Identification and physicochemical characterization of caffeine–citric acid co-crystal polymorphs

Abstract

The purpose of the present study was to identify a new caffeine–citric acid co-crystal (CA–CI) polymorph and characterize three CA–CI polymorphs. The stability order among the three CA–CI polymorphs was also determined. One new and two known CA–CI polymorphs were prepared by the liquid-assisted grinding method or the slurry methods. The three CA–CIs were then identified and characterized by powder X-ray diffraction (PXRD), thermal analysis, IR spectroscopy, Raman spectroscopy, and dynamic vapor sorption (DVS). The stability order of the CA–CIs was determined by the slurry conversion method. Each CA–CI showed distinct PXRD, IR, Raman, and DVS data. The melting points of CA–CIs were 131°C (a new form, Form III), 141°C (Form I), and 160°C (Form II). The order of thermodynamic stability was CA–CI Form II>CA–CI Form I>CA–CI Form III. CA–CI Forms I and II were relatively stable against humidity compared to CA, CI and CA–CI Form III.