Enhancement of the solubilities of polycyclic aromatic hydrocarbons by weak hydrogen bonds with water.

The thermodynamics of mobile order is applied to predict the aqueous solubility of liquid and solid aliphatic and polycyclic aromatic hydrocarbons. The solubility values are mainly determined by the magnitude of the hydrophobic effect. However, contrary to the solubilities of the alkanes, the solubilities of polycyclic aromatic hydrocarbons in water predicted in absence of solute-solvent hydrogen (H) bonds are systematically too low. This shows the contribution of weak specific interactions between the OH groups and the pi electrons of the aromatic substances. According to the theory, these interactions are characterized by a stability constant KO which can be derived from solubility data. At 25 degrees C, this constant amounts to 80 cm3/mol, the order of magnitude of which can be explained by the competition of these intermolecular bonds with the rather weak self-association bonds in the secondary chains of water.