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

The growth kinetics and morphological change of alpha and beta crystals of L-glutamic acid (L-Glu) were investigated using the single-crystal method. The growth rates of alpha crystals in the [1 1 0]. [0 1 0] and [0 0 1] directions, and that of beta crystals in the [1 0 0] direction appeared to be controlled by surface reaction. Morphological changes were observed in the growth of alpha seed crystals. The dependence of the growth rate on the relative supersaturation indicated that the growth mechanism of alpha crystals in all directions belongs to the Nuclei-above-nuclei mechanism rather than the BCF mechanism. The morphological change of a 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 beta 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 alpha crystal is several times of that of the (1 0 1) face of beta, corresponding to the selective crystallization of alpha at 298 K. The edge Gee energy of the (1 0 1) face of beta appeared to be larger than that of alpha crystals ((1 1 1) and (0 0 1) faces). Furthermore, the ratio of the growth rate of beta to that of alpha tended to increase with increased supersaturation. The higher growth rate of alpha rather than beta may be due to the large kinetic factor (NaN model) and low edge Free energy of alpha. (C) 2000 Elsevier Science B.V. All rights reserved.