Car-Parrinello molecular dynamics simulations of CaCl2 aqueous solutions

Car-Parrinello molecular dynamics (CPMD) simulations are used to investigate the structural properties of 1 and 2 molal (m) CaCl2 aqueous solutions and, in particular, the radial distribution functions, coordination numbers, and dipole moments of water molecules in the first solvation shell. According to these simulations, the first solvation shell of the Ca2+ ion consists of six water molecules, that are characterized by an increased averaged dipole moment compared to that of bulk water, and a first-shell Ca-O radial distribution function peak at 2.39 angstrom. The results are compared to those of CPMD simulations of Ca2+ (no counterions), and no significant differences are found. This indicates that the homogeneous neutralizing background charge density implicitly included in simulations of non-neutral systems appropriately mimics the presence of the counterions (at least in terms of reproducing the solvation structure properties and for the box sizes considered). Classical molecular dynamics (MID) simulations of aqueous Ca2+ using varying box sizes confirm this suggestion.. The CPMD simulations at 2 m concentration also reveal additional possibilities for the structural arrangement of water molecules and chloride ions around Ca2+. In particular, they support the stability of Ca2+-Cl-(contact) and Ca2+-H2O-Cl- (solvent-separated) ion pairs. In addition, the solvent-separated cation pair is found to occur in a deprotonated Ca2+-OH--Ca2+ form. The existence of such a species has, to our knowledge, never been invoked previously to account for experimental data on CaCl2 solutions.