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

NaCl crystals were obtained from water formamide (H-CO-NH2) solutions, either by slow evaporation at 30 degrees C or by programmed cooling of solutions saturated at 95 degrees 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.