Dielectric constant is the most important factor in determining the occurrence of ion association. A table of some typical values can be found under dielectric constant.
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. 2:2 electrolytes (q = 2, q = 2) form ion pairs more readily. Indeed, the solvent-shared ion pair [Mg(HO)]SO was famously discovered to be present in seawater, in equilibrium with the contact ion pair [Mg(HO)(SO)] Trivalent ions such as Al, Fe and lanthanide ions form weak complexes with monovalent anions.
The dielectric constant of water decreases with increasing temperature to about 55 at 100 °C and about 5 at the critical temperature (217.7 °C). Thus ion pairing will become more significant in superheated water.
Solvents with a dielectric constant in the range, roughly, 20–40, show extensive ion-pair formation. For example, in acetonitrile both contact and solvent-shared ion pairs of Li(NCS) have been observed. In methanol the 2:1 electrolyte Mg(NCS) is partially dissociated into a contact ion pair, [Mg(NCS)] and the thiocyanate ion.
The dielectric constant of liquid ammonia decreases from 26 at its freezing point (&minus;80 °C) to 17 at 20 °C (under pressure). Many simple 1:1 electrolytes form contact ion pairs at ambient temperatures. The extent of ion pairing decreases as temperature decreases. With lithium salts there is evidence to show that both inner-sphere and outer-sphere complexes exist in liquid-ammonia solutions.
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. In this solvent ion association is the rule rather than the exception. Indeed, higher associates such as tetramers are often formed. Triple cations and triple anions have also been characterized in THF solutions.
Ion association is an important factor in phase-transfer catalysis, since a species such as RPCl is formally neutral and so can dissolve easily in a non-polar solvent of low dielectric constant. In this case it also helps that the surface of the cation is hydrophobic.
In S1 reactions the carbocation intermediate may form an ion pair with an anion, particularly in solvents of low dielectric constant, such as diethylether. This can affect both the kinetic parameters of the reaction and the stereochemistry of the reaction products.