Page 231 - MODERN ELECTROCHEMISTRY

ION–SOLVENT INTERACTIONS 167

structure is not the same after an ion has entered it near the ion. Some of the water
molecules are wrenched out of the quasi-lattice and appropriated by the ion as part of
its primary solvation sheath. Further off, in the secondary solvation sheaths, the ions
produce the telltale effects of structure breaking.
What happens if, in addition to ions and water molecules, molecules of nonelec-
trolytes are also present in the system? Or what will occur if ions are added to a solution
already saturated with nonelectrolyte molecules?
One thing is certain: the fact that the ion takes the water out of circulation for a
time means that there will be on average less free water to dissolve the nonelectrolyte.
The nonelectrolyte molecules will find themselves suddenly having less water to
associate with, and some of them will shun the loneliness imposed by the water’s
preference for the newly added ions and find it energetically favorable to go back to
their parent lattice, i.e., precipitate out. This is the origin of a term from organic
chemistry—salting out. It means causing a nonelectrolyte to precipitate out of a
solution by adding an electrolyte to it to draw off available solvent molecules.
Occasionally, however, the ions are deviants and associate preferentially with the
nonelectrolyte solute, shunning the water (hydrophobic effects). In the rare instances
where these deviants appear, there is a rapid departure of the nonelectrolyte from the
parent lattice and the solubility of the former is enhanced rather than decreased. The
phenomenon is called salting in.
Two aspects of the theory of salting out are considered below. First, the effects
of the primary solvation sheath have to be taken into account: how the requisition of
water by the ions causes the nonelectrolyte’s solubility to decrease. Second, the effects
of secondary solvation (interactions outside the solvation sheath) are calculated. The
two effects are additive.

2.20.2. Change in Solubility of a Nonelectrolyte Due to Primary
Solvation
It is easy to calculate this change of solubility so long as there are data available
for the solvation numbers of the electrolyte concerned. Let it be assumed that the
normal case holds true and that ions are solvated entirely by water molecules (i.e., the
organic present is pushed out). Then recalling that 1 liter of water contains 55.55 moles,
the number of water molecules left free to dissolve nonelectrolytes after the addition
of ions is 55.55 - where is the solvation number of the electrolyte concerned.
Assuming that the solubility of the nonelectrolyte is simply proportional to the
number of water molecules outside the hydration sheath, then

where is the solubility of the nonelectrolyte before the addition of electrolyte and
S that after it.