Solvation of Ca2+ in aqueous methanol-MD simulation studies

MD simulations have been performed for solutions of CaCl2 in water and aqueous methanol. Both solvents have been described by flexible models, whereas the ions were modelled as charged Lennard-Jones spheres. An analysis of solvation shells has been based on radial and angular distribution functions. In aqueous solution the calcium ion coordinates either 8 or 9 water molecules. Addition of methanol reduces the number of the coordinated water molecules and the Ca2+ ion becomes selectively solvated by methanol. The coordination shell of the Cl− ion, consisting of more than 10 water molecules, is poorly defined, but 6 of them form almost linear H-bonds with the anion. Addition of methanol does affect neither the coordination number nor the orientation of the water molecules in the shell of Cl− and the anion is preferentially hydrated. In aqueous solution the association of unlike ions occurs. Addition of methanol reduces the association of unlike ions, but induces an aggregation of cations and formation of multi-ion aggregates, which are preferentially solvated by methanol molecules. When the methanol concentration increases these multi-ion aggregates become a nucleus of phase separation.