Materials. Maleic acid (99% chemical purity) and caffeine (99%) were sourced from Sigma-Aldrich and were used as received. Both compounds were initially characterized by X-ray powder diffraction
(XRPD). Acetone (99%, purchased from VWR International), ethanol (analytical reagent grade, from Fisher Scientific), methanol (HPLC grade, from Fisher Scientific), and distilled deionized water
were used with no further purification