Molecular packing, morphological modeling, and image analysis of cyanazine crystals precipitated from aqueous ethanol solutions

The phase behavior and morphology of cyanazine crystallized from aqueous ethanol solutions are reported. Two distinct crystalline phases were observed. A single crystal X-ray structure determination revealed a monohydrate phase, having plate morphology and containing ordered sheets of water, in addition to a previously determined anhydrous phase having needle morphology. Molecular modeling studies were used to predict the different growth morphologies of the two crystal phases using both geometric and attachment energy models. In the presence of aqueous ethanol, a slow conversion of plate crystals to needle crystals was observed at all temperatures. Particle morphology was taken as a diagnostic for the relative amounts of the monohydrate and anhydrous phases present in the crystallizing mixture; consequently, the anhydrous phase was concluded to be the thermodynamically stable phase at all temperatures studied (above 10 degreesC). Image analysis of cyanazine crystals grown from agitated supersaturated solutions showed that the initial predominant crystal morphology was highly dependent upon solution supersaturation and water weight fraction, and to a lesser extent the temperature of the precipitating solution.