Kinetic/factors/control/stability

Since the first report on the ferrocene mediated oxidation of glucose by GOx [69], extensive solution-phase studies have been undertaken in an attempt to elucidate the factors controlling the mediator-enzyme interaction. Although the use of solution-phase mediators is not compatible with a membraneless biocatalytic fuel cell, such studies can help elucidate the relationship between enzyme structure, mediator size, structure and mobility, and mediation thermodynamics and kinetics. For example, comprehensive studies on ferrocene and its derivatives [70] and polypy-ridyl complexes of ruthenium and osmium [71, 72] as mediators of GOx have been undertaken. Ferrocenes have come to the fore as mediators to GOx, surpassing many others, because of factors such as their mediation efficiency, stability in the reduced form, pH independent redox potentials, ease of synthesis, and substitutional versatility. Ferrocenes are also of sufficiently small size to diffuse easily to the active site of GOx. However, solution phase mediation can only be used if the future biocatalytic fuel cell... 

Since the two resulting antipodes have the same thermodynamic stability, the asymmetric synthesis must be kinetically controlled. Therefore, the factor controlling the enantiomeric excess must be the difference between the energies of the two diastereomeric transition states leading to one or to the other antipode respectively. Since the process leading from the olefins to the aldehydes is a multistep one, if more steps involving diastereomeric intermediates exist, the two diastereomeric transition states which determine the type of enantiomeric excess in the products must be identified. 

Stability and reactivity can be controlled by thermodynamic factors or kinetic factors. Both of these depend on the conditions and on the possibility of different routes to decomposition or reaction. 

In a reaction mixture where every step is reversible, so long as sufficient time is allowed, the final ratio of all the possible products will be determined by the relative thermodynamic stabilities of each of the compounds. Such a reaction is said to be under thermodynamic control. However, if for any reason a step is irreversible, or is in essence irreversible, because the reverse reaction is very slow with respect to the length of time that is being allowed for the reaction, then the ratio of the possible products is no longer determined by thermodynamic considerations alone, but also by the speed at which each reaction takes place, i.e. the kinetic factors. A reaction in which the product distribution is determined by the rate of formation of the various products is said to be under kinetic control. We shall now examine some of these factors. 

The morphology of latex particles is controlled by the thermodynamic and kinetic factors. The thermodynamic factors determine the ultimate stability of the multiphase system, inherent in the production of a composite latex particle, while the kinetic factors determine the ease with which such a thermodynamically favored state can be achieved. The parameters affecting the thermodynamics of the system include the particle surface polarity, the relative phase volumes, and the core particle size. The parameters affecting the kinetics of the morphological development include the mode of monomer addition (monomer starved or batch) and the use of crosslinking agents. Of course, crosslinked core/shell latexes constitute IPNs, see Section 6.4.1. 

On the whole the effect of substituents on the relative stability of isomeric arenium ions (for details see Sect. IV, 1) is described in the same terms as those used to explain the influence of substituents on the orientation and relative rates of electrophilic aromatic substitution. However, the isomeric composition of electrophilic substitution products is often controlled by kinetic factors while the equilibrium composition of isomeric arenium ions formed in aromatic compound protonation is determined by thermodynamic equilibrium. Therefore, no quantitative agreement may be observed between the relative hydrogen substitution rates at different positions of this compound and the ratio of equilibrium concentrations of the respective arenium ions formed in protonating the same compound even under identical conditions (cf. Sect. IV, 7). 

Luckily, although thermodynamics will be the factor controlling the ultimate long-term stability of an emulsion, kinetics can play an important role over the short term, and it is through kinetic pathways that most useful emulsions achieve their needed stability. It is clear, then, that while lowering the interfacial tension between phases is an important factor in the formation and stabilization of emulsions, that may not always represent the most important factor in their preparation and ultimate application. 

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