Thermodynamic Properties of Aqueous Organic Solutes in Relation to Their Structure. Part I. Free Energy of Transfer from H2O to D2O for a Series of Isomeric Ketones

Abstract

The solubilities of 18 decanones have been measured in H2O and D2O at 25 °C from their u.v. absorption spectra. The data were used to calculate ΔGt0, the standard free energy of transfer of these solutes from H2O to D2O ΔGt0 was found negative for the saturated ketones, but positive for most of the unsaturated and polycyclic isomers. A rough correlation was observed between ΔGt0 and the degree of branching in the hydrocarbon chains of the ketones: ΔGt0 increases with the degree of branching. From the temperature dependence of ΔGt0 obtained at 10, 25, and 40 °C, heats of transfer ΔHt0 were determined for typical ketones, namely 2-decanone and adamantanone. ΔHt0 was found as −2.1 kcal mol−1 for 2-decanone and ~0 ± 0.5 kcal mol−1 for adamantanone.The results are discussed in terms of two main contributions: (1) an isotope effect in the free energy of cavity formation and (2) perturbation of solvent structure by the solutes, differing in H2O and D2O. Using the scaled-particle theory to evaluate the former, the variations in the observed ΔGt0 are assigned to the structural part of ΔGt0.