3 - Electrostatic forces in electrolytes in outline

Summary

The assumptions of classical theories

The classical theory of electrolyte solutions starts with the assumption that ionic interactions in solution or between colloid particles are dictated by Coulomb, electrostatic forces alone. An ion is considered to first approximation to have a charge distribution confined to a hard sphere of a given radius. In the ‘primitive’ model the ions are immersed in water (or another solvent) within which they interact by electrostatic interactions. The solvent is treated as a passive dielectric continuum. The radius of an ion is not always just its crystallographic radius. It is an effective radius that includes one or two water layers of ‘hydration’. What occurs in the theory for the free energy of interactions involves the sum of hydrated ion radii besides the Coulomb force. The hydrated ion size is derived as a fitting parameter from comparison of theory with experiments.

For interactions between ions the long-range Coulomb interactions dominate for very dilute solutions below about 5·10⁻² M, and ion size is irrelevant. At higher electrolyte concentrations, around and above about 10⁻¹ M, the cooperative electrostatic interactions that dominate at low concentrations become progressively less important. Shorter-range forces subsumed in the ionic ‘radii’ begin to come into play. When short-range interactions between the ions become significant the molecular structure of the water in the immediate neighbourhood of the ions (hydration) becomes a dominant feature. Hydration and local hydrogen bonding are words that attempt to describe this ion-specific, local water structure induced by the ions.