Temperature dependence of interactions of an ion pair in water: A molecular dynamics study

We present a comparative study of the potentials of mean force (PMF) and the related thermodynamic properties for Na+--Cl- ion pair using a rigid simple point charge (SPC/E) water model at two different temperatures, 298 and 373 K. The PMFs were calculated using the thermodynamic perturbation theory and the long-range interactions were evaluated using an Ewald summation technique. The PMFs for both temperatures display two minima corresponding to the contact and solvent-separated ion pairs; however, they are different in details. At room temperature, the solvent separated region is favored over the contact region. On the other hand, the calculated PMF at higher temperature indicate that the contact ion pair is more favorable. These observations are in general agreement with a recent Raman study of salt solutions of the similar systems. We also carried out a calculation at room temperature using the spherical truncation technique. The calculated PMF using this technique is somewhat more repulsive compared to the calculated PMF using an Ewald summation technique. However, these PMFs have similar characteristics. When comparing the present calculations with those reported in previous works on the same systems, our calculated PMF at room temperature is identical to the results of simulations using a flexible water model. This result suggests that the more stable solvent-separated ion pair at room temperature may not be due to the inclusion of the internal motions of water molecules. It is probably due to the balancing of the intermolecular forces