Selective Adsorption/Absorption of Formamide in NaCl Crystals Growing from Solution

NaCl crystals were obtained from water–formamide (H–CO-NH 2 ) solutions, either by slow evaporation at 30 °C or by programmed cooling of solutions saturated at 95 °C, the formamide concentration ranging from 0 to 100% (in weight). Accordingly, the crystal morphology changes from {100} (pure aqueous solution) to {100} + {111} (water-formamide solutions) to {111} (pure formamide solution). X-ray powder diffraction diagrams, carried out on the bulky crystallized population, prove that formamide is not only adsorbed on the {111} NaCl octahedron but is also selectively captured within the {111} growth sectors. The excellent two-dimensional lattice coincidences between the d 101 layers of formamide and the NaCl - d 111 ones suggest that formamide can be adsorbed in the form of ordered epitaxial layers; further, the striking equivalence between the thickness of the elementary layers d 111 NaCl and d 101 formamide indicates that formamide is allowed to be buried (absorption) in the growing crystal. Moreover, empirical force field calculations carried out on reconstructed {111} NaCl surfaces, both Na + and Cl – terminated, allowed to evaluate the adhesion energy between the formamide epitaxial layers and the underlying {111} NaCl substrate. Hence, one can definitively state that formamide is not only an habit modifier of NaCl crystals, but that “anomalous NaCl/formamide mixed crystals” form, limited to the {111} NaCl growth sectors.