TY - JOUR A1 - Sarfraz, Adnan A1 - Simo, A. A1 - Fenger, R. A1 - Christen, W. A1 - Rademann, K. A1 - Panne, Ulrich A1 - Emmerling, Franziska T1 - Morphological diversity of caffeine on surfaces: needles and hexagons JF - Crystal growth & design N2 - A systematic crystal morphology study on the pharmaceutical model compound caffeine has been conducted on different surfaces: silicon, silver, soda lime glass, and silver subsurface ion-exchanged soda-lime silicate (SIMO) glasses. The morphology of the solid caffeine deposits has been investigated using environmental scanning electron microscopy (ESEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). Needle-shaped caffeine crystals have been observed by drop-casting and also by applying the rapid expansion of supercritical solutions (RESS) technique using supercritical carbon dioxide. The aspect ratio of the crystalline needles typically vary between 10 and 100, but have been observed as large as 500. The XRD data of the RESS products indicate unambiguously the presence of the thermodynamically most stable polymorph of caffeine known as the β-form. Under defined conditions we observe a unique, surface-mediated morphology for caffeine crystals with nearly perfect hexagonal shape. The relative fraction of the hexagons was seen to strongly increase especially when SIMO glasses were used. These hexagons have a distinct upper size limit depending on the solvent and substrate being used. The size distribution analysis of the hexagons yielded an average perimeter of typically 10 µm. The mechanism of the formation process of this new hexagonal motif is explained in terms of the spinodal dewetting of the thin film of caffeine solution on the surface. KW - Caffeine KW - Silicon KW - Silver KW - Ion exchanged soda lime silicate glass KW - ESEM KW - AFM KW - X-ray diffraction KW - RESS KW - Carbon dioxide KW - Supercritical fluids PY - 2012 DO - https://doi.org/10.1021/cg101358q SN - 1528-7483 VL - 12 IS - 2 SP - 583 EP - 588 PB - American Chemical Society CY - Washington, DC, USA AN - OPUS4-25457 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -