Analyzing Solution Complexation of Cocrystals by Mathematic Models and In-Situ ATR-FTIR Spectroscopy

This paper focuses on developing an experimental method, which combines mathematic models, to evaluate complexation behavior in a saturated cocrystal solution. 1:1 and 2:1 urea–oxalic acid (U-OA) cocrystals were chosen as a model. The 2:1 U-OA cocrystal solubility was first explained by mathematical models which took into consideration the solubility product (Ksp) and solution complexation. The 1:1 U-OA cocrystal solubility was also analyzed by considering 1:1 solution complexation. To test the suitability of these models and evaluate complexation behavior, the aqueous solutions of U-OA with five different stoichiometric compositions which were located on solubility curves in different cocrystal regions were measured by in situ attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) (ReactIR). The spectra of complexes were obtained by the FT-IR absorbance subtraction method. The IR spectroscopy of crystalline 1:1 and 2:1 U-OA cocrystals was also discussed in comparison with those o...