First-Principles Investigation on Ionization Strength, Volume Expansion, and Water Rotational Rigidity of Small Water Cluster Systems Formed around Sodium(I), Calcium(II), and Iron(II) Ions

Abstract

Some ionic effects on small water cluster systems formed around sodium(I), calcium(II), and iron(II) cations have been investigated using the density functional theory. By assuming that the numbers of water molecules in the first and the second water layers are 6 and 12, respectively, it is shown that (i) the Ca(II) aqueous cluster shrinks and its volume becomes similar to that of a pure (H 2 O) 18 cluster whereas the Fe(II) and Na(I) aqueous clusters expand; (ii) owing to the water dipole-dipole interactions induced by the ion in the second water layer binding, the ionization strength of the Ca(II) aqueous cluster is close to that of Fe(II) but sufficiently higher than that of Na(I); (iii) the isotropicity of s-type Ca(II) and Na(I) cation orbitals as the cause of the reduction in water rotational rigidity in the ion-water bonding has been clarified by analyzing the charge transfer and non interacting kinetic energy. By considering the three ionic effects, we predict that the Ca(II) ion is one of the more competitive water cationic impurities in the PEMFC membrane.

The effects of charge density distributions of metal ions on single water molecule dissociation of [M(H2O)6]3+ systems

“…A relevant quantity of molecular bindings and reliability of calculations will be introduced first. Recalling density functional theory (DFT) study, especially for electron population [6][7][8][9][10] and energetic analysis [11][12][13], the intermolecular binding and structural behaviour are substantially influenced by electron distributions inside the observed system [14,15]. A cation-dipole (water) binding interaction can be simply expressed using a classical electrostatic formulation [15]:…”

“…In equation (1), it is assumed that the change of the other energy contributors such kinetic energies are negligible in the ion-dipole interaction. Another interaction (reaction) energy descriptor is given with total energies of reactants and products obtained by first-principles DFT calculations energy as follows [14]: 6 ] is the total energy of the cationic water cluster. Although (2) does not provide information about local ion-dipole interaction such as the electrostatic approach in (1), but by ignoring inter-neighbour water interactions, we may obtain an average value for binding energy between the water molecules and the metal ion.…”