C O M M U N I C A T I O N S
A second possible mechanism for jointing involves “pore
pressure”, a well-recognized source of geological fracturing.16-18
Under certain environmental conditions, hydrostatic pressure is
created when water has filled vesicles in volcanic rocks and
intergrannular spaces in sedimentary rock. The pressure promotes
faulting that would not occur in dry rock. Similarly, the gemini
surfactant may exist as vesicles, micelles, or submicroscopic
aggregates that serve as nuclei for hexagonal fracturing of the
vitreous ice. Stress formation could arise, for example, if gemini
vesicles are solvated by structured water that is unable to increase
its density as rapidly as can the bulk vitreous ice during the plunge-
freezing in the liquid ethane. This jointing mechanism is similar to
its geological counterpart in that both depend on the presence of
small foreign bodies to exert stress within the solidified fluid. The
fact that other vesicle-forming compounds we have screened (e.g.,
phospholipids) failed to create columnar jointing speaks against
the mechanism. Likewise, the observation of “fences” in photo 5
favors discrete surfactant self-assembly within the ice.
Cryo-HRSEM, a method that has not been extensively applied
to colloidal suspensions, would seem to have a promising future
in discovering as yet unimagined modes of self-assembly in water.
In the first line of his One Hundred Years of Solitude, Gabriel
Garc´ıa Ma´rquez writes, “Many years later, as he faced the firing
squad, Colonel Aureliano Buend´ıa was to remember that distant
afternoon when his father took him to discover ice.” Apparently,
there is still much to discover about ice and how it responds to
colloidal additives.
Figure 1. Photos 1-4 and 6: plunge-frozen in ethane. Photo 5: plunge-
frozen in nitrogen. Labeled scale bars are given with each photo. Photo 1:
cryo-HRSEM of ice containing 3.5 wt % 1. Photo 2: close-up of photo 1.
Photo 3: 5.0 wt % 1. Photo 4: 6.5 wt % 1. Photo 5: 3.5 wt % 1. Photo 6:
3.5 wt % of analogue of 1 with two 9-carbon chains instead of two 12-
carbon chains. Note the lamellar rather than columnar morphology.
like” network, devoid of ice, seen in photo 5. When the samples
were cooled very rapidly (7 ms) in a high-pressure freezer (HPM
010),13 the structure was more heterogeneous, with only occasional
columnar features of width identical to those obtained by plunge-
freezing (not shown). This could be related to the fact that the high-
pressure method uses a totally enclosed “double planchet” so that
the sample was no longer exposed at the top, and freezing took
place more evenly. As with geological structures, unequal direc-
tional stresses might be favorable for observing the columnar
jointing. These stresses presumably arose from ice increasing its
density from 0.917 at 0 °C to 0.934 at -180 °C.
Acknowledgment. This work was supported by the U.S. Army
Research Office. We thank Ms. Elizabeth R. Wright for technical
assistance with the photography.
References
The following observations lead us to believe that the columnar
jointing is not an “artifact”:14 (a) As already mentioned, cryo-
HRSEM of pure water invariably gives featureless images. In the
presence of 1, however, reproducible columnar jointing occurs at
several concentrations. (b) After several years of cryo-electron
microscopy, working with aqueous proteins, DNA, and phospho-
lipids, we have yet to encounter a substance that could promote
formation of a similar ice morphology. (c) An analogue of 1, with
two 9-carbon chains instead of two 12-carbon chains, yields a cyro-
HRSEM image having a lamellar rather than a columnar structure
(Figure 1, photo 6). Clearly, the jointing phenomenon is imposed
onto the sample by the surfactant, and it is highly structure-specific.
(d) Cryo-SEM images of self-assembled lamellar ribbons in water
have been obtained by others15 with no hint of “artifacts”.
At least two mechanisms seem possible for the columnar jointing.
The gemini surfactant might self-assemble into a submicroscopic
network of polygonal columns which are filled first with water and
later with ice. The columns, invisible by light microscopy, should
contribute to the viscosity of the gel at higher temperatures. The
textured column walls, uncharacteristic of vitreous ice and evident
in photo 2, suggest that gemini surfactant 1 is, indeed, occupying
the column interfaces. Moreover, photo 5 provides an important
clue in that hexagonal “fences” devoid of ice are clearly observed.
This supramolecular skeleton, presumably composed of self-
assembled 1, provides a template for the unusual fracturing of the
vitreous ice. We cannot exclude the possibility that the columnar
assemblies actually formed during the freezing process, but such
an intricate self-organization would have had to occur at low
temperatures within a subsecond time frame.
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(14) An irregular polygonal pattern, obtained from a 0.5% aqueous solution
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work is not directly comparable to ours because it is a TEM image, with
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cooling. It is possible, in any case, that the image does, in fact, portray
an observable propensity of the compound to self-assemble.
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