Temperature dependence of hydrophobic hydration and entropy convergence in an isotropic model of water

Abstract

We have investigated temperature dependence of hydrophobic hydration and molecular-scale density fluctuations in an isotropic single-site model of water originally devised by Head-Gordon and Stillinger [J. Chem. Phys. 98, 3313 (1993)] using Monte Carlo simulations. Our isotropic model of water, HGS water, has the same oxygen–oxygen radial distribution function as that of the simple point charge (SPC) water at room temperature and water density. For HGS water, we find that non-Gaussian occupancy fluctuations lead to cavity formation probabilities that are considerably lower than in SPC water. Wetting of a hard-sphere solute by HGS water is also found to be significantly greater than that by SPC water. These observations can be understood in terms of differences in Hamiltonians of the two water models. Despite these differences in the details of hydration, small hydrophobic solutes display many of the well-known thermodynamic finger prints of hydrophobic hydration once the variation of density with temperature, ρ(T), along the saturation curve of real liquid water is followed for HGS water. For the hydration of small solutes, the “entropy convergence” is observed at temperatures of ≈400 K. These observations emphasize that the phase behavior of liquid water contains crucial information regarding thermodynamics of solvation phenomena.