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, DELTA(t)G-degrees(F-)H2O --> S, indicate that the solvation of the fluoride ion is dominated by hydrogen bonding. Thus, DELTA(t)G-degrees(F-)H2O --> 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 DELTA(t)G-degrees(F-)H2O --> H2O + S generally increase monotonically with declining water content. However, this results from the fortuitous cancellation of dramatic, but largely opposing, variations in DELTA(t)H-degrees and DELTA(t)S-degrees. 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.