How Does Solute-Polarization Affect the Hydrophobic Hydration of Methane?

Abstract

Abstract We simulate the solubility and solvation free energy of methane dissolved in water at infinite dilution. Molecular dynamics simulations of TIP4P-Ew model water are carried out at ambient pressure conditions over a large temperature interval, ranging from 250 K to 370 K. Solvation free energies are determined using the Widom particle insertion method. The fitted temperature dependent data is used to calculate solvation enthalpies, entropies, as well as the heat capacity of solvation. In particular we study the effect of polarizability of methane on those thermodynamic parameters. Solute polarization leads to a lowering of the solvation free energy at 298 K to 8.3 kJ mol−1, almost exactly matching the experimental value. A close inspection of the enthalpic and entropic contributions, however, reveals that this coincidence is a consequence of a compensation of enthalpic and entropic contributions, each of them deviating even larger from their respective experimental values. Surprisingly, the solute polarizability is apparently affecting the solvation entropy more strongly than the solvation enthalpy, leading to an about 5 J K−1 mol−1 smaller (less negative) solvation entropy compared to the non-polarizable model. The solute-water radial distribution functions of the polarizable particle reveals significant modifications, favoring small distances, as well as structural changes, very similar to those caused by a temperature increase. This is suggesting that the reduced negative solvation entropy of a polarizable methane particle is related to a more disordered, "high-temperature"-like hydration shell.