Solvation structures and dynamics of alkaline earth metal halides in supercritical water: A molecular dynamics study

•Stability of contact ion pairs (CIPs) increases with decrease in cation size.•For MgI2, solvent shared ion pair is more stable compared to CIP.•The hydration numbers of cations increase with the size of cations.•Orientational distribution functions of solvent molecules are sharper for smaller ions.

Constrained molecular dynamics simulations of alkaline earth metal halides have been carried out to investigate their structural and dynamical properties in supercritical water. Potentials of mean force (PMFs) for all the alkaline earth metal halides in supercritical water have been computed. Contact ion pairs (CIPs) are found to be more stable than all other configurations of the ion pairs except for MgI2 where solvent shared ion pair (SShIP) is more stable than the CIP. There is hardly any difference in the PMFs between the M2+ (M = Mg, Ca, Sr, Ba) and the X− (X = F, Cl, Br, I) ions whether the second X− ion is present in the first coordination shell of the M2+ ion or not. The solvent molecules in the solvation shells diffuse at a much slower rate compared to the bulk. Orientational distribution functions of solvent molecules are sharper for smaller ions.

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