Multiscale Approaches and Perspectives to Modeling Aqueous Electrolytes and Polyelectrolytes - Multiscale Molecular Methods in Applied Chemistry - page 255

Clusters
Liquid
Fig. 1 Average molecular dipole of water. Left part for clusters of one molecules up to the typical
coordination number of the specific ion. Right part , average molecular dipole of water molecules
in the first solvation shell of the ion in a liquid system for three different system sizes of 32, 64, and
128 water molecules. Figure adapted from [ 31 ]
one molecule. These numbers clearly show that the direct ion-water interaction is
not as dominant as expected. Furthermore, the comparison of the left side of Fig. 1
with the right side shows that, when the solvation shell is embedded into a liquid
system, the average dipole moment of the molecule in this shell converges to the
average value of bulk water, 3.0 Debye, independently from the size and the charge
of the ion. The results of Fig. 1 lead to the conclusion that water-water interaction
actually plays the principal role in the overall charge displacement of the molecules
in the first solvation shell of an ion. There are two effects that contribute to this
scenario: (1) the interaction among molecules in the solvation shell via the electro-
static repulsion between the electron clouds and (2) the interaction of those mole-
cules with the rest of the system (bulk), which creates a hydrogen bond network and
thus capture electron charge along these bonds. However, the presence of an ion
in water should still have clear effects because it perturbs the typical structural
order and density fluctuations of a pure liquid system. Although the absolute
value of the molecular dipole in the first solvation shell is mainly determined by
water-water interaction, it is expected that water molecule are oriented with
their dipoles collinear with the ion-oxygen direction and the question is whether
this ion-induced order extends to only the molecules in the first solvation shell or, at
least in part, also goes beyond.
Figure 2 shows the orientation of the molecular dipole with respect to the
oxygen-ion direction for different solvation shells. A clear signature of the orienta-
tion of the dipole along the ion-oxygen distance is represented by the peak at zero
degrees of the distribution for divalent ions, while the same is less evident, though