Precise Habit Modification of Polar dl-Alanine Crystal by Control of Supersaturation

It has long been reported that a dl-alanine crystal grows considerably faster from its flat −c end than from its pointed +c end, showing typical unidirectional growth from the −c direction and exhibiting a polar needlelike habit. Surprisingly, our new experimental results reveal that the normally slow-growing hemihedral faces at the pointed +c end can undergo fast growth at high supersaturations, contrary to the postulation of the “relay” mechanism. At the same time, a new (001) face at the +c end develops, causing the initially pointed +c end to become flat and leading to a habit modification from needlelike to rodlike with an increase of supersaturation. Furthermore, both the flat (001) face at the +c end and the (001̅) face at the −c end can grow equally fast, and hence, the usually observed unidirectional growth from the −c direction does not hold any more. These new observations are discussed and rationalized.