THE INFLUENCE OF ADDITIVES ON THE GROWTH-KINETICS AND MECHANISM OF L-ALANINE CRYSTALS

Results for the characterization of the growth kinetics and mechanism of L-alanine crystals, grown from aqueous solution in the presence and absence of tailor-made additives are reported. L-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 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 {011} crystal faces of L-alanine grow following the spiral growth mechanism, both in the presence and absence of additives. This mechanism was confirmed by examining the surface topography of crystals by atomic force microscope. The growth rate of L-alanine crystals in the presence of L-leucine and L-phenylalanine 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 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.