Investigation On Crystallization Process Of L-Alanine

As an important separation and purification method, crystallization has been widely used in many industries, such as chemical, pharmaceutical and food industries. The quality of a crystalline product is usually determined by the nucleation and crystal growth processes, which are affected by many factors. In order to effectively control product quality, it is essential to understand the effects of these factors on crystallization processes. Herein, in this work, the clustering behavior of L-alanine molecules in the solution before nucleation, and the influence of supersaturation and impurity on L-alanine crystal growth rate are investigated by combining experiments and molecular simulation.Molecular dynamics (MD) simulation of the self-association of L-alanine molecules in solution indicates that L-alanine molecules mainly exist as monomers and dimers in aqueous solution, which is verified by the experimental results of the aggregation behavior of L-alanine molecules in undersaturated solution using the ultraviolet spectroscopy method.Single crystal growth experiments of L-alanine crystal show that the relative growth rate of the (120) surface (the main side face) and the (011) surface (the main end face) increases with solution supersaturation increasing, resulting in the aspect ratio decreasing with the increase of solution supersaturation. MD simulations on the two surfaces in contact with different supersaturated solutions revealed that higher supersaturation leads to a higher ratio of binding energy of solute molecules to the (120) surface and that to the (011) surface, thus a larger relative growth rate (R(120)/R(011)).The effect of L-isoleucine on the growth of the (120) and (011) surfaces of L-alanine was also studied by combining experiments and molecular simulations. Experimental results show that the growth of (120) surface is significantly inhibited by L-isoleucine, while the growth of (011) surface is promoted, resulting in the aspect ratio of L-alanine crystal increasing with impurity concentration. Through molecular simulation, it is revealed that L-isoleucine easily occupies the step site of (120) surface and thus inhibits its growth. However, it is difficult to be adsorbed on the step site of (011) surface. Instead, it promotes the diffusion of solute molecules in the solvent interface, thereby improving the growth rate of (011) surface. The simulation results are in accordance with the experimental data.