| Abstract
| - We model the aqueous solvation of a nonpolar solute as a function of its radius. We use a simplified statisticalmechanical model of water, the Mercedes Benz (MB) model, in NPT Monte Carlo simulations. This modelhas previously been shown to predict qualitatively the volume anomalies of pure water and the free energy,enthalpy, entropy, heat capacity, and volume change for inserting a nonpolar solute into water. We find avery different mechanism for the aqueous solvation of large nonpolar solutes (much larger than a water) thanfor smaller solutes. Small solute transfer involves a large hydrophobic heat capacity; its disaffinity for coldwater (room temperature or below) is due to the ordering of the neighboring waters (entropic), while itsdisaffinity for hot water is due to the breaking of hydrogen bonds among the neighboring waters (enthalpic).In contrast, transferring large nonpolar solutes into water involves no such large changes in heat capacity orentropy. In this regard, large nonpolar solutes are not “hydrophobic”; their solvation follows classical regularsolution theory. Putting a large nonpolar surface into water breaks hydrogen bonds at all temperatures. Therefore,the traditional “iceberg” model that first-shell water structure melts out with temperature should not apply tolarge solutes. These results also explain why the free energy of creating an oil/water interface (75 cal Å-2mol-1) is greater than threefold for small molecule transfers (25 cal Å-2 mol-1). A key conclusion is thathydrophobicity depends not only on the surface area of a solute but also on its shape and curvature.
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