title: Plastic and glassy crystal states of caffeine.
abstract: The present paper focuses on the high temperature form I of caffeine and on its low temperature metastable form. Structural, dynamic, and kinetic information has been obtained by X-ray, dielectric, and calorimetric investigations. This study shows the following features: (1) The high temperature phase (I) of caffeine is in a state of dynamically orientationally disordered crystalline state (so-called "plastic, or rotator, phase"). (2) This high-symmetry hexagonal phase can be maintained at low temperature in a metastable situation. (3) Under deep undercooling of form I a glass transition occurs in the disordered crystalline state near room temperature. It is associated with the orientational freezing in of the molecular motions. Otherwise stated, the metastable state I enters into a nonergodic unstable state, so-called "glassy crystal" state. These findings rationalize the difficulties seen with caffeine in pharmaceutical science.
authors.affiliation: Laboratoire de Dynamique et Structure des Matériaux Moléculaires, UMR CNRS 8024, Université de Lille 1, Bât. P5, 59655 Villeneuve d'Ascq Cedex, France. marc.descamps@univ-lille1.fr
authors.forename: Marc
authors.initials: M
authors.lastname: Descamps
authors.forename: Natalia T
authors.initials: NT
authors.lastname: Correia
authors.forename: Patrick
authors.initials: P
authors.lastname: Derollez
authors.forename: Florence
authors.initials: F
authors.lastname: Danede
authors.forename: Frédéric
authors.initials: F
authors.lastname: Capet
mesh_terms: D002110:Caffeine / Q000737:chemistry*; D003460:Crystallization; D005898:Glass / Q000737:chemistry*; D015394:Molecular Structure; D010969:Plastics; D033362:Powder Diffraction; D014961:X-Ray Diffraction
chemical_list: D010969:Plastics; D002110:Caffeine