Supercritical fluid phase separations: Implications for detonation properties of condensed explosives

High explosive experiments offer the most extensive data on mixtures at high pressures (>10 GPa=100 kbar) and high temperatures (>1000 K). We have computed the detonation properties of two explosives, PBX-9404 (C1.4H2.75N2.57O2.69Cl0.03P0.01) and PETN (C5H8N4O12), using reliable statistical mechanical theories and realistic intermolecular potentials. The composition of the chemical species is determined by minimizing the Gibbs free energy. The calculation shows that the detonation products of explosives containing C, H, N, and O atoms can separate into N2-rich and N2-poor fluid phases and that this gas–gas phase separation can affect detonation properties at some pressures and temperatures. Since N2, CO2, and H2O molecules are major detonation products, we made a separate study on binary (N2–H2O, CO2–H2O, N2–CO2) and ternary (N2–CO2–H2O) mixtures. The results of this study show that the N2–H2O system may exhibit a fluid phase separation at pressures and temperatures relevant to a detonation environment. The predicted phase separation boundary is sensitive to the N2–H2O interaction and to the addition of CO2 molecules. We suggest static and dynamic experiments on N2–H2O mixtures that should reveal whether the predicted fluid phase separation occurs and that will help us refine the N2–H2O interaction potential.