Interaction between solute molecules in medium density solvents

Solvation properties of solutes in supercritical, medium density solvents h ave been analysed using hypernetted-chain theory with the emphasis on the s olvent-mediated interaction between solute molecules. The solvent and solut e molecules are Lennard-Jones particles, and the solute is present, at infi nite dilution. Also a pair of solute molecules separated by different dista nces has been considered using reference interaction site model theory. Mai nly, solvents at two typical densities (1.09 rho(c) and 2.91 rho(c); rho c is the critical density) that are in medium and high density regions, respe ctively, are treated. The temperature is set at 1.04T(c) (T-c is the critic al temperature). When the solute size is larger than the solvent size and t he strength of the solute-solvent attractive interaction is greater than th at of the solvent-solvent in the medium density region, the solvent structu re confined between a pair of solute molecules is largely different from th at near a single solute molecule. The confined solvent becomes denser and m ore stabilized as the distance between the solute molecules decreases, and an attractive interaction is induced between them. The interaction becomes even more attractive as the strength of the solute-solvent attractive inter action increases. The observations are qualitatively different from those i n the high density region. Another high density region, which is well below the critical temperature, has been considered, but the behaviour observed is similar to that in the high density region above the critical temperatur e.