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Freezing on heating of liquid solutions
M Plazanet1, C Floare, M R Johnson
1Institut Laue-Langevin, B.P. 156X, 38042 Grenoble Cedex, France.
The Journal of Chemical Physics
|September 9, 2004
Summary
This study reveals a unique liquid-solid phase transition in a simple solution of alpha-cyclodextrin (alpha CD), water, and 4-methylpyridine. Upon heating, the solution solidifies, demonstrating a reversible "freezing on heating" phenomenon.
Area of Science:
- Physical Chemistry
- Materials Science
- Supramolecular Chemistry
Background:
- Understanding phase transitions is crucial for materials design.
- Reversible solid-state transformations offer unique application potential.
- Cyclodextrin inclusion complexes are known for their host-guest properties.
Purpose of the Study:
- To investigate the unusual liquid-solid transition observed in alpha-cyclodextrin (alpha CD), water, and 4-methylpyridine solutions.
- To characterize the structural and dynamic changes associated with this heating-induced solidification.
- To elucidate the mechanism behind this reversible phase behavior.
Main Methods:
- Preparation of homogeneous solutions of alpha-cyclodextrin, water, and 4-methylpyridine.
- Thermal analysis to determine the transition temperature range.
- Quasielastic and elastic neutron scattering to probe molecular dynamics and crystalline order.
Main Results:
- Solutions transition from a homogeneous liquid to a solid state upon heating between 45°C and 75°C.
- Neutron scattering data indicate reduced molecular motion and established crystalline order in the solid phase.
- The liquid-solid transition is fully reversible upon cooling.
Conclusions:
- The observed phenomenon is a reversible liquid-solid transition triggered by heating.
- A rearrangement of hydrogen bonds is proposed as the underlying mechanism.
- This behavior represents a novel example of a
- freezing on heating
- process in a simple molecular solution.