Biomedical Engineering Reference
In-Depth Information
Good freezing of water depends on several factors, including two important ones:
the temperature at which the state change occurs and the rate of cooling of the
system. These factors will determine the size and nature of the seeds formed and
reduce the possibility of the migration of the water. This will block an increase in
the size of the primordial crystals. A low temperature promotes the appearance of
many nucleation sites and maximally reduces the diffusion of not-yet crystallized
water before obtaining a change in the state of the whole liquid volume. If in the
end the nucleation seeds remain a very small size (not visible at the TEM scale) and
do not provide a diffraction pattern, this is referred to as vitreous ice. Vitreous ice is
produced at temperatures lower than 77 K. Once the seeds at the nucleation centers
start to get larger, the ice becomes cubic crystalline ice.
The vitrification of pure water is obtained through a decrease in temperature, at
a rate of at least 10
9
deg/s, to a temperature less than 77 K.
This state change also depends on the pressure applied to the system when the
temperature is decreased. A strong increase in pressure (21
10
-7
Pa bars) pro-
motes the formation of vitreous ice, even with less rapid cooling. The increase in
pressure increases the viscosity of the water, and therefore slows the water's dif-
fusion and decreases the possibility of the nucleation seeds growing larger. The
vitreous ice obtained is amorphous “high-density” (
d
×
1.17) ice, whereas the
ice obtained under normal pressure is amorphous “low-density” (
d
=
0.94) ice.
Depending on the temperature decrease, the speed of this decrease, and the pres-
sure exerted on the system, different forms of crystalline ice can exist. There are at
least 12 forms.
In the case of organic systems, the material is not in pure water, but rather in
solutions and suspensions. Therefore, both free water and water are bound to the
molecules. There are fewer tetrahedrons, and there is more space between them. The
presence of solutes promotes the appearance of nucleation seeds before the nucle-
ation of the tetrahedrons. Water is immobilized by adsorption onto the molecules,
which presents the constitution of a large-scale order and stabilizes the system by
minimizing diffusion of the water. In this case, nucleation is heterogeneous. Cubic-
type crystalline glass occurs at temperatures near 200 K. In these systems, vitreous
ice will be obtained under less extreme conditions. We consider that, for a bio-
logical system having an osmolarity of 300 mOsm (e.g., mammals), vitreous ice
can be obtained on a very thin film of a solution with a temperature decrease of
10
6
deg/s down to approximately 125 K. Freezing under these conditions is referred
to as ultrarapid freezing (Fig.
5.16)
.
Like water, ice is capable of migrating if the frozen system is not maintained at a
temperature low enough to prevent the diffusion phenomenon. For pure water above
140 K, ice crystals are able to become larger, slowly at first, and then quickly as
the temperature rises to around 200 K. Then cubic ice crystals appear. This is in the
neighborhood of the
T
g
of vitreous ice (between 120 and 140 K). In a biological-type
heterogeneous system (whose diffusion rate is low), experience shows that this
phenomenon occurs around 183 K.
Once water is frozen in the best possible way, one of three preparation pathways
can be carried out to observe the sample.
=
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