A simple molecular thermodynamic theory of hydrophobic hydration

A recently developed microscopic model for associating fluids that accurately captures the thermodynamics of liquid water [Truskett et al., J. Chem. Phys. 111, 2647 (1999)] is extended to aqueous solutions with nonpolar species. The underlying association model incorporates the highly directional and open nature of water’s hydrogen-bond network, and, as a result, captures a number of the distinguishing properties of liquid water, such as the density anomaly. The model for aqueous mixtures developed herein predicts many of the thermodynamic signatures of hydrophobic hydration without resorting to empirical temperature-dependent parameters. The predicted solubility of nonpolar species is slight over a wide range of temperatures, and exhibits a minimum as a function of temperature, in accord with experiment. Hydration is opposed by a dominant entropy and favored by the enthalpy at low temperatures. At elevated temperatures these roles are reversed. Furthermore, the hydration entropies for hydrophobes of varying size converge over a very narrow temperature range. Comparison with experimental and simulation data for nonpolar solutes in water shows that the theory tends to exaggerate the solute’s transfer heat capacity at low temperature, and hence solubility minima and entropy convergence are predicted to occur at lower temperatures than observed. Our results support the emerging view that hydrophobic effects can be attributed in large part to the equation of state for pure water.