Effect of solute size and solute-water attractive interactions on hydration water structure around hydrophobic solutes

Using Monte Carlo simulations, we investigated the influence of solute size and solute-water attractive interactions on hydration water structure arou nd spherical clusters of 1, 13, 57, 135, and 305 hexagonally close-packed m ethanes and the single hard-sphere (HS) solute analogues of these clusters. We obtain quantitative results on the density of water molecules in contac t with the HS solutes as a function of solute size for HS radii between 3.2 5 and 16.45 Angstrom. Analysis of these results based on scaled-particle th eory yields a hydration free energy/surface area coefficient equal to 139 c al/(mol Angstrom (2)), independent of solute size, when this, coefficient i s defined with respect to the van der Waals surface of the solute. The same coefficient defined with respect to the solvent-accessible surface decreas es with decreasing solute size for HS radii less than similar to 10 Angstro m. We also find that solute-water attractive interactions play an important role in the hydration of the methane clusters. Water densities in the firs t hydration shell of the three largest clusters are greater than bulk water density and are insensitive to the cluster size. In contrast, contact wate r densities for the HS analogues of these clusters decrease with solute siz e, falling below the bulk density of water for the two largest solutes. Thu s, the large HS solutes dewet, while methane clusters of the same size do n ot.