Primitive model for cation hydrolysis: A molecular-dynamics study

A model of primitive cation M(Z+) in water is introduced in order to clarify the influence of ion charge on the hydration structure and dynamic properties of highly charged cations in aqueous solutions. A flexible nonconstrained model for water molecules is used. The considered model in the case of monovalent cation M(+) reduces to the realistic model for the hydration structure of Na(+). It is shown that for divalent ion M(2+) the strong cation-water electrostatic interaction leads to the formation of stable structures constituted by six water molecules octahedrally arranged around the cation. The cation-oxygen attraction and cation-hydrogen repulsion modify the octahedral hydration configuration of the model cations M(3+) and M(4+) and additional water molecules can join the hydration shell. The increase of cation charge results in the increase of O-H bond length of water molecules in the cation hydration shell. Further increase of ion-water electrostatic interaction causes the loss of some protons from the hydration shell of cations M(4+), M(5+), and M(6+) that is interpreted as a cation hydrolysis effect. For a correct description of this phenomenon the considered model is improved by modeling the effects of the charge redistribution between hydrolysis products, which essentially modified and stabilized the hydrated-hydrolyzed structure of cation. The influence of cation charge on dynamical properties of cation M(Z+) and oxygens in its hydration shell was investigated and analyzed.(c) 2005 American Institute of Physics.