Ions, ion pairs and their associates

Two ions, A and B, carrying ZA and ZB charge with centres at a distance r exhibit an electrostatic interaction energy [68] 

For small values of r (less than ca. 1 nm), the ion interaction also includes the contribution of short-distance repulsion forces and attractive (dispersion) forces. The repulsion forces are little known for simple spherical molecules their contribution can be expressed by the product br 12. The attractive forces are better defined by analogy to the repulsion forces they can be expressed as cr 6, where b and c are constants characterizing the particles. To a first approximation the overall energy can be described by 

For a pair of ions of opposite charge, the value of Utl r exhibits a minimum for a very short inter-ion distance r. For NaCl in vacuum [69], the interaction energy is about 500 kJ mol lt. 

Heterolytic dissociation of a covalent bond in the gas phase (the term ionization was recommended for this process [68a]) requires energy of the same order. The calculation is, however, subject to considerable error, and experimental verification is not easy. 

Orientation of solvent molecules in the neighbourhood of the ion, and their coordination to the ion, is known as ion solvation. The manner of solvation is very important. If the surface of the ion pair as a whole is surrounded by 

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