Ionic Association

Strong electrolytes are often assumed to exist entirely as free ions in aqueous solution. In reality, oppositely charged ions can partially associate to form ion pairs, except in many 1:1 electrolytes, like NaCl or NaOH.

Bjerrum defined an ion pair as two oppositely charged ions close enough that their electrostatic attraction exceeds the thermal energy, quantified as 2kT, where k is Boltzmann’s constant, and T is the absolute temperature. When this condition is met, ion association becomes favorable.

Ion pairing increases with higher ionic charges, as in 2:1 or 2:2 electrolytes, leading to a significant fraction of ion pairs even at low concentrations. This prediction is experimentally supported. When the percentage of cations in ion pairs is plotted against molality, the results closely match Bjerrum’s theory.

It is also strongly influenced by the solvent: water’s high dielectric constant weakens electrostatic attraction and limits ion pairing, whereas solvents with lower dielectric constants enhance ion–ion attraction, making ion-pair formation significant even for 1:1 electrolytes.