chemically modifying the composite membrane. Through the use of FTIR it was possible with an otherwise successful formulation to identify the presence of butyl glycidyl ether (BGE) as a diluent. Subsequently experimentation showed that vapor of BGE is capable of plasticizing porous polysulfone with a drop in both flux and rejection of the membrane. Collaboration with the supplier resulted in substitution of a nonvolatile glycidyl ether diluent to avoid the problem. 

A biological compound (an enzyme, usually) intended to improve the response of an electrode can be incorporated into it in two ways, namely (a) by altering the sensing surface in order to accommodate the biocatalysts [i.e. by constructing a (bio)chemically modified electrode] and (h) by using a membrane place in front of the surface electrode in order to trap the enzyme. The enzyme can be used in isolation (most often in a commercially available form) or be part of a tissue material or bacterial cells. 

Ever since an ISFET that was chemically modified by a valinomycin-containing PVC membrane was reported [141], there has been general consensus on the advantages of this type of microsensor over conventional ISEs. Some serious problems have also been acknowledged, though e.g. the low mechanical stability of the membranes, the interference of COj in the potentiometric response, the lack of a stable micro-reference electrode and the relatively high drift rate of ISFETs). Attachment of the membrane can... 

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

The fuel cell in Figure 13.9 can be conceptually viewed as a combination of a Nafion film-coated cathode and a Nafion film-coated anode. Hence, the fuel cell is, in essence, a combination of two chemically modified electrodes. This idea is, in fact, more than just a concept, because electrochemical investigations of Nafion film-coated electrodes have been used to obtain fundamental chemical and electrochemical information that is relevant to the operation of such devices [93]. For example, the kinetics of 02 reduction in fuel cells can be investigated at such modified electrodes the solubility and diffusion coefficient for 02 in Nafion and the proton conductivity of this membrane material can also be determined. Chemically modified electrodes have made analogous contributions to battery development. 

Recently several groups have tried to improve the properties of anisotropic gas separation membranes by chemically modifying the surface selective layer. For example, Langsam at Air Products and Paul et al. at the University of Texas, Austin have treated films and membranes with dilute fluorine gas [66-71], In this treatment fluorine chemically reacts with the polymer structure. By careful... 

Gunasingham H, Teo P, Lai Y, Tan S. Chemically modified cellulose acetate membrane for biosensor applications. Biosensors 1989, 4, 349-359. 

Membrane attachment can be improved by chemically modifying the gate oxide to allow covalent anchoring of the membrane [42,43]. The usual method is to react the SiOH groups at the surface with a silane, such as 3-(trimethoxysi-... 

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