Temperature and Concentration Effects on the Solvophobic Solvation of Methane in Aqueous Salt Solutions

We perform molecular dynamics (MD) simulations of aqueous salt (NaCl) solutions using the TIP4P-Ew water model (Horn et al., J. Chem. Phys. 2004, 120, 9665) covering broad temperature and concentration ranges extending deeply into the supercooled region. In particular we study the effect of temperature and salt concentration on the salvation of methane at infinite dilution. The salt effect on methane's salvation free energy, salvation enthalpy and entropy, as well as their temperature dependence is found to be semi-quantitatively in accordance with the data of Ben-Naim and Yaocobi (J. Phys. Chem. 1974, 78, 170). To distinguish the influence of local (in close proximity to ions) and global effects, we partition the salt solutions into ion influenced hydration shell regions and bulk water. The chemical potential of methane is systematically affected by the presence of salt in both sub volumes, emphasizing the importance of the global volume contraction due to electrostriction effects. This observation is correlated with systematic structural alterations similar to water under pressure. The observed electrostriction effects are found to become increasingly pronounced under cold (supercooled) conditions. We find that the influence of temperature and salt induced global density changes on the salvation properties of methane is well recovered by simple scaling relation based on predictions of the information theory model of Garde et aL (Phys. Rev. Let. 1999, 77, 4966).