From Hydrophobic to Hydrophilic Solvation: An Application to Hydration of Benzene

We report a computer simulation study on the hydration of benzene, which, despite being hydrophobic, is a weak hydrogen bond acceptor. The effect of benzene-water hydrogen bonding on the hydration free energy has been analyzed in terms of solute-solvent energies and entropies. Our calculations show that benzene-water hydrogen bonding restricts the number of arrangements possible for the water molecules resulting in a more unfavorable (negative) solute-solvent entropy change than observed for a 'nonpolar benzene' not capable of accepting water hydrogen bonds. More favorable hydration free energies of aromatic hydrocarbons in comparison with aliphatic hydrocarbons observed experimentally as well as in our calculations must therefore be a result of more favorable solute-solvent interaction energies. This result supports the view that lower aqueous solubilities of nonpolar molecules compared to polar molecules are due to a lack of favorable electrostatic interactions with water molecules. The calculated hydration free energy, enthalpy, entropy, and hydration heat capacity of benzene are in good agreement with experimentally reported values.