Role of Cation Structures for Energetic Performance of Hypergolic Ionic Liquids

In recent years, novel hypergolic ionic liquids (HILs) have attracted much attention in energy and fuel fields due to their prominent advantages, including low volatility, high thermal stability, excellent energetic performances, and outstanding designability. To investigate the influence of cation structure on the properties and performances of HILs, nine different imidazolium dicyanamide-based ionic liquids were prepared, characterized, and evaluated as potential hypergolic fuels. The heat of formation and the net proton transfer (NPT) energies were calculated by an optimized theoretical model. Ignition delay (ID) time was obtained by a droplet test with a high-speed camera. The results suggest that all ionic liquids possess high density (>1 g/mL) and high thermal stability. 1-Ethyl-3-methyl imidazolium dicyanamide has the lowest viscosity of 15.2 cP, and 1-allyl-3-methyl imidazolium dicyanamide has the shortest ID time of 25 ms. The viscosities, densities, and energy properties depend on the structure of the imidazolium cation. Different physical properties lead to the different ignition process and ID time. Low viscosity is helpful to the contact and mixing of reactants, and therefore, HILs with lower viscosity have shorter ID time. High heat of formation and NPT energy can also facilitate the hypergolic reaction. Some design strategies for HILs are proposed.