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ION–SOLVENT INTERACTIONS 85

Bockris in 1949 and by Frank and Evans in 1957. According to these workers, there
are two regions. One is in the first (and for polyvalent cations, the second) layer near
the ions, where water molecules are tightly bound and give rise to new frequencies.
Such waters accompany the ion in its movements in the solution.
There is also a broken-structure region outside the first one to two layers of water
molecules around the ion. Here the solvating waters are no longer coordinated, as in
the bulk, by other waters, because of the ion’s effect, but they are outside the primary
hydration shell, which moves with the ion. Such intermediate waters, though partly broken
out of the bulk water structure, do not accompany an ion in its diffusional motion.
Studies consistent with these ideas were first performed by Walfren in 1971 in
H 2O–D 2O mixtures. Concentrations of 1 to 4 mol were employed to get
measurable effects. Thus, at 4 mol some 40–50% of the water present is at any
moment in the primary hydration sheath!
Intermolecular effects in ionic solutions can also be studied in the Raman region
between 200 and 1000 Librational modes of water show up here. The intensity
of such peaks changes linearly with the ionic concentration. The bending mode in
the Raman spectra of alkali halides in water was studied by Weston. They may also
be interpreted in terms of models of primary hydration (water staying with the ion in
motion) and a secondary disturbed region.
Raman spectra can also be used to determine the degree of dissociation of some
molecules, namely, those that react with the solvent to give ions (e.g., HC1). If the
Raman frequency shifts for the dissociated molecules are known, then they can be used
to calculate equilibrium constants at different temperatures. Then, once the tempera-
ture coefficient of the equilibrium constant K is known, one can determine and
of the dissociation reaction (Section 2.13).
The study of the ion falls into the category of Raman studies that concentrate
on interpreting spectra caused by the solute. It illustrates the use of Raman spectra to
give structural conclusions via the study of symmetry. The free ion should have
what is termed “ symmetry” and give rise to three Raman bands corresponding to two
degenerate asymmetric stretching modes and and one symmetric stretching mode.
Irish and Davis studied the effect of solvation on the spectra of this ionand
the splitting of the band. They found that H bonding removes the degeneracy of this
mode. The symmetry change would be from to and this is interpreted to mean
that hydration effects have brought about a nonequivalence of the O atoms in a
most unexpected effect.

2.11.6. Information on Solvation from Spectra Arising from
Resonance in the Nucleus
It has been often suggested that nuclear resonance might be used to gain informa-
tion in solvation studies. Thus, in hydroxylic solvents, electron shielding around the
proton should be affected by ions and thus, in terms of changes in nuclear resonance
frequencies, solvation-bound water and free water should be distinguishable. It turns