at a small concentration of CTAB (e.g., 0.415 mM), the microstructure of the
cryogel undergoes very significant changes [192]. The cross-section of the
ice-templated pores is enlarged from 1–3 μm (Fig. 22b) to 4–5 μm (Fig. 22d) due
to the presence of the ionic surfactant. Simultaneously, the morphology of the gel–
gas interfaces also changes drastically. Although the surface of these interfaces in
surfactant-free foamed cryogel is almost smooth (Fig. 22a, b), the presence of
CTAB causes pronounced ulceration of the surface (Fig. 22c, d). A similar effect
was also observed using sodium dodecyl sulfate as an anionic surfactant additive
[192]. Thus, the presence of surfactants significantly affects the shape of the
gaseous macropores. It is evident that bubbles with such a pitted surface cannot
exist in the initial liquid foam, since such foam would be absolutely unstable.
Therefore, this unusual morphology of the gas–gel interfaces appears to be due to
the decreased surface tension between the liquid–gas interfaces and the growing
facets of ice crystals, which can evidently pierce the air bubbles. However, the
freezing solidification of the foam and consequent formation of cryogel in the
bubble walls prevent the complete destruction of the bubbles, thus “imprinting” a
certain intermediate structure of the boundary layers. Hence, surfactants similar to
CTAB in terms of their influence on the foams can be considered as auxiliary
modifiers of the architecture of gaseous pores within the foamed cryogels.
The results presented here thus show that there are very broad possibilities for
affecting the macroporosity parameters in diverse cryogels and to govern, within
certain limits, the architecture and size of the pores templated by the solvent
crystals. In addition, there are many possibilities for creating multifarious additional macropores in these gel matrices by using auxiliary porogens. In each
particular case, the approach employed depends on the purpose of the produced
cryogel and on the set of material properties required for its efficient application.
4 Conclusions
Finalizing the overview of the literature related to the general aspects of cryotropic
gelation processes, the following basic conclusions can be drawn [7–9, 105, 107,
111, 114, 148]:
1. Cryotropic gel formation is a liquid-phase process occurring in the unfrozen
liquid microphase of a macroscopically frozen system.
2. The final products of such cryostructuring are macroporous, sometimes widepore sponge-like, gel matrices, i.e., cryogels.
3. Due to concentrating of solutes in the unfrozen liquid microphase, an apparent
decrease in the critical concentration of gelation is observed in cryotropic gel
formation as compared to gelation at positive temperatures
4. Acceleration of the gel formation is usually observed in the moderately frozen
systems over a definite range of negative temperatures. The key reason for such
Basic Principles of Cryotropic Gelation
95
cryogel undergoes very significant changes [192]. The cross-section of the
ice-templated pores is enlarged from 1–3 μm (Fig. 22b) to 4–5 μm (Fig. 22d) due
to the presence of the ionic surfactant. Simultaneously, the morphology of the gel–
gas interfaces also changes drastically. Although the surface of these interfaces in
surfactant-free foamed cryogel is almost smooth (Fig. 22a, b), the presence of
CTAB causes pronounced ulceration of the surface (Fig. 22c, d). A similar effect
was also observed using sodium dodecyl sulfate as an anionic surfactant additive
[192]. Thus, the presence of surfactants significantly affects the shape of the
gaseous macropores. It is evident that bubbles with such a pitted surface cannot
exist in the initial liquid foam, since such foam would be absolutely unstable.
Therefore, this unusual morphology of the gas–gel interfaces appears to be due to
the decreased surface tension between the liquid–gas interfaces and the growing
facets of ice crystals, which can evidently pierce the air bubbles. However, the
freezing solidification of the foam and consequent formation of cryogel in the
bubble walls prevent the complete destruction of the bubbles, thus “imprinting” a
certain intermediate structure of the boundary layers. Hence, surfactants similar to
CTAB in terms of their influence on the foams can be considered as auxiliary
modifiers of the architecture of gaseous pores within the foamed cryogels.
The results presented here thus show that there are very broad possibilities for
affecting the macroporosity parameters in diverse cryogels and to govern, within
certain limits, the architecture and size of the pores templated by the solvent
crystals. In addition, there are many possibilities for creating multifarious additional macropores in these gel matrices by using auxiliary porogens. In each
particular case, the approach employed depends on the purpose of the produced
cryogel and on the set of material properties required for its efficient application.
4 Conclusions
Finalizing the overview of the literature related to the general aspects of cryotropic
gelation processes, the following basic conclusions can be drawn [7–9, 105, 107,
111, 114, 148]:
1. Cryotropic gel formation is a liquid-phase process occurring in the unfrozen
liquid microphase of a macroscopically frozen system.
2. The final products of such cryostructuring are macroporous, sometimes widepore sponge-like, gel matrices, i.e., cryogels.
3. Due to concentrating of solutes in the unfrozen liquid microphase, an apparent
decrease in the critical concentration of gelation is observed in cryotropic gel
formation as compared to gelation at positive temperatures
4. Acceleration of the gel formation is usually observed in the moderately frozen
systems over a definite range of negative temperatures. The key reason for such
Basic Principles of Cryotropic Gelation
95