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CHAPTER 3

ION–ION INTERACTIONS

3.1. INTRODUCTION

A model has been given for the breaking up of an ionic crystal into free ions which
stabilize themselves in solution with solvent sheaths. One central theme guided the
account, the interaction of an ion with its neighboring water molecules.
However, ion–solvent interactions are only part of the story relating an ion to its
environment. When an ion looks out upon its surroundings, it sees not only solvent
dipoles but also other ions. The mutual interaction between these ions constitutes an
essential part of the picture of an electrolytic solution.
Why are ion–ion interactions important? Because, as will be shown, they affect
the equilibrium properties of ionic solutions, and also because they interfere with the
drift of ions, for instance, under an externally applied electric field (Chapter 4).
Now, the degree to which these interactions affect the properties of solutions will
depend on the mean distance apart of the ions, i.e., on how densely the solution is
populated with ions, because the interionic fields are distance dependent. This ionic
population density will in turn depend on the nature of the electrolyte, i.e., on the extent
to which the electrolyte gives rise to ions in solution.

3.2. TRUE AND POTENTIAL ELECTROLYTES

3.2.1. Ionic Crystals Form True Electrolytes

An important point to recall regarding the dissolution of an ionic crystal (Chapter
2) is that ionic lattices consist of ions even before they come in contact with a solvent.
In fact, all that a polar solvent does is to use ion–dipole (or ion–quadrupole) forces to
disengage the ions from the lattice sites, solvate them, and disperse them in solution.
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