ReaxFF molecular dynamics simulations of intermediate species in dicyanamide anion and nitric acid hypergolic combustion

Ionic liquids based on the dicyanamide anion (DCA) are of interest as replacements for current hypergolic fuels, which are highly toxic. To better understand the reaction dynamics of these ionic liquid fuels, this study reports the results of molecular dynamics simulations performed for two predicted intermediate compounds in DCA-based ionic liquids/nitric acid (HNO₃) combustion, i.e. protonated DCA (DCAH) and nitro-dicyanamide-carbonyl (NDC). Calculations were performed using a ReaxFF reactive force field. Single component simulations show that neat NDC undergo exothermic decomposition and ignition. Simulations with HNO₃ were performed at both a low (0.25 g ml^-1) and high (1.00 g ml^-1) densities, to investigate the reaction in a dense vapor and liquid phase, respectively. Both DCAH and NDC react hypergolically with HNO₃, and increased density led to shorter times for the onset of thermal runaway. Contrary to a proposed mechanism for DCA combustion, neither DCAH nor NDC are converted to 1,5-Dinitrobiuret (DNB) before thermal runaway. Details of reaction pathways for these processes are discussed.