From ionic aqueous solvation shell to bulk fluid: A structural-energetic stability problem

Abstract

The aqueous solvation shell structures for two ions, Li+ and F−, were obtained through Monte Carlo simulations of systems consisting of one ion immersed in water, using the four points transferable intermolecular potential (TIP4P). The clear definition of the tridimensional structure of the solvation shells of these ions, both constituted by four water molecules, permits the calculation of the solvent molecules’ energy in the immediate vicinity of the central ion. A pseudopotential of mean force and the mean energy per molecule were also calculated. The energy profiles are easily related to the formation and the stability of the interface ion–solvent molecules. The stability of the solvation shell is due to (i) an energy difference between the molecules in the solvation shell and in the bulk phase greater, in absolute value, than the thermal energy, and (ii) an energy barrier that separates the solvation shell from the bulk. In general, most of the cationic aqueous solvation shells are similar because the cations are small ions and because the water molecule dipole vector is oriented to the cation’s center. On the other hand, the anionic aqueous solvation shells are complex structures due to the larger size of the anions and due to the fact that the typical approximate collinearity anion–H–O perturbs strongly the solvent immediate neighborhood. These facts result in monolayer cationic and bilayers anionic solvation shells. These observations were confirmed by the structure of the solvation shells of several different ions and their energies.