title: Tracing coffee tabletop traces.
abstract: Crystallization processes under different conditions are of fundamental interest in chemistry, pharmacy, and medicine. Therefore, we have studied the formation of micro- and nanosized crystals using water-caffeine (1,3,7-trimethyl-1 H-purine-2,6(3 H,7 H)-dione) solutions under ambient conditions as a relevant model system. When droplets of an aqueous caffeine solution evaporate and eventually dry on surfaces (glass, polystyrene, and polyester), stable "coffee tabletop" rings with a perimeter of typically 3 mm are formed after 20 to 50 min. Using a micro focus X-ray beam available at the BESSY muSpot-beamline, the fine structure of different caffeine needles can be distinguished. Unexpectedly, both crystal modifications (alpha- and beta-caffeine) are present, but locally separated in these rings. Furthermore, AFM studies reveal the presence of even smaller particles on a nanometer length scale. To eliminate influences of surface irregularities from the crystallization process, acoustic levitation of liquid samples was employed. Such levitated droplets are trapped in a stable position and only surrounded by air. The solvent in an ultrasonically levitated drop evaporates completely, and the resulting crystallization of caffeine was followed in situ by synchrotron X-ray diffraction. In this case, the diffraction pattern is in accordance with pure alpha-caffeine and does not indicate the formation of the room temperature polymorph beta-caffeine. Hence, our investigations open new vistas that may lead to a controlled formation of cocrystals and novel polymorphs of micro- and nanocrystalline materials, which are of relevance for fundamental studies as well as for pharmaceutical and medical applications.
authors.affiliation: BAM Federal Institute for Materials Research and Testing, Richard-Willstatter-Strasse 11, 12489 Berlin, Germany.
authors.forename: Jork
authors.initials: J
authors.lastname: Leiterer
authors.forename: Franziska
authors.initials: F
authors.lastname: Emmerling
authors.forename: Ulrich
authors.initials: U
authors.lastname: Panne
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Christen
authors.forename: Klaus
authors.initials: K
authors.lastname: Rademann
mesh_terms: D002110:Caffeine / Q000032:analysis* / Q000737:chemistry*; D003069:Coffee / Q000737:chemistry*; D003460:Crystallization; D018625:Microscopy, Atomic Force; D008855:Microscopy, Electron, Scanning; D015394:Molecular Structure; D012996:Solutions; D013499:Surface Properties; D014835:Volatilization; D014961:X-Ray Diffraction
chemical_list: D003069:Coffee; D012996:Solutions; D002110:Caffeine