Effect of Additives on the Crystal Growth of L-Aspartic Acid

Abstract

We examined the effect of the amino acid asparagine (Asn) as a tailor-made additive to control the face-selective crystal growth rate of L-aspartic acid (L-Asp). The growth rate in the c-axis direction decreased in the presence of L-Asn and D-Asn. The Kubota–Mullin growth model was found to describe the decreased crystal growth in the c-axis direction well. Different factors were found to be responsible for the effectiveness of L-Asn and D-Asn. The different growth–decreasing effects of L-Asn and D-Asn on the L-Asp crystal in the c-axis direction were due to their rates of adsorption on the active sites, which were related to the different configurations of the two additives. Based on AFM observations, a pinning mechanism was proposed as the origin of the decreased growth rate in the presence of L-Asn. We attributed this pinning mechanism to the repulsion between the lone pair of the nitrogen atom of the L-Asn NH₂ group, which was present on the crystal surface due to the adsorption of L-Asn, and the oxygen atom of the L-Asp carboxyl group.