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.