Oxidation of a Levitated 1-Butyl-3-methylimidazolium Dicyanoborate Droplet by Nitrogen Dioxide

Abstract

To develop the next generation of hypergolic, ionic-liquid-based fuels, it is important to understand the fundamental reaction mechanisms for the oxidation of ionic liquids (ILs). We consequently studied the oxidation of a levitated 1-butyl-3-methylimidazolium dicyanoborate ([BMIM]­[DCBH]) droplet by nitrogen dioxide (NO2). The properties of [BMIM]­[DCBH], including short ignition-delay times, low viscosities, and a wide liquid temperature range, make the ionic liquid especially suitable as a component of a hypergolic fuel. The chemical modifications were monitored with Fourier-transform infrared (FTIR), Raman, and ultraviolet–visible spectroscopies. To identify changes induced by the oxidation, it was first necessary to assign vibrational modes to the FTIR and Raman spectra of unreacted [BMIM]­[DCBH]. The new features in the oxidized FTIR and Raman spectra could then be identified and assigned on the basis of the possible functional groups likely to form through addition with a nitrogen and an oxygen atom of nitrogen dioxide creating a new bond with the ionic liquid. The assignments suggest that organic nitro-compounds and boron–nitrogen and boron–oxygen containing compounds were produced. A large decrease in the intensity of some [DCBH]− fundamental modes suggests the nitrogen dioxide molecule prefers to react with the anion over the cation.

Cyanoborates

“…The proton affinity of selected anions, including the DHB anion,, and proton transfer energies from organic cations to these anions in energetic ILs were investigated . A number of further properties of the DHB ion and DHB ‐ILs with respect to potential applications in the field of hypergolic ILs have been studied experimentally or by theoretical methods …”

Real-Time Measurements of pH Changes in Single, Acoustically Levitated Droplets Due to Atmospheric Multiphase Chemistry

“…An early report described photometric measurements of suspended droplets and suggested that this could be possible as well in levitated droplets. Some more recent papers ,− report reflectance spectra in studies of multiphase chemistry, but these spectra are less straightforward to interpret quantitatively.…”

Thermal Decomposition and Hypergolic Reaction of a Dicyanoborohydride Ionic Liquid

“…Characterizing the energetics of complex, rapid reactions takes a combination of experimental and computational efforts. − In the present work, this approach is used to study a dicyanoborohydride-based EIL, selected due to its known hypergolicity and inclusion of boron. Energetic ionic liquids with the dicyanoborohydride anion have low ignition delays and high propellant performance characteristics, making these chemicals a desirable fuel for bipropellant systems. ,− By incorporation of boron into the chemical structure of a fuel, the energy density can be enhanced with an increase in the fuel’s volumetric enthalpy of combustion. ,− …”