Abstract
The diffusion coefficients of alkali metal ions in molten CsCl, KCl and NaCl were determined using the capillary reservoir method from the melting points of the solvent salts to temperature 200°C higher than them. The results obtained are summarized as follows:
(1) In the same solvent, the diffusion coefficient decreases with increasing cationic radius of the solute except lithium ion. On the other hand, the diffusion coefficient decreases as the cationic radius of the solvent increases. The difference among the diffusion coefficients of solute cations becomes more pronounced when the cationic radius of the solvent is smaller.
(2) There was a close correlation between the apparent activation energy for solute diffusion and the heat of mixing of solute and solvent salts at infinite dilution. There was also a close correlation between the apparent activation energy and the size parameter [(α1−λ2)⁄λ1λ2]2 of solute and solvent cations.
(3) For the solvents with a smaller cationic size, the apparent activation energy for solute diffusion is mainly affected by the energy required for the formation of a hole large enough for diffusive displacement at the position adjacent to the cation. In the case of molten salt containing holes of a larger mean size than the size of diffusing solute cation, however, the apparent activation energy is affected both by the hole formation energy and by the short ranged interaction energy such as the polarization energy of anion.
