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Strong Additive–Surface Interaction Leads to the Unusual Revival of Growth at Solvent-Poisoned Faces of dl-Alanine Crystal

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journal contribution
posted on 20.02.2016 by Guangjun Han, Pui Shan Chow, Reginald B. H. Tan
The effects of various anions and cations dissociated from a spectrum of inorganic salts on the growth of dl-alanine polar c-axis have been systematically investigated using in situ microscopic observations on single seed crystals. Surprisingly, it is observed that both the polar −c and +c ends of dl-alanine c-axis undergo a revival of solvent-poisoned dead growth in the presence of every one of the additives used in this study. Furthermore, it is found that divalent ions (e.g., Mg2+ and SO42–) promote the growth along dl-alanine c-axis more effectively than univalent ions (e.g., Na+ and Cl). Such an ion-induced growth acceleration of dl-alanine c-axis, contrary to the usual understanding, particularly shows an unusual case where important Coulombic interaction between additive and crystal surface can actually favor face growth. This interesting phenomenon is interpreted at the molecular level, with the role of surface roughening particularly highlighted.