Solute-solute potential of mean force in simple fluids at infinite dilution

The effects of solvent density on the solute-solute potential of mean force in Lennard-Jones fluids are studied by means of the integral equation method. The solvent density plays an important role in solute-solute correlation in solution: the effect of the solute-solvent attractive interaction is quite different in the medium-density and high-density regions. In the latter the strong solute-solvent attractive interaction destabilizes the contact solute pair, and the strong solvent-solvent attractive interaction stabilizes the contact solute pair, which is expected from both ‘desolvation’ and the ‘solvophobic’ effect. In the medium-density region, however, curious phenomena are observed that cannot be interpreted by a simple picture: for example, the solute-solvent attractive interaction stabilizes both the solvent-separated pair and the contact pair of solute molecules under some conditions. The characteristics of the medium-density region are discussed in terms of the density fluctuation around the solute molecule.