Calculation of the potential of mean force for chloride-argon and sodium-argon pairs in water

As a model for understanding hydrophilic-hydrophobic interactions in aqueou s solutions, the potential of mean force for Cl-...Ar and Na+...Ar pairs in water was computed by molecular dynamics simulations. The Cl-...Ar potenti al is slightly modified by the solvent, with the appearance of al shallow s econdary minimum associated to,the stabilization of a solvent separated pai r in addition to the stable contact pair. The Na+...Ar potential is conside rably modified from the gas to the liquid. The depth of the attractive well is divided by a factor 3. At short distances, the repulsive part of the po tential is dominated by the solvent-induced electrostatic repulsion rather than by the direct Pauli repulsion. The difference of the solvation propert ies of the two pairs is explained by a simple electrostatic model and is at tributed to a size rather than a charge effect. (C) 2000 Elsevier Science B.V. All rights reserved.