Short-Range Interactions of Aqueous Ions

Data on the 1–1 fluoride complex ion with noble-gas-type cations are examined for information on short-range ion—ion and water—ion interactions. A four-term numerical correlation of the standard free energies with the size and charge of the ion is developed based on the approach that electric saturation is not directly important to the complex formation. Deviations from the classical point charge Coulomb energy are expressed by a term independent of temperature and the dielectric medium, exponential in the separation distance, and containing only fundamental quantities which suggest wave-mechanical interaction between cation and anion. The total standard entropy change separates into three terms; the temperature dependence of the macroscopic dielectric constant, the removal of coordinated water, and the cratic term for the change in the number of solute particles. A screened charge model of the hydrated ion is proposed in which hydration is the loss of the quantum-mechanical free energy of the gaseous ions by the formation of covalent bonds with the water. The ion charge is delocalized, spreading out to the coordinated water. The degree of hydration depends on the free energy of the water, with the free energy for coordination of the last water molecule being less than 1 kcal.