Influence of additives on the morphology and growth kinetics of L-alanine crystals.

Abstract

We have studied the growth kinetics and morphology of scL-alanine crystals, grown from aqueous solution in the presence and absence of several scL-amino acids, scD-leucine, and benzoic acid. The crystal morphology was characterized by optical goniometry, the different Miller indices were assigned to the well developed crystal faces. scL-Alanine crystals grown from water in the absence of additives are prismatic, elongated along the crystallographic c-axis bounded by the $\{$120$\}$, $\{$010$\}$, $\{$110$\}$, $\{$210$\}$ and $\{$011$\}$ faces. The growth rate of scL-alanine crystals was measured by monitoring the evolution of the crystal size distribution the change of size of individual crystals as a function of time. Results from both methods are in agreement. It was determined that the growth of scL-alanine crystals, both in the presence and in the absence of additives, is a surface-integration controlled process. From examining the growth rate dependence on supersaturation, it was determined that the $\{$120$\}$, $\{$010$\}$, and possibly $\{$011$\}$, crystal faces of scL-alanine grow following the spiral growth mechanism. The growth rate of scL-alanine crystals in the presence of hydrophobic scL-amino acids such as scL-leucine, scL-phenylalanine, and scL-valine at concentrations as low as 0.02 m (0.3% w/w) is drastically reduced and the crystals are bounded by the $\{$120$\}$ and $\{$011$\}$ faces. The morphology of scL-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. The inhibitory effect of these additives is 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.