Mass transport of substrate

As depicted in Figure 2.8, mass transport of substrate from the bulk water phase takes place through a fluid boundary layer (liquid film) and into a biofilm followed by a combined diffusion and utilization of the substrate in the biofilm. 

The principle of the mass transport of substrates/nutrients into the immobilized enzyme/cells, through a solid, porous layer (membrane, biofilm) or through a gel layer of enzyme/cells is the same. The structure, the thickness of this mass-transport layer can be very different, thus, the mass-transport parameters, namely diffusion... 

Kinetics of Immobilized Enzymes. Another major factor in the performance of immobilized enzymes is the effect of the matrix on mass transport of substrates and products. Hindered access to the active site of an immobilized enzyme can affect the kinetic parameters in several ways. The effective concentration of substrates and products is also affected by the chemistry of the matrix especially with regard to the respective partition coefficients between the bulk solution and the matrix. In order to understand the effects of immobilization upon the rate of an enzyme-catalyzed reaction one must first consider the relationship between the velocity of an enzyme-catalyzed reaction and the... 

Any immobilization method applied must guarantee as far as possible that the activity of the biomolecule is maintained, that the accessibility of the substrate to the active sites of the bound biomolecules is not sterically hindered, and that mass transport of substrates and products through the enzyme layer is possible. It should also be ensured that the biological com-... 

To understand the interplay of enzyme catalysis and mass transfer within polymer film, it is essential to develop models that take account of these effects, then compare the models predictions with experiment. Fig. 9.13 illustrates the physicochemical processes involved in the enzymic turnover of substrate to product within a polymer film. Such processes include mass transport of substrate and product either to or from the film, partition of these species across the polymer-solution interface, transport of reactants and products within the film (by diffusion), and electrochemical reaction with enzymic products at the electrode surface. Effects of migration of charged species within the film are usually ignored. 

The high porosity may also be advantageous when cells are kept within such gels, since it gives good mass transport of substrates to the cells and waste products from the cells. This can apply to cell separatimi, ceU adsorption, and even cell immobilization. A special aspect of the latter is ceU culture for use in tissue engineering. 

When enzymes are immobilised within smart hydrogels, then cyclic changes in external stimuli can lead to on-off activity of the enzyme due to the cyclic collapse and re-swelling of the hydrogel pores. This action can also be used to enhance mass transport of substrate into and product out of the immobilised enzyme hydrogels. 

Catalytic voltammetry reports on the intrinsic properties of the enzyme when the response is free from hmitation by (a) relatively slow rate of interfacial electron transfer and (b) mass transport of substrate to the adsorbed enzyme from the bulk of the electrolyte solution. The latter is... 

The Vinax and effective diffusion eoefficient (Dgff) were both found to be higher for enzyme immobilization via direct adsorption than for immobilization with poly-pyrrole, indicating that the codeposition of enzymes in the pyrrole monomer created an architecture that hindered enzyme loading or mass transport of substrate. Further... 

The response of the immobilized enzyme electrode can be made independent of the enzyme concentration by using a large excess of enzyme at the electrode surface. The electrode response is limited by the mass transport of the substrate. Using an excess of enzyme often results in longer electrode lifetimes, increased linear range, reduced susceptibiUty to pH, temperature, and interfering species (58,59). At low enzyme concentrations the electrode response is governed by the kinetics of the enzyme reaction. 

In general, the substrate temperature will remain unchanged, while pressure, power, and gas flow rates have to be adjusted so that the plasma chemistry is not affected significantly. Grill [117] conceptualizes plasma processing as two consecutive processes the formation of reactive species, and the mass transport of these species to surfaces to be processed. If the dissociation of precursor molecules can be described by a single electron collision process, the electron impact reaction rates depend only on the ratio of electric field to pressure, E/p, because the electron temperature is determined mainly by this ratio. 

Mass transport of precursors through the boundary layer to the growth surface (3), on the substrate. 

This complex easily looses CO, which enables co-ordination of a molecule of alkene. As a result the complexes with bulky phosphite ligands are very reactive towards otherwise unreactive substrates such as internal or 2,2-dialkyl 1-alkenes. The rate of reaction reaches the same values as those found with the triphenylphosphine catalysts for monosubstituted 1-alkenes, i.e. up to 15,000 mol of product per mol of rhodium complex per hour at 90 °C and 10-30 bar. When 1-alkenes are subjected to hydroformylation with these monodentate bulky phosphite catalysts an extremely rapid hydroformylation takes place with turnover frequencies up to 170,000 mole of product per mol of rhodium per hour [65], A moderate linearity of 65% can be achieved. Due to the very fast consumption of CO the mass transport of CO can become rate determining and thus hydroformylation slows down or stops. The low CO concentration also results in highly unsaturated rhodium complexes giving a rapid isomerisation of terminal to internal alkenes. In the extreme situation this means that it makes no difference whether we start from terminal or internal alkenes. 

Derived methods. A mercury-film electrode (MFE) is superior to an HMDE because stirring of the solution can be performed much more vigorously, thereby enhancing the efficiency of the mass transport of analyte to the electrode. Faster stirring is allowed because there is no longer the chance that the mercury drop will be displaced and thus lost by the solution s movement. The usual substrate employed for the mercury film is graphite. ... 

Mass transport of the initial reactants and reaction products to the substrate surface... 

Two different variants of the electrocatalytic process are analyzed here. The first one corresponds to first-order conditions and in this case one-electron and two-electron charge transfers coupled to the chemical reaction are discussed under SWV and Voltcoulometry conditions [19, 83, 95-97], After that, a second-order catalytic scheme is presented in which the mass transport of the substrate of the chemical reaction is considered [98, 99]. 

A radial-flow-converter substrate design has recently been introduced by Bosal. This is based on improving the mass transport of reactants to the walls by increasing cell density to as high as 1,600 cells inch, as has been shown by others [28]. Bosal seeks... 

Because a gradient in surface tension induces a net mass transport of liquids, as mentioned in Section 2.2, it was anticipated that spatially controlled photoirradiation of a CRA-CM-modified substrate results in differences in surface energies so that the motion of a liquid droplet can be guided by light. 

Mass transport of reactant gaseous species to vicinity of substrate ... 

To understand the temperature dependence of the growth rate Grove [44] proposed a simple model as shown in Figure 4.20. In this model it is assumed that the mass transport of the reactant gaseous species across the boundary layer only depends on the mass diffusion. As a result there is a concentration gradient of the gaseous species. The flux (Fj) of mass transport from the gas phase to the substrate surface is written as [45]... 

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