Solvent size vs cohesive energy as the origin of hydrophobicity.

Two main physical explanations of hydrophobicity seem to be currently competing. The classical, intuitive view attributes it to the fact that interactions between water molecules are much stronger than those between water and nonpolar groups. The second, "heretical" view attributes it to the small size of the water molecule which increases the entropic cost of opening up a cavity to accommodate the solute. Here we examine the solvation of methane in water and in model liquids that lack one or more of water's properties and report a detailed decomposition of the solvation free energy, enthalpy, entropy, and heat capacity in these solvents. The results fully support the classical view. It is found that fluids with strong intermolecular interactions favor expulsion of methane to its pure phase or to CCl(4), whereas fluids with weak intermolecular interactions do not. However, the specific thermodynamic signature of the hydrophobic effect (entropy driven at room temperature with a large heat capacity change) is a result of the hydrogen-bonding structure of water.