Concentration gradients, interpretation kinetic observations

The relative rates were very different when the reactions were carried out in the microemulsion. Now the methyl-substituted benzyl bromide reacted fastest followed by the unsubstituted benzyl bromide. The ferf-butyl and the isopropyl substituted substrates reacted with the same rate. These results were interpreted as follows. The methyl substituted and, even more, the unsubstituted benzyl bromides have non-negligible water solubility. For these substrates reaction in the bulk water domain occurs in parallel to the interfacial reaction and the observed rate is the sum of the two processes. The ferf-butyl and the isopropyl substituted benzyl bromides, on the other hand, react only at the interface. A kinetic expression has been derived for a reaction that occurs partly at the interface and partly in the aqueous domain [ 16]. In this treatment the microemulsion has been divided into two sub-volumes, oil and water, instead of three. The surfactant sub-volume is divided equally between the oil and water sub-volumes. Assuming again that the reactant concentration is the same in the bulk as at the interface, i.e. there is no concentration gradient for the reactants, it can be shown that the rate constant of the total reaction can be written as... 

It is of course also conceivable that the surface processes arc faster than the mass transport. In that case we cannot observe any of the surface processes, since the surface is fully equilibrated with the adjacent solution, and all that remains is a concentration gradient in the solution which controls the kinetics. In order to be able to interpret experimental data we must therefore have a prediction of the adsorption and desorption rates for the equilibrated case. Deviations from this are then indicative of slow surface processes. 

In all of the multistep immobilized enzyme work done to date, theoretical or experimental, for modelling purposes or for applications, there exists one common factor the chemical reactions are affected by the diffusive processes so that the macroscopically observed kinetics are strongly perturbed by the incorporation of the enzymes into a gel. This perturbation is caused by the development of localized concentrations and concentration gradients within the gel which are quite different from that found in free solution. Only one instance appears to have been reported where exact modelling of real experimental data has been attempted. All other work has been either purely theoretical or qualitative interpretations of limited experimental data. There is still much to be learned of the role played by the gel matrix in affecting the overall kinetic performance of gel entrapped multienzyme systems before they can be well designed for applications or used with any confidence in a quantitative way as models for living systems. 

---