Fluoride solvation-the case of the missing ion

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 Fsolvation 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.