Super- and subcritical hydration of nonpolar solutes. I. Thermodynamics of hydration

Hydration thermodynamics of nonpolar solutes in high-temperature water is investigated by computer simulations. The excess chemical potentials of the methane and hard sphere solutes are evaluated over a wide range of density and temperature, and the thermodynamic origin of the enhanced affinity of the nonpolar solute for super- and subcritical water is identified. It is shown that when the density is medium to high in the high-temperature conditions, the enhanced affinity results from the elevated temperature and represents the nonspecific aspect of super- and subcritical water. The excess chemical potentials are further decomposed into the enthalpic and entropic components. It is found that when the system is moved from the ambient state to a high-temperature state, the accompanying change is unfavorable for the enthalpic component and is favorable for the entropic component. The thermodynamics of cavity formation is also pursued in connection to the size distribution of cavities in pure solvent water. The utility of the scaled-particle theory is then demonstrated over a wide range of thermodynamic conditions, and the effective diameter of the water molecule is assigned within the framework of the scaled-particle theory. (C) 2000 American Institute of Physics. [S0021-9606(00)50418-8].