Solubilities of and Transformations between the Anhydrous and Hydrated Forms of l -Serine in Water−Methanol Solutions

Methanol is used in the industrial crystallization of l -serine to reduce solubility and thereby to enhance the recovery from aqueous solutions in which l -serine has been synthesized. Earlier research had shown that the crystal form obtained from such solutions can be either the anhydrous or the monohydrate form of l -serine (Gatewood, M. D. M.S. Thesis, Georgia Institute of Technology, 1992). Solubilities of these species have been determined in the present work by equilibration of crystals with methanol−water solvents having different compositions and at different temperatures. It was found that over certain ranges of temperature and methanol concentration, l -serine crystals changed in shape from rod-like to hexagonal plates. Powder X-ray diffraction was used to determine that the rod-like crystals were anhydrous l -serine, while the hexagonal crystals were l -serine monohydrate. Although the effect of methanol on the solubility of l -serine had been known, the role solvent composition plays in determining which species is formed had not been elucidated. Furthermore, the variation in temperature where a transition occurs between one stable form and another has not been reported.