Monte Carlo simulations of M+Cl− (H2O)n (M = Li, Na) clusters and the dissolving mechanism of ion pairs in water

The dissolving mechanism of ion pairs in water was analyzed by means of radial distribution functions and the pair energies obtained from Monte Carlo simulations for M+Cl−(H2O)n (M = Li, Na) (n = 1–8) at room temperature. The minimum-energy geometries of these clusters for n ⩾ 3 are water-shared ion pairs. Although the interionic distances for Na+Cl− (H2O)n clusters are about 0.2 Å longer than those for Li+Cl− (H2O)n for n = 1–2, this reversed at n = 4. This trend originates because the water-shared structure for the Li+Cl− (H2O)n clusters suddenly becomes dominant at n = 4, whereas for the Na+Cl− (H2O)n clusters, the change is more gradual.