Aliphatics vs. aromatics hydration thermodynamics

By comparing the hydration thermodynamics of benzene with that of a hypothetical aliphatic hydrocarbon having the same accessible surface area (ASA) of benzene, Makhatadze and Privalov concluded that the whole difference is due to the weak H-bonds that water forms with the aromatic ring. The formation of such H-bonds would be characterized by a negative Gibbs energy change, slightly increasing in magnitude with temperature, and a positive entropy change over a large temperature range. The latter thermodynamic feature is not physically reliable for the formation of H-bonds. In the present article, by using a statistical mechanical dissection scheme of hydration, a microscopic interpretation for the numbers obtained by Makhatadze and Privalov is proposed. The difference in hydration Gibbs energy should be attributed to the different strength of van der Waals interactions that benzene can do with water, owing to the larger polarizability of the aromatic ring with respect to an aliphatic hydrocarbon of equal size. In addition, the difference in hydration entropy should account for the different extent of H-bond reorganization upon the insertion of benzene and the corresponding aliphatic hydrocarbon in water.