The interfacial structure around ferric and ferrous ions in aqueous solution: The nature of the second hydration shell

The second ionic hydration shells of the Fe2+, Fe3+ cations are studied by Monte Carlo simulations using TIP4P water. The second hydration shell is constituted by about 13 water molecules for Fe2+ and 15 for the Fe3+ ion forming hydrogen bonds with the hydrogen atoms of the first shell. The basic average structure is described by a truncated tetrahedron. The water mean molar configurational energy indicates that the central ion attracts strongly the hydration molecules which themselves strongly repel each other so that the combination of the attractive and repulsive energies maintains the structure of the first shell and defines the structure of the second shell. The role of the second shell is to complete the transition between the very rigid highly ordered first shell and the bulk fluid, The solvation structures are determined in the first place by the structural tetrahedral properties of the water molecule so that the key of the hydration phenomenon is to be found mainly in the second shell.