isolated and preserved for further detailed study ex situ. In the case of solid state
reactions, usually performed at elevated temperatures, the isolation of intermediates
is mainly performed by quenching, i.e., fast cooling of the reaction mixture to the
temperature of reaction termination. In the case of precipitation reactions in aqueous solutions, similar quenching can be performed by fast cooling of the reaction
medium to liquid nitrogen temperature. Elimination of frozen solvent by freezedrying thus allows further investigation of reaction intermediates by various solid
state analysis methods. It was shown that this approach is useful in the study of the
formation and transformation of nanoparticles in aqueous solutions, such as TiO 2
[87, 88], ZrO 2 [89], ZnO [90], and calcium phosphate and its derivatives [91, 92].
Hydrothermal processing of aqueous solutions is an efficient and widely used
preparation method for nanocrystalline individual and complex oxides. An essential
feature of this method is the high level of crystallographic ordering in the resulting
nanopowders. Although the applications of oxide nanoparticles mainly demand
crystallographically ordered species, many other chemical synthesis methods for
such nanoparticles often result in amorphous or poorly crystalline products. Thermal processing of amorphous powders at elevated temperatures in order to enhance
their crystallinity is usually accompanied by significant agglomeration and grain
growth of nanocrystallites. Similar problems also exist in hydrothermal processing
during isolation of the nanocrystalline reaction products from the liquid reaction
medium. The high surface energy of these powders results in their significant
aggregation and agglomeration. Therefore, freezing and subsequent freeze-drying
are often applied as a final stage of the hydrothermal synthesis process to prevent
aggregation and agglomeration [93–101].
Another important example of a successful application of cryogenic approaches
to particle isolation concerns the separation of magnetic nanoparticles. Magnetic
properties of ferri- and ferromagnetic particles in the nanosize range depend
essentially on their size. The magnetization behavior of superparamagnetic, small
single-domain, and multidomain particles is substantially different. However, the
transition from superparamagnetic particle rotation to the domain wall movement
mechanism can occur during grain growth within a 100 nm size range. This
difference in magnetic behavior is essential for the biomedical applications of
magnetic nanoparticles and, therefore, has attracted growing attention during the
last few years. The influence of particle size on the magnetization behavior of
nanoparticles has an important role in their agglomeration processes. Depending on
the character and extent of the interparticle contact, the closely located magnetic
particles could demonstrate individual, mutually independent, or cooperative magnetic behavior. These particles are often produced and stored in aqueous or
nonaqueous suspensions but their detailed studies can only be carried out in the
solid state. Taking into account these features, isolation of magnetic particles by
freezing and freeze-drying of suspensions and magnetic liquids is widely used in
sample preparation for magnetic studies of such particles [102–108], as well as for
the investigation of other kinds of suspension-derived ultrafine solids [109, 110]. In
the case of hydrophobic particles and, hence, nonaqueous suspension media, the
continuous phase can be first substituted with another solvent having a moderate
Inorganic Cryogels
231
reactions, usually performed at elevated temperatures, the isolation of intermediates
is mainly performed by quenching, i.e., fast cooling of the reaction mixture to the
temperature of reaction termination. In the case of precipitation reactions in aqueous solutions, similar quenching can be performed by fast cooling of the reaction
medium to liquid nitrogen temperature. Elimination of frozen solvent by freezedrying thus allows further investigation of reaction intermediates by various solid
state analysis methods. It was shown that this approach is useful in the study of the
formation and transformation of nanoparticles in aqueous solutions, such as TiO 2
[87, 88], ZrO 2 [89], ZnO [90], and calcium phosphate and its derivatives [91, 92].
Hydrothermal processing of aqueous solutions is an efficient and widely used
preparation method for nanocrystalline individual and complex oxides. An essential
feature of this method is the high level of crystallographic ordering in the resulting
nanopowders. Although the applications of oxide nanoparticles mainly demand
crystallographically ordered species, many other chemical synthesis methods for
such nanoparticles often result in amorphous or poorly crystalline products. Thermal processing of amorphous powders at elevated temperatures in order to enhance
their crystallinity is usually accompanied by significant agglomeration and grain
growth of nanocrystallites. Similar problems also exist in hydrothermal processing
during isolation of the nanocrystalline reaction products from the liquid reaction
medium. The high surface energy of these powders results in their significant
aggregation and agglomeration. Therefore, freezing and subsequent freeze-drying
are often applied as a final stage of the hydrothermal synthesis process to prevent
aggregation and agglomeration [93–101].
Another important example of a successful application of cryogenic approaches
to particle isolation concerns the separation of magnetic nanoparticles. Magnetic
properties of ferri- and ferromagnetic particles in the nanosize range depend
essentially on their size. The magnetization behavior of superparamagnetic, small
single-domain, and multidomain particles is substantially different. However, the
transition from superparamagnetic particle rotation to the domain wall movement
mechanism can occur during grain growth within a 100 nm size range. This
difference in magnetic behavior is essential for the biomedical applications of
magnetic nanoparticles and, therefore, has attracted growing attention during the
last few years. The influence of particle size on the magnetization behavior of
nanoparticles has an important role in their agglomeration processes. Depending on
the character and extent of the interparticle contact, the closely located magnetic
particles could demonstrate individual, mutually independent, or cooperative magnetic behavior. These particles are often produced and stored in aqueous or
nonaqueous suspensions but their detailed studies can only be carried out in the
solid state. Taking into account these features, isolation of magnetic particles by
freezing and freeze-drying of suspensions and magnetic liquids is widely used in
sample preparation for magnetic studies of such particles [102–108], as well as for
the investigation of other kinds of suspension-derived ultrafine solids [109, 110]. In
the case of hydrophobic particles and, hence, nonaqueous suspension media, the
continuous phase can be first substituted with another solvent having a moderate
Inorganic Cryogels
231