New Cocrystallization Method: Non-photochemical Laser-Induced Nucleation of a Cocrystal of Caffeine–Gallic Acid in Water

This paper reports for the first time the crystallization of cocrystals (caffeine–gallic acid) in water using the non-photochemical laser-induced nucleation (NPLIN) technique. The nucleation temporal control of NPLIN and the induction time reduction (70 times shorter than spontaneous nucleation) allow the obtention of cocrystals on demand, with excellent repeatability. Prior to NPLIN experiments, solubility measurements, metastable zone limit determination, and spontaneous nucleation (SN) are carried out. Supersaturated solutions at three different molarities of caffeine (0.0158, 0.0167, and 0.0179 M) are prepared and dissolved at 338 K. Supersaturated solutions are exposed to a 532 nm-wavelength nanosecond pulsed laser at 296 K. Some hours later, crystals can be filtered. The impact of supersaturation on probability of nucleation has been studied, and the result shows a higher supersaturation, which implies a higher probability of nucleation. Crystalline form characterization has employed a combination of Raman scattering, high-performance liquid chromatography, thermogravimetry, differential scanning calorimetry, and powder X-ray diffraction; the sizes of the cocrystals are too small for a single-crystal X-ray diffraction experiment. Two different cocrystal forms are obtained: a new polymorph of hemihydrate CAFGAL.0.5H2O from spontaneous nucleation by precipitation and a new hydrate form CAFGAL.1.5H2O from NPLIN and from spontaneous nucleation of a supersaturated solution without evaporation. These results open a promising way to crystallize cocrystals in the context of the pharmaceutical industry.