Impact of Solvents on the Crystal Morphology of CL-20/TFAZ Cocrystals: A Predictive Study

The cocrystallization technique has been widely applied in the fields of energetic materials (EMs) to settle the inherent trade-off between high energy and low sensitivity in current high-energy molecules. Despite its widespread application, the mechanistic understanding of cocrystals growing from solutions remains largely underexplored. This paper presents a mechanistic model grounded in the spiral growth mechanism to predict the crystal morphologies of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and 7H-trifurazano [3,4-b:3′,4′-f:3″,4″-d]azepine (TFAZ) cocrystals. In this model, it was assumed that CL-20 and TFAZ molecules incorporated into the crystal lattice simultaneously from solution as preformed growth units. The binding energies between the CL-20 molecule and TFAZ molecule were calculated to determine the most potential growth units. The predicted morphologies closely align with the experimental determinations supporting the model’s validity. Furthermore, the study found that the crystal habits were significantly influenced by the choice of solvents, due to variations in interfacial energetics affecting the growth process.