Shape and size of simple cations in aqueous solutions: A theoretical reexamination of the hydrated ion via computer simulations

The simplest representation of monoatomic cations in aqueous solutions by m eans of a sphere with a radius chosen on the basis of a well-defined proper ty (that of the bare ion or its hydrate) is reexamined considering classica l molecular dynamics simulations. Two charged sphere-water interaction pote ntials were employed to mimic the bare and hydrated cation in a sample of 5 12 water molecules. Short-range interactions of trivalent cations were desc ribed by Lennard-Jones potentials which were fitted from ab initio calculat ions. Five statistically independent runs of 150 ps for each of the trivale nt spheres in water were carried out in the microcanonical ensemble. A comp arison of structural and dynamical properties of these simple ion models in solution with those of a system containing the Cr3+ hydrate ([Cr(H2O)(6)]( 3+)) is made to get insight into the size and shape definition of simple io ns in water, especially those that are highly charged. Advantages and short comings of using simple spherical approaches are discussed on the basis of reference calculations performed with a more rigorous hydrated ion model [J . Phys. Chem. B 102, 3272 (1998)]. The importance of nonspherical shape for the hydrate of highly charged ions is stressed and it is paradoxically sho wn that when spherical shape is retained, the big sphere representing the h ydrate leads to results of ionic solution worse than those obtained with th e small sphere. A low-cost method to generate hydrated ion-water interactio n potentials taking into account the shape of the ionic aggregate is propos ed. (C) 1999 American Institute of Physics. [S0021-9606(99)51703-0].