Energetics of interactions of aromatic hydrocarbons with water.

The thermodynamics of transfer of aromatic (benzene, toluene) and aliphatic (ethane, propane, butane) hydrocarbons from the gas phase into water in the temperature range 5-125 degrees C have been analyzed in order to determine the net hydration effect of these compounds. In the case of the aromatic hydrocarbons the enthalpic contribution predominates over the entropic contribution to the Gibbs energy of hydration. This results in a negative value of the hydration Gibbs energy of aromatic hydrocarbons, in contrast to the positive Gibbs energy of hydration of aliphatic hydrocarbons. The different sign of the hydration Gibbs energies indicates that the mechanism causing hydrophobicity of aromatic hydrocarbons has different nature than that causing the hydrophobicity of aliphatic hydrocarbons. The comparison of hydration of aliphatic and aromatic hydrocarbons leads to the following thermodynamic parameters for these additional interactions between the benzene ring and water at 25 degrees C: enthalpy -5.4 kJ/mol, entropy 26.8 J/K mol and Gibbs energy -13.4 kJ/mol. The large enthalpic contribution to the Gibbs energy of hydration of aromatic hydrocarbons probably comes from the ability of the aromatic ring to accept hydrogens from water, forming hydrogen bonds.