Fluoride solvation-the case of the missing ion

Abstract- The thermodynamics of fluoride ion solvation in nonaqueous and mixed aqueous solvents are reviewed. Gibbs energies of transfer, at ^ O (F-) H20+, indicate that the solvation of the fluoride ion is dominated by hydrogen bonding. Thus, at ^ o ( ~-) H 2 0- S is unfavourable for virtually all solvents, especially dipolar aprotics, and correlates well with solvent acceptor strength. The corresponding enthalpy and entropy data are few and are of low quality but suggest F- solvation is entropy controlled: the only ion known to be so. In mixed aqueous solvents the values of A t G (F-) H20-H20+S generally increase monotonically with declining water content. However, this results from the fortuitous cancellation of dramatic, but largely opposing, variations in A,Ho and A t s o. The complex changes in these parameters are related to subtle changes in ion-solvent and solvent-solvent interactions. Finally, a relationship between fluoride and hydroxide solvation is proposed. 1