A theoretical study of like ion pairs in solution

Theoretical studies of ionic distributions in aqueous solutions have been carried out. The spatial pair distribution in ions in solution and the distribution of water near pairs of ions have been computed. Influences of solvent structure on ion pairing and the structural mechanisms of the pairing (or unpairing) process are investigated. We present a detailed picture of ionic distributions in solution which are compared with existing experimental structural studies such as those produced by neutron diffraction difference measurements and NMR experiments. The results of molecular dynamics simulations on the free energy of association for like-charged ions in water are presented. The influence of the solvent on the free energy surfaces (potentials of mean force) of Cl_-Cl_- and Na₊Na₊ is determined by using a dynamic trajectory in conjunction with a non-Boltzmann sampling approach for the interionic degree of freedom. The potential of mean force for the anionic pair C1_-C1_- in water reveals a minimum near ion contact due to the presence of several bridging water molecules, whereas the cationic Na₊Na₊ potential of mean force is found to be essentially repulsive. The results are in general agreement with site-site integral equation results on the same models.