Physicochemical characterization of 9,10-anthraquinone 2-carboxylic acid.

The physicochemical properties of 9,10-anthraquinone-2-carboxylic acid (AQCA) were investigated by thermal analysis, powder X-ray diffraction pattern, solubility, and partition coefficient. The chemical structure of AQCA was confirmed by the data from UV, Fourier transform IR (FTIR), and NMR analyses. Solubility and partition coefficient data were pH dependent. A new methanolic solvate of AQCA that lost 9.34% of its initial weight from 70 to 100 degrees C was found by thermogravimetric analysis. Sublimation of AQCA occurred at temperatures > 250 degrees C and needle-like crystals (Form I) were formed. The water solubility of AQCA increased with temperature; its heat of solution was 32.7 kJ/mol. A differential scanning calorimetry-FTIR microscopic system correlated a thermal phase transition with structural changes due to loss of methanol from the solvate, and three-dimensional FTIR spectra indicated that the process was complete by 100 degrees C. The kinetics of desolvation and sublimation from each crystal form of AQCA were determined. The activation energy of desolvation of methanol from solvate was 425.8 kJ/mol, and the activation energy of sublimation from Form I or desolvated crystals was 182.7 kJ/mol.