Hypergolic Combustion of Nitric Acid with Protonated Dicyanamide and with Nitro-Dicyanamide-Carbonyl: A Reactive Molecular Dynamics Study

Ionic liquids are of interest as much less volatile and less toxic replacements for current hypergolic fuels.  One such ionic liquid is dicyanamide (DCA), which when it reacts with nitric acid (HNO₃) has the predicted reaction mechanism DCA -> protonated DCA (DCAH) -> nitro-dicyanamide-carbonyl (NDC) -> 1,5-Dinitrobiuret (DNB) -> combustion.  To better understand the reaction dynamics of DCA, we used a ReaxFF reactive force field to perform molecular dynamics simulations of mixtures HNO₃ with DCA (DCAH) and with NDC.  To investigate the reaction in a dense vapor and liquid phase, simulations were performed at both low (0.25 g/ml) and high (1.00 g/ml) densities.  DCAH and NDC each react hypergolically with HNO₃ with increased density leading to shorter times for the onset of thermal runaway.  Neither DCAH nor NDC are converted to 1,5-Dinitrobiuret (DNB) before thermal runaway, suggesting a new reaction path may need to be considered.