Understanding Growth Morphology Changes of γ-Glycine and DL-Alanine Polar Crystals in Pure Aqueous Solutions

The growth kinetics of gamma-glycine and DL-alanine crystals in pure aqueous solutions was studied systematically using in situ microscopic observations on single crystal seeds. It was found that the growth behaviors of these two polar crystals are very similar. A fairly large dead supersaturation zone is observed respectively for their growth along the polar c-axis. Over a wide supersaturation range. the growth of these crystals along the polar c-axis remains remarkably slower than that along other axes, leading to a drastic morphological change from needle-like to prismatic pyramidal shape. This slow growth and the existence of the dead zone are attributed to the preferential adsorption of solvent water at the polar c ends, which is supported by predicted binding energies for solvent-surface interactions. With the increase in supersaturation, however, the growth along the polar c-axis, predominantly at the -c end, changes from the slowest to the fastest thereby yielding the usual needle-shaped morphology. This observed growth acceleration is explained based on structural features of the faces. The implication of these growth phenomena for the previously reported competitive formation of alpha-glycine and gamma-glycine is discussed.