Design of Selective Layers

This may work well if the process involves only electrically neutral species. However, when ions are discriminated on the basis of size, the partitioning process is affected by the Donnan potential. This potential, which we discuss more fully in Chapter 6, develops at the membrane/electrolyte interface. Another possibility is to discriminate on the basis of charge, as shown in Fig. 7.10 (see Chapter 7). Again, a porous barrier membrane is used, although here it would contain fixed, electrically charged moieties. When placed in front of the transducer, it rejects the like-charged species by electrostatic repulsion. In other words, it is a form of ion exchange membrane. 

The selection, preparation, and properties of a selective layer depend largely on the type of transducer at which they will be used, as well as on the application. Those aspects are discussed in the context of the individual transduction principles. Only certain common features and procedures are included in this section. 

Vacuum deposition techniques, such as sputtering, electron beam evaporation, and plasma deposition are common. Photopolymerization and laser-assisted depositions are used for preparation of specialized layers, particularly in the fabrication of sensing arrays. Most commercial instruments have thickness monitors (Chapter 4) that allow precise control of the deposition process. 

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Linear Energy Solubility Relationships (LSERs) are useful in the design of selective layers for mass and optical gas sensors. 

Two product barrier layers are formed and the continuation of reaction requires that A is transported across CB and C across AD, assuming that the (usually smaller) cations are the mobile species. The interface reactions involved and the mechanisms of ion migration are similar to those already described for other systems. (It is also possible that solid solutions will be formed.) As Welch [111] has pointed out, reaction between solids, however complex they may be, can (usually) be resolved into a series of interactions between two phases. In complicated processes an increased number of phases, interfaces, and migrant entities must be characterized and this requires an appropriate increase in the number of variables measured, with all the attendant difficulties and limitations. However, the careful selection of components of the reactant mixture (e.g. the use of a common ion) or the imaginative design of reactant disposition can sometimes result in a significant simplification of the problems of interpretation, as is seen in some of the examples cited below. 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

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