ABSTRACT
What is an ion-pair? In order to shed light on the theory that governs ion-pairing, the process first will be treated phenomenologically.
In a naïve picture, an ion-pair may be described as a couple of oppositely charged ions temporarily held together by electrostatic attraction, without establishment of a durable chemical bond. An ion-pair is a diverse chemical species, but the distinction between ion-pairs and free ions that interact via long-range non-specific ion-ion forces is questionable. As a rule, species are regarded as ion-pairs if the distance (r) between two oppositely charged ions bathed in solution is lower than a cut-off length (R). Obviously the ion-pair partners cannot approach each other more closely than the distance of closest approach (a) because of the strong repulsive interactions of their electron shells. The ions are deemed to be paired if their distance apart is between a and R. Moreover the lifetime of the ion-pair should be at least longer than the time required to thermally diffuse over such distance, since a simple encounter of a cation and an anion does not involve ion-pair formation. Lifetimes as low as ~1 ns have been reported [1].
