Page 104 - MODERN ELECTROCHEMISTRY

ION–SOLVENT INTERACTIONS 47

attenuated to virtually zero. The normal structure of water has been re-attained: it is
that of bulk water.
In the region between the solvent sheath (where the ionic influence determines
the water orientation) and the bulk water (where the ionic influence has ceased to
dominate the orientation of water molecules), the ion still has some orienting influence
on the water network: it tries to align the water dipoles parallel to the spherically
symmetrical ionic field, and the water network tries to convert the water in the
in-between region into a tetrahedral arrangement (Fig. 2.12). Caught between the two
types of influences, the in-between water adopts some kind of compromise structure
that is neither completely oriented nor yet fixed back into the undisturbed water
structure shown in Fig. 2.10. The compromising water molecules are not close enough
to the ion to become oriented perfectly around it, but neither are they sufficiently far
away from it to form part of the structure of bulk water; hence, depending on their
distance from the ion, they orient out of the water network to varying degrees. In this
intermediate region, the water structure is said to be partly broken down.
One can summarize this description of the structure of water near an ion by
referring to three regions (Fig. 2.12). In the primary, or structure-enhanced, region
next to the ion, the water molecules are oriented out of the water structure and
immobilized by the ionic field; they move as and where the ion moves. Then, there is
a secondary, or structure-broken (SB), region, in which the normal bulk structure of

Fig. 2.12. The neighborhood of an ion may be consid-
ered to consist of three regions with differing solvent
structures: (1) the primary or structure-forming region, (2)
the secondary or structure-breaking region, and (3) the
bulk region.