Imidazolium based energetic ionic liquids for monopropellant applications: a theoretical study

A large variety of 1-ethyl-3-methylimidazolium ([EMIm](+)) based energetic ionic liquids (ILs) have been studied via their ion pair ([EMIm](+)[X](-)) formation using high accuracy G3MP2 method and density functional theory (DFT) methods M06L, M05-2X, M06-2X and B3LYP. The selected X- includes nitrogen rich derivatives of tetrazolate and triazolate, dinitramine, dicyanamide as well as conventional anions BF4- and PF6-. The nitrogen enrichment in the system produces energetic ionic liquids (EILs) which showed comparable and in some cases superior thermochemical, fluid and specific impulse (Isp) properties than conventional ionic liquids. The binding energy values for [EMIm]+[X] - are in the range 336-400 kJ mol(-1) at DFT levels while the atomization procedure used to compute their heat of formation (Delta H-f degrees) at the G3MP2 level produced results in very close agreement with available experimental data (maximum deviation < 5%). The Delta H-f degrees of conventional ILs is negative whereas that of EILs (167-559 kJ mol(-1)) confirmed their high energy state. The predicted Isp of all EILs are slightly lower compared to hydrazine in monopropellant systems whereas a significant increase in Isp is observed with the addition of hydroxyl ammonium nitrate (HAN). A good linear correlation between Isp and the wt% of (N + O) content of the EIL is also observed. Our results suggest that imidazolium based energetic ionic liquids have attractive thermochemical properties for use as green substitutes to hazardous hydrazine for monopropellant application in spacecraft technology.