Solvation free energies of alcohols in water: temperature and pressure dependences

Solvation free energies mu* of amphiphilic species, methanol and 1,2-hexanediol, are obtained as a function of temperature or pressure based on molecular dynamics simulations combined with efficient free-energy calculation methods. In general, mu* of an amphiphile can be divided into and , the nonpolar and electrostatic contributions, and the former is further divided into and which are the work of cavity formation process and the free energy change due to weak, attractive interactions between the solute molecule and surrounding solvent molecules. We demonstrate that mu* of the two amphiphilic solutes can be obtained accurately using a perturbation combining method, which relies on the exact expressions for and and requires no simulations of intermediate systems between the solute with strong, repulsive interactions and the solute with the van der Waals and electrostatic interactions. The decomposition of mu* gives us several physical insights including that mu* is an increasing function of T due to , that the contributions of hydrophilic groups to the temperature dependence of mu* are additive, and that the contribution of the van der Waals attraction to the solvation volume is greater than that of the electrostatic interactions. The three important contributions to the solvation free energy of alcohols in water are quantified as functions of temperature and pressure based the perturbation combining method and other step-wise methods.