Hydration of aromatic hydrocarbons

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