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

Abstract

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 2D-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 NaCl d 111 and formamide d 101 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.

Three study cases of growth morphology in minerals: Halite, calcite and gypsum

“…On the contrary, even if the mother solution is unsaturated with respect to formamide, its molecules are forced to organize themselves in 2D-d101 layers by the crystal field imposed by the {111}-NaCl growing substrate, so generating a 2D-epitaxy NaCl/formamide. The experimental evidence revealed by XRPD has been confirmed either by the calculated geometrical constraints to be fulfilled for the epitaxy to occur, or by the calculation of the interaction energy between a 2D-d101 layer of formamide and the {111}-NaCl bulky substrate [7].…”

“…Habit change occurring on NaCl crystals when formamide is added during growth from aqueous solution. Reprinted with permission from Pastero et al [7]. Copyright © 2012, ACS.…”

“…First, we observed [7] (Fig. 6) the products of our crystallization by a Scanning Electron Microscope, which is an instrument that allows to obtain an "objective information."…”

Habit Change of Monoclinic Hydroxyapatite Crystals Growing from Aqueous Solution in the Presence of Citrate Ions: The Role of 2D Epitaxy

“…Thus, we aim at searching for potential epitaxies between the most important faces of HAp (host) and Ca-Cit-TH (guest), as it was successfully done in preceding researches [21,[25][26][27][28][29][30].…”

Calcium Carbonate Polymorphs Growing in the Presence of Sericin: A New Composite Mimicking the Hierarchic Structure of Nacre

“…As demonstrated in many previous papers, the 2D-epitaxial growth may be considered a common thread for the modification of a crystal growth habit and phase stabilization [40][41][42][43][44]. In the case of sericin and calcium carbonate polymorphs, we started from the considerations by Mann [8] about the matching of the Ca-Ca distances in the 001 plane of aragonite along the directions [010] and [100].…”