baisley at alumni.rice.edu
Sat Sep 25 20:34:11 PDT 2004
I recall hearing a long time ago (early 70s? even earlier?) about a solid that
would become skinnier when compressed (vertically). Something about the
crystalline structure that worked like a mechanical contraption with odd
arrangements of linkages. The effect on the solid, and our lives, was slight.
Now there's word of a liquid that freezes when heated. And melts again when
cooled. Tres interesante.
"And if you expect me to tell you how this discovery will modify our lives,
you're going to be disappointed. I've not a slightest idea about it ..."
samedi 25 septembre 2004
A Liquid that Goes Solid when Heated
There are some sure things in life, such as death and taxes. When you are
heating a solid, you expect it will melt and when you're boiling water, you're
pretty certain that it will turn into vapor. But what about a liquid that
becomes solid when it's heated? Of course, it has already been done, for
example in the chemical process of polymerization. But now, PhysicsWeb writes
that a team of French physicists has discovered a law-breaking liquid that
defies the rules. When you heat it between 45 and 75°, it becomes solid. But
the process is fully reversible, and this is a world's premiere. When you
decrease the temperature, this solid melts and turns again into a liquid. I'm
not sure of the implications of such a phenomenon, but it's fascinating. Read
Here is the summary from PhysicsWeb.
Physicists in France have discovered a liquid that "freezes" when it is
heated. Marie Plazanet and colleagues at the Université Joseph Fourier and the
Institut Laue-Langevin, both in Grenoble, found that a simple solution
composed of two organic compounds becomes a solid when it is heated to
temperatures between 45 and 75°, and becomes a liquid when cooled again. The
team says that hydrogen bonds are responsible for this novel behaviour.
Ready for the scientific details?
Plazanet and colleagues prepared a liquid solution containing a-cyclodextrine
(alpha-CD), water and 4-methylpyridine (4MP). Cyclodextrines are cyclic
structures containing hydroxyl end groups that can form hydrogen bonds with
either the 4MP or water molecules.
At room temperature, up to 300 grams of alpha-CD can be dissolved in a litre
of 4MP. The resulting solution is homogenous and transparent, but it becomes a
milky-white solid when heated. The temperature at which it becomes a solid
falls as the concentration of alpha-CD increases.
Neutron-scattering studies revealed that the solid phase is a "sol-gel" system
in which the formation of hydrogen bonds between the alpha-CD and the 4MP
leads to an ordered, rigid structure. At lower temperatures, however, the
hydrogen bonds tend to break and reform within the alpha-CD, which results in
the solution becoming a liquid again.
The research work has been published by The Journal of Chemical Physics in its
September 15, 2004 issue under the name "Freezing on heating of liquid
solutions." Here is a link to the abstract.
We report a reversible liquid-solid transition upon heating of a simple
solution composed of a-cyclodextrine (alpha-CD), water, and 4-methylpyridine.
These solutions are homogeneous and transparent at ambient temperature and
solidify when heated to temperatures between 45° and 75°. Quasielastic and
elastic neutron scattering show that molecular motions are slowed down in the
solid and that crystalline order is established. The solution "freezes on
heating." This process is fully reversible, on cooling the solid melts. A
rearrangement of hydrogen bonds is postulated to be responsible for the
If you are interested by the subject, visit a university library, or buy the
article for $22.
And if you expect me to tell you how this discovery will modify our lives,
you're going to be disappointed. I've not a slightest idea about it, even if I
find fascinating that scientists always find new ways to break rules and shake
[Additional note for physicists: I've been forced to use the "alpha-CD"
notation here, because neither my publishing software nor my browsers seem to
be able to understand the correct notation, which is "αCD."]
Sources: Belle Dumé, PhysicsWeb, September 24, 2004; The Journal of Chemical
Physics, September 15, 2004, Volume 121, Issue 11, pp. 5031-5034
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