Aqueous Solvation of Methane from First Principles

Structural, dynamical, bonding, and electronic properties of water molecules around a soluted methane molecule are studied from first principles. The results are compatible with experiments and qualitatively support the conclusions of recent classical molecular dynamics simulations concerning the controversial issue on the presence of "immobilized" water molecules around hydrophobic groups: the hydrophobic solute slightly reduces (by a less than 2 factor) the mobility of many surrounding water molecules rather than immobilizing just the few ones which are closest to methane, similarly to what was obtained by previous first-principles simulations of soluted methanol. Moreover, the rotational slowing down is compatible with the one predicted on the basis of the excluded volume fraction, which leads to a slower hydrogen bond exchange rate. The analysis of simulations performed at different temperatures suggests that the target temperature of the soluted system must be carefully chosen, in order to avoid artificial slowing-down effects. By generating maximally localized Wannier functions, a detailed description of the polarization effects in both solute and solvent molecules is obtained, which better characterizes the solvation process.