Solvent binding to benzamide crystals: Morphology, induced twinning and direct observation by surface X-ray diffraction

Single crystals of benzamide which form hydrogen-bonded (001) bilayers are obtained from ethanol or 1-propanol solutions. The added presence of amide cosolvent such as X-CONH2 X = CH3 or CF3, yields very thin (001) plates which are twinned. Grazing incidence X-ray diffraction (GIXD) measurements from the (001) surface of pure benzamide indicates that the surface almost exclusively exposes the phenyl substituents. The observed GIXD data from the (001) surface of the affected crystals revealed lamellar twinning. Structural modeling of the data showed that the top surface layer contains about 15% adsorbed amide cosolvent, all oriented with their -X moieties exposed to air. A simple structural model has been invoked to account for the solvent-induced twinning.