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ION–SOLVENT INTERACTIONS 173

manufacture upon salting in. However, the salting in with which they are associated
is not the rare deviant phenomenon arising when the dipole moment of the nonelec-
trolyte is greater than that of water. It possesses the characteristic of always being
associated with organic electrolytes in which the ions are large. Here, even when water
has a dipole moment greater than that of the nonelectrolyte, which would be expected
to give salting out, salting in occurs.

2.20.5. Cause of Anomalous Salting In

The picture given above seems satisfactory as a first approximation and for dilute
solutions, but it has this one disturbing feature; namely, there are situations where
theory predicts salting out but experiment shows salting in. In such cases, the theory
seems to favor ions being surrounded by water whereas in fact they move away from
water (hydrophobic effects). Now, it will be recalled that only ion–dipole forces have
been reckoned with in treating the interactions among the particles populating the
primary solvation shell. Thus, the ion–water and ion–nonelectrolyte forces have been
considered to be of the ion–dipole type of directional forces which orient polar
particles along the ionic field. Perhaps this restriction is too severe, for there are also
nondirectional forces, namely, dispersion forces.
These dispersion forces can be seen classically as follows: The time-average
picture of an atom may show spherical symmetry because the charge due to the
electrons orbiting around the nucleus is smoothed out in time. However, an instanta-
neous picture of, say, a hydrogen atom would show a proton “here” and an electron
“there”—two charges separated by a distance. Thus, every atom has an instantaneous
dipole moment; of course, the time average of all these oscillating dipole moments is
zero.
Now, an instantaneous dipole in one atom will induce an instantaneous dipole in
a contiguous atom, and an instantaneous dipole–dipole force will arise. When these
forces are averaged over all instantaneous electron configurations of the atoms and
thus over time, it is found that the time-averaged result of the interaction is finite,
attractive, and nondirectional. Forces between particles that arise in this way are called
dispersion forces.
Dispersion forces give rise to an interaction energy in which the potential energy
of interaction varies as where r is the distance between the centers of the two
substances interacting. Thus, the equation for the dispersive energy of interaction may
be written as where is a constant independent of r. The rapid decrease of such
forces with increase of distance from the origin makes it unnecessary to consider
dispersion interactions outside the primary solvation shell; by then, they have already
decreased to an extent that they no longer warrant consideration. Inside the primary
hydration sheath, the dispersion interaction can be treated in the same way as the
ion–dipole interaction. That is, in the replacement of a water molecule by a nonelec-
trolyte molecule, one must take into account not only the difference in ion–dipole