Solid–Liquid\nPhase Equilibrium and Dissolution\nProperties of 1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane Form\nα in Pure Organic Solvents

For solid-state energetic materials, it might be able\nto exist\nin different crystal structures, which is well-known as polymorphism.\nDifferent polymorphs of the crystals exhibit different physiochemical\nproperties, such as solubility, dissolution rate, morphology, <i>etc</i>. In this work, the solid–liquid phase equilibrium\nof 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane form α (α-HMX)\nover various temperature ranges (298.15 to 358.15 K) was experimentally\ndetermined using the dynamic method in 12 pure solvents, including\nacetone, pyridine, acetic acid, 2-butanone, cyclopentanone, acetonitrile,\nmethyl acetate, ethyl acetate, isopropyl acetate, ethyl acetoacetate,\nmethyl propyl ketone, and dimethyl sulfoxide (DMSO), at atmospheric\npressure. The experimental results indicate that the solubility of\nα-HMX increases with increasing temperature in all test solvents.\nThe experimental solubility data were correlated with four thermodynamic\nmodels, which are the modified Apelblat equation, the λ<i>h</i> equation, the Wilson model, and the NRTL model. In addition,\nthe dissolution thermodynamic properties, such as the enthalpy, entropy,\nand Gibbs free energy change, were also calculated with the experimental\ndata and NRTL model. All of these results lay the foundation for the\nstudy of preparation and phase transformation of different crystal\nforms of HMX and provide guidance for designing suitable crystallization\nmethods in different solvents.