Pair Formation and Non-Coulombic Forces

The theory of ion-pair formation in nonaqueous solutions has been substantially advanced by the work of Barthel, who demonstrated how important it is to take into account the non-Coulombic forces at small ionic distances in addition to the Coulom-bic ones used by Bjerrum. These non-Coulombic forces are represented by the mean force potential W (r) in the region a r  

Non-Coulombic forces are those that are responsible for different degrees of association of electrolytes in isodielectric solvents. For example, one can see the importance of non-Coulombic forces in respect to ion-pair formation when one compares the temperature dependence of the association constants of alkali metai salts and the tetraalkylammonium salts in protic solvents. The association constants of aikali metal halides show a monotonicaily increasing when plotted against 

Association Constants, Gibbs Energies, Enthalpies, and Entropies of Ion-Pair Formation of Alkaii Metai and Tetraaikylammonium Iodides in 1-Propanol from Temperature-Dependent Conductivity Measurements 

Source Reprinted from J. Barthcl, R. Wachter, G Schmeer, and H. Hilbinger, J. Sol Chem. IS 531, 1986. 

In Table 4.27, one can see the free energies, enthalpies, and entropies of ion-pair formation. The enthalpies and entropies of the alkali metal salts are on the order of 16 kJ mor and 100 J K mol , respectively. In contrast, the small enthalpies and entropies of the tetraaikylammonium ions reflect lesser solvation of the cations in the protic solvents. 

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