Quasichemical lattice fluid model for water and hydrophobic solvation

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. Similarly, the unusual thermodynamics of hydrophobic solvation are attributed largely to the influence of nonpolar solutes on the temperature-dependent rotational entropy. Parameters in the model are chosen by least-squares fitting to experimental volumetric data. The description of pure water requires four parameters: the hydrogen-bond energy, the energy of non-hydrogen-bonded water-water contacts, the lattice contact number, and the volume of a water molecule. Solutes are described by solute-water contact energies and by parameters related to molecular volume and surface area.