Investigating Additive Effects on α-Glycine Growth through the Measurement of Facet Specific Growth Rates

Abstract

The presence of trace amounts of additives during crystal growth can have a significant impact on the physical properties of the crystallizing substrate (e.g., crystal morphology, purity, polymorphic phase, or growth kinetics). In this work, we report the growth of α-glycine crystals (α-gly) in the presence of a variety of diverse additives: two l-amino acids, two organic acids, α-iminodiacetic acid, and two chloride salts. Growth rate data from imaging, together with analytical techniques such as X-ray photoelectron spectroscopy (XPS) and fluorescence microscopy, are used to observe which facet growth is impacted by the additive and to what extent. Relating these findings to the α-gly crystal structure provides explanations for the observed effects. Specifically, the growth inhibition of the (02̅0) facet α-gly in the presence of l-tryptophan and l-methionine shows how the prochirality of glycine results in two symmetrically equivalent facets growing at different rates. In the presence of malonic acid and salicylic acid, growth of the {011} facets is inhibited as a result of the interaction of deprotonated acids at the {011} surfaces. We find α-iminodiacetic acid to be an extremely effective inhibitor of α-gly, stopping the growth of both the {011} and {020} facets. We correlate the effectiveness of α-iminodiacetic acid to its structural similarity to gly, allowing it to easily block the growth of two α-gly facets. Finally, we observe the incorporation of the metal ions Fe(II), Cu(II), and Zn(II) into the {011} facets of α-gly. Interestingly, in the cases of Cu(II) and Zn(II), the incorporation of the metals into the α-gly lattice does not cause a noticeable change in the growth rates. The formation of coordination complexes containing the metal ions and glycine ligands allows for the observed incorporation of the metals into the α-gly lattice with limited disturbance to its crystal growth.

Growth rate data of α-glycine facets in the presence of a varied selection of additives is presented together with a molecular-level rationalization of the mechanisms.