ABSTRACT
Much attention has been directed since olden times towards ion solvation, which is a key concept for understanding various chemical processes with electrolyte solutions. In 1920, a theoretical equation of ion solvation energy was first proposed by Born [1], who considered the ion as a hard sphere of a given radius (r) immersed in a continuous medium of constant permittivity and then defined as the electrostatic energy for charging the ion up to ze (z, the charge number of the ion; e, the elementary charge):
(1)
where NA is the Avogadro constant, and the permittivity of vacuum. Since the term “1” in the parentheses of the RHS of Eq. (1) is 1/[the relative permittivity of vacuum (=1, by definition)], (Born) can also be regarded as the electrostatic energy required for the transfer of the ion from vacuum to the medium having the permittivity of Accordingly, the Gibbs free energy of ion transfer i.e., the resolvation energy for the transfer of an ion from one medium (e.g., organic solvent, O) to another (e.g., water, W) can be expressed as
(2)
where and are the relative permittivities of O and W, respectively. The Born equation thus derived is based on very simple assumptions that the ion is a
sphere and that the solvents are homogeneous dielectrics. In practice, however, ions have certain chemical characters, and solvents consist of molecules of given sizes, which show
various chemical properties. In the simple Born model, such chemical properties of ions as well as solvents are not taken into account. Such defects of the simple Born model have been well known for at least 60 years and some attempts have been made to modify this model. On the other hand, there has been another approach that focuses on shortrange interactions of an ion with solvent molecules.
