Preferential solvation of Ca(2+) in aqueous solutions containing ammonia: A Molecular dynamics study

Ca2+ aqueous solutions containing different proportions of ammonia have been studied by means of molecular dynamics simulations. Previously developed ab initio effective pair potentials, in the framework of the polarizable continuum model, and only tested at a cluster computation level, have been employed to describe ion-ligand interactions. Structural and dynamic changes present in the neighborhood of the ion as a function of the ammonia concentration have been followed. Results show a preferential solvation for ammonia, even at very low concentrations. For the pure aqueous solution, calcium ion is coordinated by eight water molecules, while the presence of ammonia favors an equilibrium between an octa and enna-coordinated situation when this ligand becomes predominant, confirming the prediction of cluster calculations. However, the increase in the coordination number is followed by an intrinsic loss of stability for the identifiable solvated structures because of the larger tendency of ammonia to participate in solvent exchange phenomena. Solvent exchange events show, for the most simple case (water-water exchange), a marked mechanistic variety. (C) 2002 American Institute of Physics.