A Liquid That Goes Solid When Heated

By Roland Piquepaille

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 more...

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 "αCD."]

Sources: Belle Dumé, PhysicsWeb, September 24, 2004; The Journal of Chemical Physics, September 15, 2004, Volume 121, Issue 11, pp. 5031-5034