Röthel Christian, Radziown Michal, Resel Roland, Grois Andreas, Simbrunner Clemens, Werzer Oliver
Institute of Pharmaceutical Sciences , Department of Pharmaceutical Technology , Karl-Franzens Universität Graz , Universitätsplatz 1 , 8010 Graz , Austria . Email:
Institute of Solid State Physics , Graz University of Technology , Petersgasse 16 , 8010 Graz , Austria.
CrystEngComm. 2017 Jun 7;19(21):2936-2945. doi: 10.1039/c7ce00515f. Epub 2017 Apr 27.
Defined crystal growth is highly demanded for technological applications but also fundamental research. Within this work, the crystal growth of the asymmetric molecule caffeine was studied on single crystalline surfaces of muscovite mica, sodium chloride and potassium chloride. While elongated needle-like crystals grow on muscovite mica and sodium chloride, smaller individual "bird-like" structures were observed on potassium chloride. Depending on the surface type and temperature, the disk-shaped caffeine molecules prefer either an edge-on or flat-on orientation with respect to the surface, but in each case, a defined crystallographic relation between the surface and caffeine crystallites was determined by using the X-ray pole figure technique. On muscovite mica and sodium chloride, needle-like crystallites with edge-on oriented molecules aligned mainly with the unit cell -axis (which coincides with the long needle axis) along the [1-10], [100], [110] and [110], [1-10] directions, respectively. Crystals consisting of flat-on oriented molecules on KCl showed also defined alignments with respect to the substrate, but due to the altered molecule-substrate contact, the -axis aligned along [110] and [1-10]. Growth at elevated temperatures enabled changes in the crystal growth whereby more defined structures formed on NaCl. On KCl, the bird-like structures remained very similar, while caffeine on the mica surface at elevated temperatures resulted in even additional texture forming with the caffeine molecules now also favoring a flat-on orientation with respect to the surface. The systematic variation of various system parameters demonstrates how sensitive the growth behavior of caffeine on this variety of substrates is.
确定的晶体生长不仅在技术应用中，而且在基础研究中都有很高的需求。在这项工作中，研究了不对称分子咖啡因在白云母、氯化钠和氯化钾单晶表面上的晶体生长。虽然在白云母和氯化钠上生长出细长的针状晶体，但在氯化钾上观察到较小的单个“鸟状”结构。根据表面类型和温度，盘状咖啡因分子相对于表面倾向于边缘取向或平躺取向，但在每种情况下，通过使用X射线极图技术确定了表面与咖啡因微晶之间确定的晶体学关系。在白云母和氯化钠上，分子边缘取向的针状微晶主要沿[1-10]、[100]、[110]和[110]、[1-10]方向与晶胞轴（与长针轴重合）对齐。由氯化钾上平躺取向分子组成的晶体也显示出相对于衬底的确定排列，但由于分子-衬底接触的改变，轴沿[110]和[1-10]对齐。高温下的生长使晶体生长发生变化，从而在氯化钠上形成更规则的结构。在氯化钾上，鸟状结构保持非常相似，而高温下云母表面上的咖啡因甚至导致形成额外的织构，此时咖啡因分子也更倾向于相对于表面的平躺取向。各种系统参数的系统变化表明咖啡因在各种衬底上的生长行为是多么敏感。
