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ION–SOLVENT INTERACTIONS 45

position at one instant and in the next instant it may become held as a unit of the
network. 5
In the 1970s and 1980s, calculational approaches (in addition to the X-ray studies)
were added to the tools for the attack on the structure of water. In the molecular
dynamics approach, classical mechanics is used to calculate the successive movements
of molecules in the structure. Such an approach is dependent on the correctness of the
equation that represents the energies of interaction between the particles. The basic
equation for these interactions is the “Lennard-Jones 6–12” potential.

The first term represents the attraction between two molecules and the second the
repulsion that also occurs between them.
One of the results of these calculations is that the number of water molecules in
an ordered structure near a given water molecule drops away rapidly from the original
molecule considered. The similarity of the liquid to solid ice does not remain too far
from a given water molecule, i.e., the long-range order present in the solid is soon lost
in the liquid.

5
Radial distribution functions are met along the path between the results of X-ray and neutron diffraction
examinations of water and the deriving of structural information, which is more difficult to do with a liquid
than with a solid. Radial distribution functions are, e.g.,

and can be seen as proportional to the intensity of the reflected X-ray beam as a function of the incident
angle, θ. Thus, in Eq. (2.1) sin The significance of g(r)dr allows one to calculate the
number of oxygen atoms between r and r + dr. In this way one can derive the number of nearest neighbors
in the liquid from any central O. The value is 4 for ice and, rather curiously, increases as the temperature
is increased (it is 4.4 at 286 K). This may be due to the disturbing effect of pure molecules, which
increase in number with temperature. Their presence would add to the intensity due to the regular pattern
and account for values greater than 4.2.