One Polymorph and Various Morphologies of Phenytoin at a Silica Surface Due to Preparation Kinetics

The preparation of solid crystalline films at surfaces is of great interest in a variety of fields. Within this work the preparation of pharmaceutically relevant thin films containing the active pharmaceutical ingredient phenytoin is demonstrated. The preparation techniques applied include drop casting, spin coating, and vacuum deposition. For the solution processed samples a decisive impact of the solution concentration and the applied film fabrication technique is observed; particular films form for all samples but with their extensions along different crystallographic directions strongly altered. Vacuum deposition of phenytoin reveals amorphous films, which over time crystallize into needle-like or particular-type structures whereby a nominal thickness of 50 nm is required to achieve a fully closed layer. Independent of all preparation techniques, the resulting polymorph is the same for each sample as confirmed by specular X-ray diffraction scans. Thus, morphologies observed via optical and atomic force microscope techniques are therefore a result of the preparation technique. This shows that the different time scales for which crystallization is obtained is the driving force for the various morphologies in phenytoin thin films rather than the presence of another polymorph forming.