Solvation structures and dynamics of the magnesium chloride (Mg(2+)-Cl(-)) ion pair in water-ethanol mixtures

We have performed constrained molecular dynamics simulations of magnesium chloride in water-ethanol mixtures. From the potentials of mean force (PMFs) of the Mg(2+)-Cl(-) ion pair, we notice that, as the mole fraction of ethanol increases, the depths of the minima of the contact ion pair (CIP) and solvent assisted ion pair (SAIP) increase, but the depth of the CIP minimum increases more in comparison to the SAIP minimum. This shows that ion pairing becomes more favorable with an increase in the mole fraction of ethanol. Significant differences in the PMFs between the Mg(2+) and the Cl(-) ion (depending upon whether the second Cl(-) ion is present in the first coordination shell of the Mg(2+) ion or not) seem to have been reported for the first time in this work. The local mole fraction of water molecules in the first solvation shell of ions is generally greater than in the bulk. The diffusional behavior of solvent molecules in solvation shells of the ion-pair indicates that the ions as well as the first solvation shells of the ions diffuse at much slower rates. Also, the diffusion constant of bulk water in the mixtures is greatly reduced compared to the pure solvent value.