Hydration of aromatic hydrocarbons

The transfer of benzene and toluene from gas phase into water is a spontane ous process at room temperature under Ben-Naim's standard conditions. This strongly contrasts with the behavior of aliphatic hydrocarbons, for which t he Gibbs energy of transfer is large and positive. To understand this diffe rence in behavior between the aromatics and aliphatics, we analyze the hydr ation of benzene and toluene in a large temperature range using available e xperimental and computer simulation data. We introduce a small but importan t modification in the definition of the solvent reorganization enthalpy in order to handle slightly polar solutes such as the aromatic hydrocarbons, w hich form weak hydrogen-bonds with water. The analysis shows that, for arom atic hydrocarbons, the van der Waals interaction energy overwhelms the Gibb s energy cost for cavity creation at room temperature, thus rendering the h ydration process spontaneous. The formation of the weak hydrogen-bonds betw een the aromatic ring and water appears to be largely compensating and not to contribute significantly to the hydration Gibbs energy.