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°C, and becomes a liquid when cooled again. The team says that hydrogen bonds are responsible for this novel behaviour (M Plazanet et al. 2004 J. Chem. Phys 121 5031).
Solids usually melt when they are heated, and liquids turn into gas, although exceptions do exist when heating leads to chemical changes that cannot be reversed, such as polymerisation. However, a reversible transition in which a liquid becomes a solid when heated has never been observed until now.
Plazanet and colleagues prepared a liquid solution containing α-cyclodextrine (αCD), 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 αCD 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 αCD increases.
Neutron-scattering studies revealed that the solid phase is a “sol-gel” system in which the formation of hydrogen bonds between the αCD and the 4MP leads to an ordered, rigid structure. At lower temperatures, however, the hydrogen bonds tend to break and reform within the αCD, which results in the solution becoming a liquid again.
Molecular dynamics simulations by Plazanet and co-workers confirmed that the cyclodextrine ring becomes distorted as it is heated up to close to the solidification temperature. The hydrogen bonds within the αCD break and the hydroxyl groups rotate towards the outside, which allows a network of bonds to form between the different molecules. The team has found a number of cyclodextrine/pyridine systems that also become solid when heated, and is now looking more closely at the structure of the sol-gel system to understand the solidification mechanism in more detail.