Freezing on heating of liquid solutions

We report a reversible liquid-solid transition upon heating of a simple solution composed of a-cyclodextrine (alphaCD), water, and 4-methylpyridine. These solutions are homogeneous and transparent at ambient temperature and solidify when heated to temperatures between 45degrees and 75degrees. 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. (C) 2004 American Institute of Physics.