The most important factor to determine the extent of ion association is the dielectric constant of the solvent.
This usage is common in coordination chemistry and denotes a complex between a solvated metal cation and an anion.
The essential difference between the three types is the closeness with which the ions approach each other: fully solvated > solvent-shared > contact.
[10][11] This is described by Coulomb's law: where F is the force of attraction, q1 and q2 are the magnitudes of the electrical charges, ε is the dielectric constant of the medium and r is the distance between the ions.
The entropy term is similar for electrolytes of the same type, with minor differences due to solvation effects.
Therefore, it is the magnitude of the enthalpy term that mostly determines the extent of ion association for a given electrolyte type.
Dielectric constant is the most important factor in determining the occurrence of ion association.
Water has a relatively high dielectric constant value of 78.7 at 298K (25 °C), so in aqueous solutions at ambient temperatures 1:1 electrolytes such as NaCl do not form ion pairs to an appreciable extent except when the solution is very concentrated.
Solvents with a dielectric constant in the range, roughly, 20–40, show extensive ion-pair formation.
[17] The dielectric constant of liquid ammonia decreases from 26 at its freezing point (−80 °C) to 17 at 20 °C (under pressure).
With lithium salts there is evidence to show that both inner-sphere and outer-sphere complexes exist in liquid-ammonia solutions.
[18] Of the solvents with dielectric constant of 10 or less, tetrahydrofuran (THF) is particularly relevant in this context, as it solvates cations strongly with the result that simple electrolytes have sufficient solubility to make the study of ion association possible.
[20] Ion association is an important factor in phase-transfer catalysis, since a species such as R4P+Cl− is formally neutral and so can dissolve easily in a non-polar solvent of low dielectric constant.
In SN1 reactions the carbocation intermediate may form an ion pair with an anion, particularly in solvents of low dielectric constant, such as diethylether.
Anions containing a CN group, such as cyanide, cyanate and thiocyanide have a vibration frequency a little above 2000 cm−1, which can be easily observed, as the spectra of most solvents (other than nitriles) are weak in this region.
Instead, the formation of a contact ion pair is seen to depend more on the energy needed to displace a solvent molecule from the primary solvation sphere of the cation.