4
1 A Historical Review of the Structures of Water and Ice
1.2.1 Scattering of X-Rays by Liquid Water
Bernal and Fowler used X-rays to identify the local molecular environment of H 2 O
molecules in a condensed state. They assumed that water molecules have spherical
symmetry with a triangular arrangement of the hydrogen and oxygen nuclei. Five
characteristic points were chosen for this model of the water molecule based on earlier
spectroscopic data [10]. The oxygen was placed at the center of the sphere as shown in
Fig. 1.2. Two hydrogen atoms (protons) were placed in the y-z-plane about 1 Å from
the oxygen atom, and 1.5 Å from each other so that the H–O–H angle was about 105
◦ .
Two more points, which represent electron densities that compensate for the density
of the protons, were settled in the x-z-plane. This tetrahedral model molecule of H 2 O
became the structural unit of Bernal–Fowler water. It has been suggested that such a
molecule has some preferential orientations with respect to surrounding molecules,
due to electrostatic interactions between the specific points of its positively and
negatively charged parts. The search for these preferred configurations became the
main subject of Bernal–Fowler work and anticipated future studies of molecular
dynamics.
Bernal and Fowler noticed that an experimental X-ray pattern (see Fig. 1.3) that
was scattered by ambient water (cure 1 in Fig. 1.3b + open symbols) and the model
spectrum of amorphous quartz with a tridymite structure (curve 3) have much in
common.
3 In particular, the positions of the minima and maxima of the intensity
distribution are approximately the same. Authors assumed that water, by analogy
with quartz, has a coordination of atoms as in amorphous SiO 2 , and suggested a
quartz-like structure of water, shown in Fig. 1.4. The asymmetric non-linear shape
of water molecules allowed them to form a tetrahedral molecular coordination as
shown in Fig. 1.4a. The assembly of pentawater complexes gives an open hexagonal
structure shown in Fig. 1.4b, which is similar to the structure of quartz shown in
Fig. 1.4c.
In order to obtain the correct density of water, Bernal and Fowler assumed that the
volume of voids between molecules is approximately equal to the occupied volume.
As a result, a good coincidence between the experimental curve 1 and the theoretical
curve 3 was achieved. In such a way, the tetrahedrally coordinated arrangement of
water (see Fig. 1.4a), self-assembled into hexagonal rings, was introduced to the
water model as the most suitable configuration of the hydrogen and oxygen atoms
satisfying the X-ray data. However, this model structure corresponds to the most
probable position of atoms (F-structure) and does not account for any molecular
dynamics.
Although a good coincidence of Bernal–Fowler model with X-rays diffraction data
has been achieved, the quartz-like water model is static by nature. Electro-diffusion
data, rheological properties, and thermal effects were missing. Assuming that water
consists of long-lived H 2 O molecules similar to those observed in steam with small
mutual deformations, the model offers no explanation of the experimentally observed
3 Note that in this kind of experiment, water atoms are not directly observed, because X-rays interact
with the electronic density around the oxygen atoms, but not the atoms themselves.
1 A Historical Review of the Structures of Water and Ice
1.2.1 Scattering of X-Rays by Liquid Water
Bernal and Fowler used X-rays to identify the local molecular environment of H 2 O
molecules in a condensed state. They assumed that water molecules have spherical
symmetry with a triangular arrangement of the hydrogen and oxygen nuclei. Five
characteristic points were chosen for this model of the water molecule based on earlier
spectroscopic data [10]. The oxygen was placed at the center of the sphere as shown in
Fig. 1.2. Two hydrogen atoms (protons) were placed in the y-z-plane about 1 Å from
the oxygen atom, and 1.5 Å from each other so that the H–O–H angle was about 105
◦ .
Two more points, which represent electron densities that compensate for the density
of the protons, were settled in the x-z-plane. This tetrahedral model molecule of H 2 O
became the structural unit of Bernal–Fowler water. It has been suggested that such a
molecule has some preferential orientations with respect to surrounding molecules,
due to electrostatic interactions between the specific points of its positively and
negatively charged parts. The search for these preferred configurations became the
main subject of Bernal–Fowler work and anticipated future studies of molecular
dynamics.
Bernal and Fowler noticed that an experimental X-ray pattern (see Fig. 1.3) that
was scattered by ambient water (cure 1 in Fig. 1.3b + open symbols) and the model
spectrum of amorphous quartz with a tridymite structure (curve 3) have much in
common.
3 In particular, the positions of the minima and maxima of the intensity
distribution are approximately the same. Authors assumed that water, by analogy
with quartz, has a coordination of atoms as in amorphous SiO 2 , and suggested a
quartz-like structure of water, shown in Fig. 1.4. The asymmetric non-linear shape
of water molecules allowed them to form a tetrahedral molecular coordination as
shown in Fig. 1.4a. The assembly of pentawater complexes gives an open hexagonal
structure shown in Fig. 1.4b, which is similar to the structure of quartz shown in
Fig. 1.4c.
In order to obtain the correct density of water, Bernal and Fowler assumed that the
volume of voids between molecules is approximately equal to the occupied volume.
As a result, a good coincidence between the experimental curve 1 and the theoretical
curve 3 was achieved. In such a way, the tetrahedrally coordinated arrangement of
water (see Fig. 1.4a), self-assembled into hexagonal rings, was introduced to the
water model as the most suitable configuration of the hydrogen and oxygen atoms
satisfying the X-ray data. However, this model structure corresponds to the most
probable position of atoms (F-structure) and does not account for any molecular
dynamics.
Although a good coincidence of Bernal–Fowler model with X-rays diffraction data
has been achieved, the quartz-like water model is static by nature. Electro-diffusion
data, rheological properties, and thermal effects were missing. Assuming that water
consists of long-lived H 2 O molecules similar to those observed in steam with small
mutual deformations, the model offers no explanation of the experimentally observed
3 Note that in this kind of experiment, water atoms are not directly observed, because X-rays interact
with the electronic density around the oxygen atoms, but not the atoms themselves.