Exchange concept in water-in-oil microemulsions: consequences for ‘slow’ chemical reactions

Abstract

Water-in-oil (w/o) microemulsions are thermodynamically stable dispersions of water droplets in an oil-continuous medium. Water and solutes exchange between droplets in a random process whereby pairs of droplets fuse transiently and then separate again. An equation is derived for the rate of an irreversible chemical reaction between two water-soluble reactants A and B, initially isolated in separate droplets, by applying the steady-state approximation (i.e. by assuming that the concentration of intermediate droplet species containing both A and B, generated in the fusion process, is constant under initial rate conditions). The equation shows how the rate of the chemical reaction can be expected to depend on the rate of the fusion process and the size and concentration of the droplets. The theoretical basis for the utilization of very fast chemical reactions in the measurement of droplet fusion rates, exploited in earlier published studies, is discussed in the context of the rate equation. The analysis is extended to the case of chemical reactions involving a reversible step (i.e. of the type A + B ⇄ C → P). The potential for the additional perturbation of reaction rates arising from the influence of properties of the microemulsion aqueous environment not associated with dynamics of droplet interactions is emphasized. With the emphasis on ‘slow’ chemical reactions, a revision of nomenclature is recommended in order to distinguish the kinetics of the fusion process itself from those of exchange, which should be regarded as a multiple-step process incorporating both droplet fusion and solute-transfer events.