Precipitation mechanism of stable and metastable polymorphs of L‐glutamic acid

Precipitation of the polymorphic compound L‐glutamic acid was initiated by premixing aqueous solutions of sodium L‐glutamate and sulfuric acid. Samples of the supersaturated solution were either subjected to vigorous post‐stirring or left under quiescent conditions. For low supersaturation (S ≤ 13) without post‐stirring aggregated platelet‐shaped crystals of the stable beta polymorph formed while post‐stirring generated large prismatic crystals of the metastable alpha polymorph. For high supersaturation (S ≥ 17) first smooth spheres were observed which transformed into rough spherulitic crystals of the beta phase. For high supersaturation it is proposed that the spheres are a metastable phase consisting of droplets formed by liquid‐liquid phase separation. Subsequently from these spheres crystals of the stable beta phase nucleate. For low supersaturation without post‐stirring aggregated beta platelets form according to the same mechanism, while with post‐stirring concentration fluctuations are equalized and metastable alpha crystals nucleate from the homogenized solution.