Hydrophobicity in a simple model of water: Entropy penalty as a sum of competing terms via full, angular expansion

The entropy penalty of solvation for nonpolar solutes dominates the hydroph obic effect at room temperature. We find that this entropy arises from a co mpetition between a relatively localized "two-body" term, and a contributio n arising from non-pairwise-decomposable three-body and higher-order terms. We use a full, angular dependent, expansion of solute-water correlation fu nctions over the full range of fluid temperatures for a two-dimensional mod el of water. This water model has been shown to capture many of the basic a nomalies of water and aqueous solutions of sparingly soluble nonpolar molec ules, including the volume anomalies of water and the thermal anomalies of the hydrophobic effect. Our results show that for hot liquid water, the two -body approximation is sufficient to estimate the transfer entropy, but in cold liquid water, which is the main regime for biological hydrophobic inte ractions, the two-body assumption substantially overestimates the degree of ordering in water. (C) 2001 American Institute of Physics.