Numerical simulations of solvation in simple polar fluids: dependence on the thermodynamic state below and above the critical point Original Research Article

The dependence of solvation dynamics on the thermodynamic state of the solvent is studied numerically for simple model polar solvents. The solvent is described by the Stockmayer model, characterized by Lennard-Jones and dipolar intermolecular interactions. The solute–solvent coupling is given by a nonpolar (Lennard-Jones) and, for a charged solute, by a charge-dipole interaction. We study thermodynamic states which are 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 fluctuations in the electrostatic potential induced by the solvent at the solute position and the fluctuations in the nonpolar part of the solute–solvent interaction. The relaxation of these fluctuations corresponds, within linear response theory, to the dynamics of nonequilibrium solvation, and the applicability of linear response can be glimmed from comparing the results obtained for charged and uncharged solutes. For a few selected thermodynamic states, we also simulate the corresponding nonequilibrium solvation, starting from either a neutral or a charged solute. We find that both static and dynamical aspects of the solvation process are strongly affected by the density of the neat solvent. Effects of temperature are less pronounced. On lowering the solvent density, the relaxation of dynamic fluctuations gets increasingly more dependent on the solute charge, i.e. the validity of a linear response description decreases. The main characteristics of the dynamics can be largely traced to aspects of static structure. In addition, the effect of proximity to the critical point on the solvent static and dynamic response is examined.