Thermodynamics of solvation of 18-crown-6 and its alkali-metal complexes in various solvents

Heats of solution of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) in acetonitrile, 1,2-dichloroethane, N,N-dimethylformamide,dimethyl sulfoxide, nitromethane, propylene carbonate, pyridine and water were measured at 25degreesC and the enthalpies of the transfer of 18-crown-6 from water to the aprotic solvents were derived. The thermodynamic quantities, DeltaG(1)(o), DeltaH(1)(o) and TDeltaS(1)(o), for the formation of the [M(18-crown-6)](+) (M+ = Na+, K+, Rb+, Cs+, NH4+) complexes were determined by titration calorimetry in dimethyl sulfoxide containing 0.1 mol dm(-3) (C2H5)(4)NClO4 as a constant ionic medium at 25degreesC. These thermodynamic quantities suggest that the complexation of 18-crown-6 with the alkali-metal ions mainly reflects the different solvation of 18-crown-6 and also the different degree of solvent structure.