Solid Forms, Crystal Habits, and Solubility of Danthron

The polymorphism, crystal habits,
and solubility of 1,8-dihydroxyanthraquinone
(danthron) were investigated in acetic acid, acetone, acetonitrile, n-butanol, and toluene. The solubility was determined for
the commercially available form (FI) from 293.15 K to 318.15 K by
the gravimetric method. The influence of solvents on crystal habit
and polymorphic form has been investigated. Three different crystal
habits of danthron were obtained from slow evaporation and cooling
experiments. By evaporation, thin squares of FI were obtained from n-butanol and toluene solutions while both FI and fine needles
of FII were obtained from acetone and acetonitrile solutions. In addition,
needle-shaped solvate crystals were obtained from acetic acid solutions
and the structure of the solvate was solved by single crystal X-ray
diffraction. From cooling crystallization experiments, mixtures of
FI and FII were often obtained from various solvents, but FI and FII
possess distinct habits which can be easily distinguished by visual
comparison. Slurry conversion experiments have established that FI
is the thermodynamically stable polymorph of danthron at ambient conditions.
Differntial scanning calorimetry (DSC) and high-temperature powder
X-ray diffraction (PXRD) have shown that both FI and FII will transform
into a high-temperature form (FIV) around 435 K to 439 K before this
form melts at 468.5 K. FI, FII, and FIV have been characterized by
transmission and high-temperature PXRD, scanning electron microscopy,
infrared spectrometry, Raman spectrometry, thermogravimetric analysis,
and DSC. The solubility of danthron FI in the pure organic solvents
of the present work and in the temperature range investigated is below
4.3 % by weight and decreases in the order toluene, acetone, acetonitrile,
and n-butanol.