Page 103 - MODERN ELECTROCHEMISTRY

46 CHAPTER 2

Knowing the distribution functions, g(r), as a function of r from experiment or
calculation from a central particle, it is possible to calculate how the water molecules
spread out from a central particle. With knowledge of this function, it is possible to
calculate various properties of liquid water (e.g., its compressibility), and these can
then be compared with experimental values as a test of the calculation.

2.4.1. How Does the Presence of an Ion Affect the Structure of
Neighboring Water?

The aim here is to take a microscopic view of an ion inside a solvent. The central
consideration is that ions orient dipoles. The spherically symmetrical electric field of
the ion may tear water dipoles out of the water lattice and make them point (like
compass needles oriented toward a magnetic pole) with the appropriate charged end
toward the central ion. Hence, viewing the ion as a point charge and the solvent
molecules as electric dipoles, one obtains a picture of ion–dipole forces as the principal
source of ion–solvent interactions.
Owing to the operation of these ion–dipole forces, a number of water molecules
in the immediate vicinity of the ion (the number will be discussed later) may be trapped
and oriented in the ionic field. Such water molecules cease to associate with the water
molecules that remain part of the network characteristic of water (Section 2.4.3). They
are immobilized except insofar as the ion moves, in which case the sheath of
immobilized water molecules moves with the ion. The ion and its water sheath then
become a single kinetic entity (there is more discussion of this in Section 2.4.3). Thus,
the picture (Fig. 2.11) of a hydrated ion is one of an ion enveloped by a solvent sheath
of oriented, immobilized water molecules.
How about the situation far away from the ion? At a sufficient distance from the
ion, its influence must become negligible because the ionic fields have become

Fig. 2.11. An ion envel-
oped by a sheath of ori-
ented solvent molecules.