NUMERICAL SIMULATIONS OF SOLVATION IN SIMPLE POLAR FLUIDS - DEPENDENCE ON THE THERMODYNAMIC STATE BELOW AND ABOVE THE CRITICAL-POINT

The dependence of solvation dynamics on the thermodynamic state of the solvent is studied numerically for simple model polar solvents. The s olvent is described by the Stockmayer model, characterized by Lennard- Jones and dipolar intermolecular interactions. The solute-solvent coup ling is given by a nonpolar (Lennard-Jones) and, for a charged solute, by a charge-dipole interaction. We study thermodynamic states which a re representative of the liquid and vapor phases, of the neighborhood of the critical point, and of the supercritical region of the solvent. Statics and dynamics are studied by investigating equilibrium fluctua tions in the electrostatic potential induced by the solvent at the sol ute position and the fluctuations in the nonpolar part of the solute-s olvent interaction. The relaxation of these fluctuations corresponds, within linear response theory, to the dynamics of nonequilibrium solva tion, and the applicability of linear response can be glimmed from com paring the results obtained for charged and uncharged solutes. For a f ew selected thermodynamic states, we also simulate the corresponding n onequilibrium solvation, starting from either a neutral or a charged s olute. We find that both static and dynamical aspects of the solvation process are strongly affected by the density of the neat solvent. Eff ects of temperature are less pronounced. On lowering the solvent densi ty, the relaxation of dynamic fluctuations gets increasingly more depe ndent on the solute charge, i.e. the validity of a linear response des cription decreases. The main characteristics of the dynamics can be la rgely traced to aspects of static structure. In addition, the effect o f proximity to the critical point on the solvent static and dynamic re sponse is examined. (C) 1998 Elsevier Science B.V. All rights reserved .