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ION–SOLVENT INTERACTIONS 131
Inserting numerical values for at 298.16 K becomes
Substituting from Table 2.17 for for the SCW molecules near ions
can be calculated. The quantity represents the sum of the ion–dipole, ion–quadru-
pole, and ion–induced dipole interactions.
In the computation of the rotational entropies of SCW and NSCW near an ion,
the rotation is restricted to libration about the axis perpendicular to the dipole. The
third rotation, i.e., about the dipole axis, does not change the orientation of the dipole
and may be better calculated as if it were a free rotation. The partition function for this
is
where is the moment of inertia of water molecules about the dipole axis and is
1.9187 × Using this partition function in the general equation for entropy
in terms of partition function, one calculates
Thus,
and
The term can be calculated from
and therefore
where
