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

摘要

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_Mld_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.