Growth kinetics and morphological change of polymorphs of L-glutamic acid

Abstract

The growth kinetics and morphological change of α and β crystals of L-glutamic acid (L-Glu) were investigated using the single-crystal method. The growth rates of α crystals in the [1 1 0]. [0 1 0] and [0 0 1] directions, and that of β crystals in the [1 0 0] direction appeared to be controlled by surface reaction. Morphological changes were observed in the growth of α seed crystals. The dependence of the growth rate on the relative supersaturation indicated that the growth mechanism of α crystals in all directions belongs to the Nuclei-above-nuclei mechanism rather than the BCF mechanism. The morphological change of α crystals is due to the differences in the dependence of the growth rate on the supersaturation between (1 1 1) and (1 1 0) faces, indicating the important contribution of the kinetic process to the morphological change. The growth mechanism of β crystals is also due to the nuclei-above-nuclei mechanism. At the same supersaturation, the growth rate of the (1 1 1) face of an α crystal is several times of that of the (1 0 1) face of β, corresponding to the selective crystallization of α at 298 K. The edge free energy of the (1 0 1) face of β appeared to be larger than that of α crystals ((1 1 1) and (0 0 1) faces). Furthermore, the ratio of the growth rate of β to that of α tended to increase with increased supersaturation. The higher growth rate of α rather than β may be due to the large kinetic factor (NaN model) and low edge free energy of α.