Polycyclic Aromatic Hydrocarbons: Solvation, Solubility, and Hydrophobic Effects from Monte Carlo Simulations

Free energies of solvation in water and liquid cyclohexane
have
been computed for 20 arenes ranging in size from cyclobutadiene to
coronene. Monte Carlo statistical mechanics (MC) was used with free-energy
perturbation theory (FEP) and the OPLS-AA force field. The computed
results for free energies of hydration are in close agreement with
experimental data giving an average error of 0.4 kcal/mol. Some discrepancies
are found for larger arenes for which the experimental data have greater
uncertainties owing to low solubility. The free energies of solvation
and free energy of transfer from cyclohexane to water all display
strong correlations with the solvent-accessible surface area (SASA)
or volume of the arene. In contrast to the hydration of alkanes, the
free energies of hydration of arenes become much more favorable with
increasing size covering an 11 kcal/mol range. The free energies of
solvation in cyclohexane are still more favorable, resulting in a
5 kcal/mol range for the resistance to transfer of arenes from cyclohexane
to aqueous solution. Strong correlations are also found between the
aqueous solubility of arenes and the free energies of solvation in
cyclohexane and water. The reported results provide fundamental thermodynamic
data for solution-phase properties of arenes, with relevance to materials
and environmental science.