[FoRK] Volte-face

Wayne Baisley 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 
observed phenomenon.

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 
our certitudes.

[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 "&#945;CD."]

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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