Molecular model of hydrophobic solvation

The physical basis for the "hydrophobic effect" is studied using a simple s tatistical mechanical model of water, the "MB" model, in which water molecu les are represented as Lennard-Jones disks with hydrogen bonding arms. Usin g a four-state framework developed by Muller [Acc. Chem. Res. 23, 23 (1990) ], and extended by Lee and Graziano [J. Am. Chem. Soc. 118, 5163 (1996)], w e find the model reproduces the fingerprints of hydrophobicity, namely, the large positive heat capacity, and temperatures T-H and T-S at which the en thalpy and entropy of transfer, respectively, are zero. Further, the behavi or can be interpreted readily in terms of hydrogen bonds that are either ma de or broken in the bulk or in the first solvation shell around a nonpolar solute. We find that inserting a nonpolar solute into cold water causes ord ering and strengthening of the H bonds in the first shell, but that the rev erse applies in hot water. This provides a physical interpretation for the crossover temperatures T-H and T-S. (C) 1999 American Institute of Physics. [S0021-9606(99)51141-0].