Effect of ionic strength on the kinetics of ionic and micellar reactions in aqueous solution

Abstract

The effect of electrostatic forces on the rate of reaction between ions in aqueous solutions of intermediate ionic strength is studied in this paper. We consider the kinetics of reactions involving simple ionic species (1–1 and 2–2 electrolyte systems) as well as kinetic processes mediated by the presence of micellar ions (or other charged organizates). In the regime of ionic strength considered, dielectric saturation of the solvent in the vicinity of the reacting ions must be taken into account and this is done by introducing several models to describe the recovery of the solvent from saturation to its continuum dielectric behavior. To explore the effects of ion size, charge number, and ionic strength on the overall rate constant for the process considered, we couple the traditional theory of ionic reactions in aqueous solution with calculations of the electrostatic potential obtained via solution of the nonlinear Poisson–Boltzmann equation. The great flexibility of the nonlinear Poisson–Boltzmann theory allows us to explore quantitatively the influence of each of these effects, and our simulations show that the short-range properties of the electrostatic potential affect primarily kinetically controlled processes (to varying degrees, depending on the ionic system considered) whereas the down-range properties of the potential play a (somewhat) greater role in influencing diffusion-controlled processes. A detailed examination is made of ionic strength effects over a broad range of ionic concentrations. In the regime of low ionic strength, the limiting slope and intercept of the curve describing the dependence of log kD on I1/2/(1+I1/2) may differ considerably from the usual Debye–Hückel limiting relations, depending on the particular model chosen to describe local saturation effects. The results in the regime of high ionic strength for all models considered converge to the Smoluchowski limit, and thus differ significantly from the traditional Debye–Hückel results. As a specific application of our theory, we consider the reaction between pyrene solubilized in cetyltrimethylammonium bromide (CTAB) and e−aq. We estimate the chemical rate kR and then calculate the dependence of the overall rate on the ionic strength; the results obtained are found to be in excellent agreement with experiment.