Inhomogeneous Fluid Approach to Solvation Thermodynamics. 1. Theory

Abstract

The solvation energy and entropy at infinite dilution consist of a solute−solvent term and a solvent reorganization term representing the contributions of changes in solvent structure upon solute insertion. In the standard, homogeneous treatment of solutions, changes in solvent structure are expressed through derivatives of the homogeneous pair correlation function, which are very difficult to obtain by simulation. Tractable expressions for the solvation energy and entropy are here derived by viewing the solution as an inhomogeneous system with the solute fixed at a certain point. The solvent reorganization terms in the inhomogeneous view contain two contributions: the local, “correlation” contributions, which are due to correlations between the solute and the solvent and dominate at high densities, and the nonlocal, “liberation” contributions, which are due to the effective dilution of the solvent caused by the thermal motion of the solute and dominate at low densities. The liberation contributions are independent of the nature or size of the solute and depend only on the properties of the solvent. For common liquid solvents the nonlocal terms are negligible and the solvation properties arise almost entirely from effects localized around the solute. The new expressions are tested by calculations of the solvent reorganization energy and entropy in ideal hard-sphere and Lennard-Jones mixtures (solute identical to solvent). The solvent reorganization energy and entropy make distinct and significant contributions to the solvation free energy. The theory can be applied to truly inhomogeneous systems as well as small solute solvation, thus providing a connection between interfacial phenomena and microscopic solvation.