Crystal Morphology Prediction and Anisotropic Evolution of 1,1-Diamino-2,2-dinitroethylene (FOX-7) by Temperature Tuning

Abstract

Temperature-induced morphological changes are one of the strategies for designing crystal shapes, but the role of temperature in enhancing or inhibiting crystal growth is not well understood yet. To meet the requirements of high density and low sensitivity, we need to control the crystal morphology of the energetic materials. We studied the crystal morphology of 1,1-diamino-2,2-dinitroethylene (FOX-7) in dimethyl sulfoxide/water mixed solvent by using the modified Hartman-Perdok theorem. Molecular dynamics simulations were used to determine the interaction of FOX-7 and solvents. The results showed that the crystal shape of FOX-7 is hexagonal, the (101) face is the largest exposed face and is adjacent to six crystal faces at 354 K. As the temperature goes down, the area of the (001) face is significantly reduced. The crystal morphology of FOX-7 at 324 K has a smaller aspect ratio of 4.72, and this temperature is suitable for tuning the morphology from slender hexagon into diamond. The prediction results are in remarkable agreement with the experiments. Moreover, we predicted the evolution path of FOX-7 morphology by Gibbs-Curie-Wulff theorem and explained the variation of crystal shape caused by different external conditions in the actual crystallization process. Crystallization as a green separation process is characterized by low energy consumption and high efficiency 1-3. The growth shape that a crystal obtains in the course of its formation is also highly sensitive to growth conditions, and reflects the growth mechanism, as does the surface morphology 4-7. Therefore, the growth shapes make it possible to judge the formation conditions and to correct the growth parameters in experiment. Based on the periodic bond chain (PBC) theory, Hartman and Perdok stated that the growth rate of a face is lower if fewer chains of strong bonds cross this face 8,9. This is the well-known Hartman-Perdok theorem (H-P theorem). The equilibrium crystal morphology can be determined by calculating the attachment energy. Gibbs stated that the polyhedral features of the crystal shape reduce the total surface energy. Curie defined that the normal growth rate of the crystal face is in proportion to the surface free energy 10. Wulff further proved that the distance from the crystal center to crystal face is proportional to the specific surface free energy in equilibrium 11. This relationship is also called the Gibbs-Curie-Wulff theorem 12,13. The prediction and regulation of crystal morphology is essential in the fields of pharmaceuticals 14,15 , catalysis 16,17 , functional ceramics 18 , thin film materials 19 , energetic materials 20,21 , etc. For example, the crystal morphologies influence the sensitivity of the energetic materials. In the formation of a drug product, crystals with high aspect ratio shapes are troublesome in the subsequent processing steps. In the field of catalysis, it is efficient for the catalyst to expose more active crystal face. Anisotropic morphologies, especially the large aspect ratio of one-dimensional needl...

Different Reactivity of Birnessite and Cryptomelane toward Isoprene: Effect of Structure, Morphology, and Exposed Faces

“…According to the Gibbs–Curie–Wulf theorem, the normal growth rate of the crystal face is proportional to the surface free energy . This theory suggests that the (001) surface on the cross section is more active than the ( hk 0) surface along the nanorods.…”

“…According to the Gibbs−Curie−Wulf theorem, the normal growth rate of the crystal face is proportional to the surface free energy. 34 This theory suggests that the (001) surface on the cross section is more active than the (hk0) surface along the nanorods. Thus, in the oxidation of CO, cryptomelane with short nanorods shows higher catalytic activity than that with long nanowires.…”

Preparation of self-assembled FOX-7 nanosheets and their performance

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Crystal regeneration – a unique growth phenomenon observed in organic crystals post breakage

“…Non-mechanistic (thermodynamic) theories that form the basis of equilibrium morphology prediction have developed drastically since the late 1800's from considering surface energetics, crystal geometry, through to lattice energies and synthonic (intermolecular) interactions. 10,11 Although longestablished, these theories are considered system specific and fail to take into consideration the crystallisation environment Although mechanical breakage is a common occurrence in industrial crystallisers, there exists limited knowledge on the consequent growth of broken crystals. Owing to this, specific post-breakage growth kinetics are rarely accounted for during process modelling, resulting in unreliable predictions of crystal shape and size.…”

“…Non-mechanistic (thermodynamic) theories that form the basis of equilibrium morphology prediction have developed drastically since the late 1800's from considering surface energetics, crystal geometry, through to lattice energies and synthonic (intermolecular) interactions. 10,11 Although long-established, these theories are considered system specific and fail to take into consideration the crystallisation environment which can radically influence the crystal shape and morphology. 12 As a result, mechanistic (kinetic) models such as the layered growth (spiral growth and Birth & Spread mechanism) and rough growth theories are used to demonstrate that the anisotropic growth of crystal facets depends on the solution properties, solute surface kink density, and the hydrodynamic environment.…”