Ion-Ion and Ion-Neutral Interactions in Solution and Measurements of Dielectric Constants

Data on dielectric constants of electrolyte solutions are used to evaluate directly the r^(−4) term in the asymptotic expression for the free energy of interaction of two ions in solution for large separation distance r. Use is made of the fact that for large separations each ion is in a uniform field due to the other, and that information about ions in uniform fields is obtainable from measurements of dielectric constants. For a Z:Z electrolyte, for example, the r^(−4) term is found to be Z^2e^2δ/8πϵ_0r^4, assuming the effect of overlapping solvent structures to be of shorter range; ϵ_0 is the dielectric constant of the solvent and δ is the measured decrement in dielectric constant per unit concentration of added electrolyte. A similar result obtained when one of the particles is uncharged, δ now referring to the decrement observed when the neutral is added to solution. Typical values of the term are given for various substances using the data on δ's. This determination of the r^(−4) term permits some evaluation of ion—image force theories