Molecular dynamics simulations on aqueous solutions of rare gases

Molecular dynamics simulations for water and aqueous solutions of apolar rare gases have been carried out in order to study the dependence of hydration structures on solute species. The solute species under investigation were neon and xenon. The structures and thermodynamic properties relevant to the hydration processes were analyzed based upon the quenching of the system to local potential energy minima. It was found that the two solutes give rise to qualitatively different hydration structures; the introduction of Xe atoms make a significant water structure enhancement, increasing the population of pentagonal hydrogen bond network. The solution of neon with usual size parameter and combining rule failed to reproduce the experimentally observed properties. The structural change in quenched state was seen in the Ne solution where a somewhat larger size parameter was adopted. In normal mode analysis, the potential energy was expanded up to quadratic order around quenched configurations. Free energy and entropy changes in the hydration were evaluated for these aqueous harmonic systems. It was shown that a substantial part of the negative entropy change in hydration of apolar molecules experimentally observed arises from the differences in molecular rearrangement and anharmonic rather than harmonic motions.