Solution Thermodynamic Studies. 6. Enthalpy-Entropy Compensation for the Complexation Reactions of Some Crown Ethers with Alkaline Cations: A Quantitative Interpretation of the Complexing Properties of 18-Crown-6

Article abstract of DOI:10.1021/ja00389a002

The interactions of 18-crown-6, 15-crown-5, and 12-crown-4 with Na⁺ and K⁺ were studied in methanol and water as solvents at 25 deg C. ΔG⁰ values for both 1:1 and 2:1 complexation reactions were determined by potentiometric titrations.Used in conjunction with these values, calorimetric measurements led to ΔH⁰ and ΔS⁰ values.The thermodynamic parameters obtained cannot be correlated with the cations or the crown ethers "hole" sizes in any 1:1 or 2:1 reactions.Moreover, the ΔG⁰ values are the results of quite different but permanently compensating combinations of the ΔH⁰ and ΔS⁰ values.These arise from several thermodynamic processes in which the role of the solvent must be considered.In the case of 18-crown-6, we present a quantitative interpretation in which this crown ether develops interactions that are stronger with Na⁺ than with K⁺.