Molecular dynamics study of water clusters containing ion pairs: From contact to dissociation

Abstract

We have studied the potential of mean force between pairs of monovalent ions immersed in water clusters composed of up to 64 molecules at 200 K using constrained molecular dynamics techniques. Two different Hamiltonians for the water particles were investigated: one has fixed-point charges while the other has induced atomic dipoles which explicitly introduce effects due to fluctuations in the electronic density of the molecules. The qualitative behaviors of both models present similarities. For the case of pairs of equally charged ions, the solvent reactive field introduces a net attraction between the ions that prevents the dissociation of the clusters over a wide range of interionic distances. Similar binding effects are found for neutral ion pairs where the solvent reinforces the ionic attraction when the interionic distance attains values comparable to the cluster size. The correct thermodynamic interpretation of the calculated averages is restricted to small interionic distances; beyond this range proper sampling of all relevant fluctuations is not possible. Polarization effects in the water Hamiltonian introduce significant changes in the equilibrium structures: the clusters exhibit less structure and present a lower degree of ionic solvation.