Self-diffusion and viscosity of molten alkali halides

Abstract

In this paper we report a theoretical calculation of the self-diffusion coefficients and shear viscosity coefficients of some molten alkali halides on the basis of the kinetic theory of dense fluids proposed by Rice and Allnatt, and Rice and Kirkwood. The “effective” pair potentials between like or unlike ions are composed of three parts : a Coulomb attraction or repulsion (involving a local dielectric constant), an intermultipole attraction (the van der Waals potentials), and a core repulsion. The calculation is performed without the assumption of the pseudo one-component fluid, which has often been used for the interpretation of the properties of ionic melts. The pair correlation functions are taken from the diffraction results. The hard and soft friction coefficients are calculated separately. The predicted self-diffusion and viscosity coefficients are in good agreement with experimental results. The contributions of like ions to the friction coefficients and viscosities of some molten salts are important together with those of unlike ions. This trend is especially displayed in the NaI melts, in which anion-anion interactions are appreciable.