On the Kinetic Theory of Dense Fluids. XVI. The Ideal Ionic Melt

In this paper a theory is constructed to describe to first order the properties of a fused salt. The Rice—Allnatt theory of simple dense fluids is used in conjunction with the following assumptions: (1) oppositely charged ions are of nearly equal size and have identical electronic properties, except for the sign of the charge; (2) a negative ion is, on the average, surrounded by positive ions and vice versa; (3) short-range repulsive interactions occur only between pairs of oppositely charged ions; (4) the role of the Coulomb potential is to determine the fluid density and local fluid structure, but the Coulomb potential makes an insignificant contribution to the direct dissipation of energy and momentum. These conditions are taken to define an ideal ionic melt, and integrodifferential equations for the singlet and pair distribution functions of this system are derived. From these there is obtained an expression for the ion mobility in a weak electric field. This expression predicts a deviation from the Nernst—Einstein relation. A recipe for calculating the thermal conductance and shear viscosity is outlined. Some comparisons of theory with experiment are made. In particular, the theory can account for the magnitude of the deviation of the mobility—diffusion relationship from the Nernst—Einstein relation.