Probing the influence of electronic effects of organic additives on the morphology of sodium chloride crystals: a combined experimental and computational study

Abstract

The influence of urea, thiourea, formamide and thioformamide on the morphology of sodium chloride has been studied both experimentally and computationally. The oxo analog additives have a pronounced effect on the morphology of sodium chloride to change from cubic to octahedron crystals, however, the thio analogs are ineffective to change the habit of sodium chloride. The striking difference in the behaviour of similar oxo and thio additives on the habit of sodium chloride has been examined employing molecular electrostatic potential (MESP) surface and 3D periodic surface density functional calculations. MESP analysis showed that the sites of interaction for oxo and thio analogs would be different with the electrostatic polar {111} surface of sodium chloride. The model CBS-QB3 calculations supported the electrostatic potential analysis for the sites of interaction of these four additives with NaCl. Periodic surface calculations performed with slab models suggest that the thio analog binds weakly to {111} surface of NaCl (> 10.0 kcal mol⁻¹) and water molecules can possibly compete towards binding with such surfaces. To the best of the author's knowledge, this is the first systematic study to unravel the importance of electronic effects towards the habit of sodium chloride crystals with additives of similar molecular structures and volume. Furthermore, these results support the model of effective interaction of additives with the certain face of sodium chloride crystals for their change in the morphology. (R. Speidel, Neues Jahrbuch fur Mineralogie, 1961, Part 4, 81; N. Cabrera, D. A. Vermilyea, Growth and Perfection of Crystals, John Wiley & Sons Inc., 1958, 393; Bunn, C. W. Proc. Roy. Soc. A 1933, 141, 567.)