Quasichemical Lattice Fluid Model for Water and Hydrophobic Solvation

Abstract

A quasichemical lattice fluid model for water and mixtures of water with nonpolar solutes is presented. The model provides an analytic expression for the free energy, from which additional thermodynamic functions including the equation of state and the chemical potentials of water and solute are derived. The volumetric behavior predicted by the model is similar to that of real water. For example, it exhibits a density maximum. The model also yields the signature features of hydrophobic solvation, including low solubility, negative entropy of solvation in cold water, and positive heat capacity of solvation. Key features of water described by the model include a significant rotational contribution to the entropy resulting from the combinatorics of directed hydrogen bonding, and a temperature- and density-dependent ratio of intact vs broken hydrogen bonds. The unusual volumetric properties of water are attributed largely to the influence of free volume on the temperature-dependent rotational entropy. Similar...