Page 176 - MODERN ELECTROCHEMISTRY

116 CHAPTER 2

Fig. 2.37. Models for the region near an ion. (Reprinted from J. O.’M. Bockris and P.
P. S. Saluja, J. Phys. Chem. 76: 2298,1972.)

The dynamic solvation number (as distinguished from the static coordination
number) is the number of these water molecules that remain with the ion for at least
one diffusive movement. When an ion arrives at a new site, it may remain there long
enough to influence a number of the surrounding water molecules to come out of the
water structure and become part of the primary solvation shell that moves with the ion.
Conversely, it may remain at a given site for a time so brief that it does not have an
effect on the waters that are relatively stable and fixed tightly in the water structure
that surrounds it. In the solvent structure these latter may still be thought of as part of
the coordinating waters of the ion, but their dipoles have not had sufficient time to
rotate into the attractive position such as that of the solvationally
coordinated waters represented by the letters SCW.