Fluoride solvation-the case of the missing ion

Hefter, G.T. (1991) Fluoride solvation-the case of the missing ion. Pure and Applied Chemistry, 63 (12). pp. 1749-1758.

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Abstract

The thermodynamics of fluoride ion solvation in nonaqueous and mixed aqueous solvents are reviewed. Gibbs energies of transfer , ∆tG⁰ (F -) H20-s , indicate that the solvation of the fluoride ion is dominated by hydrogen bonding. Thus, ∆tG⁰ (F -) H20-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 ∆tG⁰ (F -) H20-s generally increase monotonically with declining water content. However, this results from the fortuitous cancellation of dramatic, but largely opposing, variations in ∆tH⁰ and ∆tS⁰. 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.