Effects of molecular structure and growth kinetics on the morphology of l-alanine crystals

Abstract

The crystal morphology of l-alanine crystals was characterized by optical goniometry, the different Miller indices being assigned to the well developed crystal faces. l-Alanine crystals grown from water in the absence of additives are prismatic, and are elongated along the crystallographic c-axis bounded by the {120}, {010}, {110}, {210} and {011} faces. The growth rate of l-alanine crystals was measured by monitoring the change of size of individual crytals as a function of time. Examination of both the growth rate dependence on supersaturation and the structure of the crystal surface, reveals that the {120}, {010}, and{01} crystal faces of l-alanine grow following the spiral growth mechanism, in both the presence and absence of additives. The growth rate of l-alanine crystals in the presence of hydrophobic l-amino acids at concentrations as low as 0.02 m (i.e., 0.3 g additive/100 g solvent or 0.18 g additive/100 g l-alanine) is drastically reduced and the crystals are bounded by the {120} and {011} faces. The morphology of l-alanine crystals depends on the relative growth rate of the faces and the growth rate of a crystal face is decreased by the adsorption of the additive. The molecular structure of a face determines the availability of sites that favor the adsorption of the additives. The available sites and their energy, on a given crystal face, determine the extent of adsorption.