Enthalpy-entropy and cavity decomposition of alkane hydration free energies: Numerical results and implications for theories of hydrophobic solvation

This study reports the first complete description of the solution thermodynamics of a series of linear, branched, and cyclic alkanes in water by computer simulations, including the enthalpy and entropy changes in addition to the solvation free energies. We have also obtained a complete thermodynamic description of the solvation of the associated alkane cavities. Our results lead to the following key observations: (i) The theoretical prediction that hydration entropy and solvent reorganization are weakly coupled to solute-solvent dispersion interactions is confirmed by computer simulations. (ii) The weak correlation between solute-solvent dispersion interaction energies with solute surface area explains the large relative solubilities of cyclic alkanes and the large difference between the free energy/surface area relations observed for gas to water transfer processes compared to processes involving conformational rearrangements. (iii) The work of cavity formation iri water is determined in about equal measure by unfavorable entropic and solvent reorganization energy effects. The findings obtained in this work have important implications for theories of hydrophobicity and suggest an approach to parametrize the free energies of apolar hydration and association.