The Problem of Concentration and Reactivity in Reversed Micelles and Water-In-Oil Microemulsions

Abstract

Microemulsions have aroused much interest as a novel medium for chemical reactions. It is usual to attempt to compare reactivity in the dispersed system with homogeneous solvent media. For water-soluble reactants, insoluble in the dispersion medium, the aim would be to compare reactivity in the aqueous droplets of the microemulsion with bulk aqueous solution. In the case of first-order reactions the measured rate constants (units −s^−l) may, in most cases, be compared directly in the two types of system, (e.g. k_cat for an enzyme reaction). However, in the case of a bi-molecular process, the calculation of the second-order rate constant (units −dm³mol⁻¹s⁻¹) involves consideration of the concentration of species in the microemulsion, and the question arises as to whether the overall concentration (moles dm⁻³ of total solution) or the concentration in the water should be employed. In the latter case there are two possibilities: a) the reactant concentration in a discrete droplet and b) the concentration in moles dm⁻³ of the dispersed aqueous phase. In principle it is possible to define and use any concentration scale. The problem is to decide on the most applicable units in order to best interpret the data in terms of a comparison of reactivity in the microemulsion and bulk solvent media.