Density and orientational structure of water around a hydrophobic solute: effects due to the solute size

The major concern is the structure of water around a hydrophobic solute. Effects due to the solute size are systematically investigated using the reference hypernetted-chain (RHNC) theory combined with the water model, a hard sphere in which a point dipole and a point quadrupole of tetrahedral symmetry are embedded. The structure exhibits drastic change when the solute size d(M)/d(S) (d(M) and d(S) are diameters of the solute and a water molecule, respectively) increases from 1 to 2. As the solute size increases further, the density structure converges to the planar-wall limit quite rapidly, while the convergence of the orientational structure is slower. The orientation of molecules neighboring the solute can be characterized by a mixture of the two opposite orientations unique to d(M)/d(S)=1 and to the planar-wall limit. They contribute to the structure almost equally for 2 < d(M)/d(S)< 3. As d(M) increases, the contribution from the orientation for the planar-wall limit becomes larger and it dominates for d(M)/d(S)>= 5. (c) 2004 Elsevier B.V. All rights reserved.