Pair-correlation entropy of hydrophobic hydration: Decomposition into translational and orientational contributions and analysis of solute-size effects

We develop an efficient method to evaluate the translational and orientational contributions to the solute-water pair-correlation entropy that is a major component of the hydration entropy. A water molecule is modeled as a hard sphere of diameter d(S)=0.28 nm in which a point dipole and a point quadrupole of tetrahedral symmetry are embedded. A hard sphere of diameter d(M), a hydrophobic solute, is immersed at infinite dilution in the model water. The pair-correlation entropy is decomposed into the translational and orientational contributions in an analytical manner using the angle-dependent Ornstein-Zernike integral equation theory. The two contributions are calculated for solutes with a variety of sizes (0.6 <= d(M)/d(S)<= 30). The effects of the solute-water attractive interaction are also studied. As d(M) becomes larger, the percentage of the orientational contribution first increases, takes a maximum value at d(M)=D-M (D-M/d(S) depends on the strength of the solute-water attractive interaction and is in the range of 1.4-2), and then decreases toward a limiting value. The percentage of the orientational contribution reduces progressively as the solute-water attractive interaction becomes stronger. The physical origin of the maximum orientational restriction at d(M)=D-M is discussed in detail. (c) 2006 American Institute of Physics.