Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 100

5.2 Freezing Principles

Freezing is the transformation of water into ice, i.e., the passage from the liquid
phase to the solid phase. This state change is the result of two phenomena occurring
simultaneously: nucleation and diffusion. Nucleation is the production of small ice-
formation sites, constituting the seeds of the first crystals. Diffusion is the ability of
water or ice to move. The seeds have a significant attraction to the free water or ice
molecules that migrate and increase the size of the crystals.
Water's particularity comes from its molecular structure. In a water molecule, an
oxygen atom is bound to two hydrogen atoms by covalent bonds involving a pair
of electrons. The O

O angle is in the neighborhood of 104.74 ◦ . The molecule
is electrically neutral but is polarized. Indeed, the density of electrons is greater
near the oxygen nucleus than near the hydrogen nuclei. When two water molecules
are present, they tend to unite through an electrostatic bond between a positively
charged hydrogen nucleus and the electron cloud surrounding the oxygen nucleus.
This bond, called a hydrogen bond, tends to make the H

− −

H

H alignment. At a
given moment, a water molecule is surrounded by four neighboring molecules,
which are ordered according to a tetrahedral geometry as a result of the hydrogen
bonds. Liquid water is constantly reorienting itself, but has an instantaneous short-
range order. If the temperature falls below 273 K, the tetrahedral structure stiffens
and becomes a long-range order, constituting a solid with a hexagonal crystal struc-
ture. Homogeneous nucleation occurs. Since water is very mobile, the nucleation
sites will get larger in order to form large-dimensional crystals (Fig. 5.15) . This is
ordinary hexagonal ice ( I h ).

− O −

Fig. 5.15 Two possible
arrangements of a water
molecule

1 water molecule

Tetrahedral Pentamer

In the solid-liquid system created at the start of the phase transformation, the
crystals are unstable and are very quickly redissolved, contributing calories to the
system due to their fusion; this is then a metastable state. It is necessary to fight this
caloric contribution and surpass this surfusion stage and the eutectic point (corre-
sponds to the temperature at which the largest crystals are formed) by performing
quenching at a very low temperature.