Makhatadze and Privalov have analyzed the thermodynamics of transfer of aliphatic and aromatic hydrocarbons from the gas phase into water. Finding that the hydration free energy of aliphatic and aromatic hydrocarbons trave different signs, they conclude that the mechanism causing hydrophobicity of these solutes is of a different nature. Here, we offer an alternative analysis of the dissolution of these non-polar compounds into water based on a recently published interpretation scheme for thermodynamic transfer functions. Our analysis shows that the hydrophobicity of aromatic and aliphatic hydrocarbons is qualitatively the same, i.e. its causes are the same namely the extremely high cohesive energy of water which overcomes the favorable solute-solute and solute-water interactions. However, both analyses conclude that the experimentally observed quantitative difference between the interactions of water with aliphatic and aromatic hydrocarbons, can be assigned to the formation of aromatic ring-water H-bonds.

We re-examine the numerical value and physical significance of TS , the temperature where the entropy of transfer Δ WLCP fitting function. It is concluded that the interpretation of TS as the temperature where hydration ceases cannot be sustained. As previously reported [R.L. Baldwin, N. Muller, Proc. Natl. Acad. Sci. USA, 89 (1992) 7110], hydration must vanish at a temperature T1 >TS, where its experimental manifestation, i.e., ΔWL CP, is zero. We discuss the concept of water relaxation around a non-polar solute molecule and its relation to the hydration process.