Solid–Liquid Phase Equilibrium and Dissolution Properties of 1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane Form α in Pure Organic Solvents

For solid-state energetic materials, it might be able to exist in different crystal structures, which is well-known as polymorphism. Different polymorphs of the crystals exhibit different physiochemical properties, such as solubility, dissolution rate, morphology, etc. In this work, the solid–liquid phase equilibrium of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane form α (α-HMX) over various temperature ranges (298.15 to 358.15 K) was experimentally determined using the dynamic method in 12 pure solvents, including acetone, pyridine, acetic acid, 2-butanone, cyclopentanone, acetonitrile, methyl acetate, ethyl acetate, isopropyl acetate, ethyl acetoacetate, methyl propyl ketone, and dimethyl sulfoxide (DMSO), at atmospheric pressure. The experimental results indicate that the solubility of α-HMX increases with increasing temperature in all test solvents. The experimental solubility data were correlated with four thermodynamic models, which are the modified Apelblat equation, the λh equation, the Wilson model, and the NRTL model. In addition, the dissolution thermodynamic properties, such as the enthalpy, entropy, and Gibbs free energy change, were also calculated with the experimental data and NRTL model. All of these results lay the foundation for the study of preparation and phase transformation of different crystal forms of HMX and provide guidance for designing suitable crystallization methods in different solvents.