Solvation of Ions and Molecules

When an ionic compound dissolves in a polar solvent such as water, the ions become strongly solvated. The ion-dipole forces produce a layer of solvent molecules (the primary solvation sphere) surrounding each ion. This layer can cause other solvent molecules in the immediate vicinity to become oriented as well. Although the primary solvation sphere may seem to be rather firmly attached to the ion, there is considerable interchange between these molecules and the bulk solvent in a dynamic process in most cases. For strongly solvated ions like [Cr(H206] , the exchange of coordinated water molecules and those of the bulk solvent is very slow. As we shall describe later, some desolvation of ionic species may be required before a reaction of the ion can take place. 

Theoretical treatment of the solvation of ions is quite difficult. If we could use a simple electrostatic approach in which polar molecules interact with a charged ion, the problem would be much simpler. However, the fact that the polar solvent molecules interact with an ion causes their 

When an electrostatic approach to the interaction between an ion and a polar solvent is used and the dielectric constant, s, is assumed to be the same as the bulk solvent, the free energy of hydration (AGh) of an ion of radius r can be shown to be 

---

The description of solvation of ions and molecules in solvent mixtures is even more complicated. Besides the interaction between solvent and solute, the interaction between unlike solvent molecules plays an important supplementary role. This leads to large... 

---