• Decroix, Anne-Amandine

Abstract

We present the study of the physical state of solid caffeine, a well-known pharmaceutical and food-industry compound, in order to understand its original polymorphism and the associated instabilities. Experimental characterisations have been obtained by DSC, XRD, NMR and dielectricrelaxation.The study of the molecular mobility of anhydrous phases shows a very slow complex dynamic at room temperature. Two similar dynamical processes are observed in phases I and II, called AI and AII, and a process slower in phase I called BI.The phases transformations (from I to II and II to I) have been studied to determine the origin of the extraordinary property of caffeine to be maintained in situation of high metastability. This investigation reveals anomalous slow transformation processes, influenced by the slow molecular mobility. The results suggest a first order I→II transformation kinetic, controlledbythenucleationrate,anddirectlygovernedbytheBIdynamicalprocess.A coherentmodelhasalsobeenproposedfortheII→Itransition,suggestingatransformation from the surface, with a high influence of the microstructure.Milling operations confirm the microstructural influence for the II→I transition. It has also been shown that milling induces I→II transformation but also II→I transformation according to the milling intensity.

Topics

• surface
• compound
• phase
• mobility
• x-ray diffraction
• grinding
• milling
• differential scanning calorimetry
• Nuclear Magnetic Resonance spectroscopy