Growth and Morphology of L-Alanine Crystals: Influence of Additive Adsorption

Abstract

The effect of L-amino acids, as additives, on the crystal growth and morphology of L-alanine crystals has been studied. The crystal growth of L-alanine is described by the spiral growth mechanism. From examining the growth rate dependence on supersaturation at constant additive concentration, it is concluded that there is no change in the growth mechanism due to the presence of the different additives. L-Alanine crystals were grown both in the absence and in the presence of additives. The crystal morphology was characterized by optical goniometry assigning the different Miller indices to the well-developed crystal faces. The addition of L-amino acids selectively inhibits the development of certain L-alanine crystal faces. L-Alanine crystals grown in the presence of nonpolar amino acids, such as L-leucine, L-phenylalanine, and L-valine, at concentrations as low as 0.20 m (0.3%, w/w) develop the {120} faces, whereas the {010}, {110}, and {210} faces are not developed. The effect of these additives on the morphology of L-alanine is explained at the molecular level based on crystallographic considerations. The molecular structure of a face will determine the availability of sites that favor the adsorption of the additives. The availability of sites and their energy, on a particular crystal face, will determine the extent of adsorption. The growth rate of a crystal face is decreased by the adsorption of the additive. The inhibitory effect of these additives can be explained by a Langmuir isotherm, assuming that the inhibition of the growth rate is proportional to the degree of surface coverage and that the crystal surface is homogeneous with respect to the energy of adsorption sites.