Abstract
The Gibbs free energy (ΔG°tr), enthalpy (ΔH°tr), and entropy changes (ΔS°tr) for transfer of 18-crown-6 (18C6) from water to various polar nonaqueous solvents were precisely determined by solvent extraction. By using these data, and the known values of thermodynamic quantities for transfer of alkali metal ions (M+) and those for complexation in water and the nonaqueous solvents, we calculated the thermodynamic parameters of transfer of the M(18C6)+ complexes from water to the solvents from the Ph4As+/Ph4B- assumption via a thermodynamic cycle. The ΔH°tr and ΔS°tr values of the M(18C6)+ complexes vary considerably with the solvent and with the alkali metal ion, and also those of 18C6 do with the solvent. The values of ΔH°tr and ΔS°tr are all positive and large, reflecting strong hydrogen bonding between 18C6 and water. 18C6 complexes with Rb+ and Cs+ are enthalpically more stable in a given solvent than those with Na+ and K+. This shows that Na+ and K+ in the complexes are more effectively shielded by 18C6 from surrounding solvents than Rb+ and Cs+. Among all the M(18C6)+ complexes, positive ΔH°tr value from water to DMF is found only for Na(18C6)+. It is concluded from this and the other results that the Na(18C6)+ complex undergoes hydrophobic hydration more strongly than the other complexes do.
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