Cationic effect on properties related to thermal stability and ignition delay for hypergolic ionic liquids

Hypergolic ionic liquids (HILs) are considered as a promising alternative of toxic hydrazine derivatives in the field of liquid bipropellants. The anionic structure plays an important role in determining the hypergolic performance of HILs. Nevertheless, the structure–activity relationship between the cationic structure and ignition properties is still unclear. In this study, the aromaticity, electrostatic potentials (ESPs), natural bond orbital (NBO) analysis, and molecular orbital gap of 25 ionic liquids based on dicyanamide anion were studied to gain more insight into the correlation between cationic structure and hypergolic properties. For the HILs based on the same type of cation with different substituents, better aromaticity leads to better thermal stability of HILs. For the HILs with different types of cations, when the ESPs distribution is relatively concentrated around a certain value, the thermal stability of HILs is relatively higher. The second-order perturbation energy (E(2)) would be responsible for ignition delay (ID) time. The larger the E(2), the more stable the system, and the longer the ID time of HILs. It is also proved for the first time that the energy gap between the HOMO of cations and LUMO of HNO₃ could not be used as a method to determine whether this cation-based IL is hypergolic. All these results can be fully justified with experimental data and have important theoretical instructive significance in the design of new high-performance HILs.