Kinetics of partly diffusion-controlled reactions. Part 23.—The case of ionic reactions

Abstract

Diffusion-controlled reactions have been investigated for many years, and have been modelled for the case where Brownian particles move in a medium of hard spheres. We here present models based upon liquid physics concepts and known interaction potentials for cases where the transport phenomenon is due to a random walk hampered by the electrostatic potential and screen effect.

Our model assumes the diffusing particles to be spheres, of identical radius and with a central charge. A ‘continuum’ model is developed which combines the radial configurational function of hard spheres and the Debye–Hückel interaction potential. The solution of a classical integro-differential system then allows us to compute the apparent rate constant of the reaction.

This model is applied to the interpretation of the fluorescence quenching of fluorescein ions by iodide ions, a well known system for which several unsatisfactory models (especially static or non-diffusional quenching) have been proposed. The use of the present model, where coupling between diffusion and reactivity is taken into account, makes it possible to find a satisfactory agreement with experiment. However, theoretical refinements are yet required for a better understanding of the phenomenon occurring at low relative permittivity.