Surface Induced Order of Solution Processed Caffeine Needles on Silica and Muscovite Mica

In this work the orientational order of needle-shaped caffeine crystallites at silica and muscovite mica surfaces is investigated by X-ray diffraction experiment and optical microscopy. The investigation of thin films reveals the formation of caffeine in the anhydrous monoclinic β-polymorphic structure independent of the surface with the disclike caffeine molecules assemble in an edge-on conformation. The silica surface provides isotropic properties for crystal growth which leads to a fiber texture arrangement of the needles; a majority of needles have a 010 texture with all other directions being randomly distributed. The mica surface with its highly regular surface structure induces defined textures and azimuthal order of the caffeine needles resulting in a fully defined epitaxial order. The edge-on and disclike caffeine molecules align along the [100]m, [110]m, and [11̅0]m real space directions of the mica substrate and show a mirror symmetry around the [110]m. The experiments on these two surfaces show that the type of surface is able to induce order during caffeine needle growth