Page 239 - MODERN ELECTROCHEMISTRY

ION–SOLVENT INTERACTIONS 175

are applicable and salting out is the norm, with salting in a rare exception. When the
dispersive interactions predominate, it is the other way around; salting in (and
hydrophobic effects) becomes the norm.
What factors of structure tend to make the dispersive forces dominate the situ-
ation? Clearly, they will be more likely to have a main influence upon the situation if
the nonelectrolyte is large (because then the distortion polarizability is large), but there
are many situations where quite large nonelectrolytes are still salted out. The dispersive
interaction contains the product of the polarizability of both the ion and the nonelec-
trolyte (or the ion and water, depending upon which interaction one is considering),
so that it is when both the ion and the nonelectrolyte are large (hence, both the are
large) that the dispersive situation is likely to dominate the issue, rather than the
ion–dipole interaction.
In accordance with this it is found that if one maintains the nonelectrolyte
and varies the ion in size, though keeping it of the same type, salting in
begins to dominate when the ion size exceeds a certain value. A good example is the
case of the ammonium ion and a series of tetraalkylammonium ions with increasing
size, i.e., where R is etc. Here, the salting in begins with the
methylammonium ion (its is evidently large enough), the ammonium ion alone
giving salting out. The degree of salting in increases with an increase in the size of the
tetraalkylammonium cation. Thus, the observations made concerning the salting in of
detergents, emulsions, and antibiotics by organic ions are, in principle, verified. An
attempt has been made to make these considerations quantitative.
This discussion of the effect of ions upon the solubility of nonelectrolytes is
sufficiently complicated to merit a little summary. The field is divided into two parts.
The first part concerns systems in which the dispersive interactions are negligible
compared with the dipole interactions. Such systems tend to contain relatively small
ions acting upon dissolved molecules. Here salting out is the expected phenomenon—
the solubility of the nonelectrolytes is decreased—and the reverse phenomenon of
salting in occurs only in the rare case in which the nonelectrolyte dipole moment is
greater than the dipole moment of the solvent. In the other group of solubility effects
caused by ions, the ions concerned tend to be large and because distortionpolarizability
increases with size, this makes the dispersive activity between these large ions and the
nonelectrolyte become attractive and dominate this situation so that the organic
molecule is pulled to the ions and the water is pushed out. Then salting in (solubility
of the nonelectrolyte increases) becomes the more expected situation.

2.20.6. Hydrophobic Effect in Sotvation
Solvation entropies (Section 2.15.13) are negative quantities. Since
this means that the disorder in solution (the entropy) is less than that of the ion

43
For example, it may be benzoic acid, which is considered a nonelectrolyte because it dissociates to a very
small degree.