Membrane methods steady layers

In the filtration-type methods (the first three techniques listed above), components accumulate as a steady-state (polarization) layer at a barrier or membrane [4] this occurs in much the same way as in field-flow fractionation or equilibrium sedimentation. However, there are several complications. First, fresh solute is constantly brought into the layer by the flow of liquid toward and through the filter. This steady influx of solute components can be described by a finite flux density term J0. Second, components can be removed from the outer reaches of the layer by stirring. Third, the membrane or barrier may be leaky and thus allow the transmission of a portion of the solute, profoundly affecting the attempted separation. In fact, one reason for our interest in layer structure is that leakiness depends on the magnitude of the solute buildup at the membrane surface. As solute concentration at the surface increases, more solute partitions into the membrane and is carried on through by flow. 

Volumetric steady-state gas permeation tests at elevated temperatures are typically used to characterize the performance of paUadium membranes. Electrochemical methods are also effective, even at high temperatures [90, 166]. Permeation through palladium depends on a solution-diffusion mechanism, including the steps of chemisorption and dissociation into atoms, absorption into the metal, diffusion through the metal lattice, transfer from the bulk metal to the opposite side, and recombination into molecules for desorption [167, 168]. Difiusion of molecular hydrogen through boundary layers adjacent to the surface is also necessary. 

This first of several [ilarined volumes discusses rume uilihrium tliermodynamics tuid kinetics, particularly eii/ymc catalysis, for processes and systems in the steady state. Methods of mathematical modeling by means oi network simulations are also treated, since t iey serve to assess the tran sient behavior of a system on its way to a steady state. Water as a ubiquitous constituent plays an essential role in biodectrodiemical systems, hence its structure is carefully evaluated, both in the fmre state and in the ionic hydration shell. Similarly, the interface between water and a membranous or biocolluidal phase is uf major im[>ortance. The phenomena occiir-rinj at such interfaces, including diffuse double layers, as well as binding and adsorption of solutes, are extensively examined. 

The ratio e /Ds determines the response time of the biosensor because it expresses the time taken for the enzymatic layer to reach the steady state. This response time can be reduced by modifying the membrane so that it is extremely permeable to the substrate (increase Ds) or very thin (decrease e). Reduction in enzymatic membrane thickness is the most effective method because this value is squared in the expression for the response time. In reality, an excessive decrease in the thickness of the membrane will also affect its mechanical properties. Moreover, the response time of the biosensor can never be less than the response time of the transducer. 

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