Confinement of molecular materials using a solid-state loading method: a route for exploring new physical states and their subsequent transformation highlighted by caffeine confined to SBA-15 pores

Abstract

Using the innovative solid-state loading (milling-assisted loading, MAL) method to confine caffeine to cylindrical pores (SBA-15, ∅ = 6 nm) gives the opportunity to explore the original physical states of caffeine and their subsequent transformation using low-frequency Raman spectroscopy, powder X-ray diffraction and microcalorimetry investigations. It was shown that MAL makes possible the loading of the selected form in the polymorphism of caffeine. While form II has similar structural and dynamics properties in confined and bulk forms, the confined rotator phase (form I) exhibits clear differences with the bulk form inherent to its orientational disorder. Interestingly, the two confined forms of caffeine undergo an exothermic disordering transformation upon heating into a physical state at the border between a nanocrystallized orientationally disordered phase and an amorphous state, not existing in the bulk form. The melting of this new physical state was observed at 150 °C, i.e. 85 degrees below the melting temperature of the bulk form I, thus demonstrating the confinement of caffeine. It was also found that the liquid confined to pores of 6 nm mean diameter recrystallizes upon cooling, which can be explained by the very disordered nature of the recrystallized state.