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ION–SOLVENT INTERACTIONS 201

2.27. OVERVIEW OF IONIC SOLVATION AND ITS FUNCTIONS

2.27.1. Hydration of Simple Cations and Anions

The first thing that was dealt with in this chapter was the fundamental question
of how things dissolve. Materials have stability in the solid lattice and therefore in the
solution there must be energies of interaction that compensate for the lattice energies
and thus make it comfortable for the ion to move into the solution. This energy of
interaction of solvent and ion is called the solvation energy. Solvation and the
corresponding water-related hydration are fields having a great breadth of application
and indeed one can even see a relevance to environmental problems such as acid rain
because the pH of natural waters depends on the stabilization of certain ions in solution,
and this depends primarily upon their solvation energies.
It is useful to divide the methods of investigating hydration into four groups. First
of all, there are the many methods in which one looks at the ionic solution in the
equilibrium state; for example, one can study its compressibility or one can study heats
and entropies of hydration and fit the values to various model systems.
Another approach involves measuring the transport of the ion concerned under
an applied electric field, its mobility. This approach has a unique aspect: it provides
the individual properties of ions directly so long as they are in dilute solution.
Spectra of all kinds (infrared, Raman, nuclear magnetic resonance, neutron
diffraction) have been applied to electrolytic solutions for a long time, and the contrast
between the indications from such measurements (which involve the grave disadvan-
tage of having to be carried out at high concentrations) and those of other (concordant)
methods of investigation (usually carried out in dilute solutions to diminish the effects
of ion–ion interactions) in measuring solvation numbers is troubling. It seems likely
that the spectroscopic results report the much smaller values obtained in solvation
studies at higher ionic concentrations (i.e., lower water concentration). The thermo-
dynamic methods (e.g., partial molar volumes and the Debye vibrational potential)
give much higher results than the spectroscopic methods because they measure the
situation at high concentrations of water (i.e., low ionic concentrations) or dilute
solutions. However, increasingly neutron diffraction and MD methods are being able
to provide information on the time spent by water molecules in the first and second
layer near an ion. By comparing these times with those known for the time an ion
spends between its movements in solution, it is becoming increasingly possible to use
spectroscopic methods (particularly the recent neutron diffraction methods) to make
a distinction between coordination numbers and numbers of waters which travel with
the ion, the hydration number.
Finally, there is the theoretical method of approaching ionic solvation including
the molecular dynamics simulations. These have become increasingly used because
they are cheap and quick. However, MD methods use two-body interaction equations
and the parameters used here need experimental data to act as a guide for the
determination of parameters that fit.