Spectroscopic Study on the Intermediates and Reaction Rates in the Oxidation of Levitated Droplets of Energetic Ionic Liquids by Nitrogen Dioxide

Abstract

To optimize the performance of hypergolic, ionic-liquid-based fuels, it is critical to understand the fundamental reaction mechanisms of ionic liquids (ILs) with the oxidizers. We consequently explored the reactions between a single levitated droplet of 1-butyl-3-methylimidazolium dicyanamide ([BMIM][DCA]), with and without hydrogen-capped boron nanoparticles, and the oxidizer nitrogen dioxide (NO). The apparatus consists of an ultrasonic levitator enclosed within a pressure-compatible process chamber interfaced to complementary Fourier-transform infrared (FTIR), Raman, and ultraviolet-visible spectroscopic probes. First, the vibrational modes for the Raman and FTIR spectra of unreacted [BMIM][DCA] are assigned. We subsequently investigated the new structure in the infrared and Raman spectra produced by the reaction of the IL with the oxidizer. The newly produced peaks are consistent with the formation of the functional groups of organic nitro-compounds including the organic nitrites (RONO), nitroamines (RR'NNO), aromatic nitro-compounds (ArNO), and carbonitrates (RR'C═NO), which suggests that the nitrogen or oxygen atom of the nitrogen dioxide reactant bonds to a carbon or nitrogen atom of [BMIM][DCA]. Comparison of the rate constants for the oxidation of pure and boron-doped [BMIM][DCA] at 300 K shows that the boron-doping reduces the reaction rate by a factor of approximately 2. These results are compared to the oxidation processes of 1-methyl-4-amino-1,2,4-triazolium dicyanamide ([MAT][DCA]) with nitrogen dioxide (NO) studied previously in our laboratory revealing that [BMIM][DCA] oxidizes faster than [MAT][DCA] by a factor of about 20. The present measurements are the first studies on the reaction rates for the oxidation of levitated ionic-liquid droplets.

Controlled Chemistry via Contactless Manipulation and Merging of Droplets in an Acoustic Levitator

“…Since nitric acid partly dissociates in aqueous solution to form the nitrate (NO 3 – ) and hydronium (H 3 O + ) ions, the Raman spectrum for the aqueous nitric acid droplet was identified by comparison with the 10 Raman active vibrational modes of molecular nitric acid and the 5 Raman active modes of the nitrate anion (Table S3). , For the Raman spectrum of the [BMIM]­[DCA] droplet, the vibrational modes for the numerous observed peaks are assigned in Table S4 by comparison with the wavenumbers for the 9 normal modes of the [DCA] − anion and the 75 normal modes of the [BMIM] + cation . Clearly, the Raman spectrum of the merged droplet differs significantly from the spectra of the unreacted nitric acid and [BMIM]­[DCA] droplets.…”

“…The levitator experimental apparatus has been discussed in detail previously. , In the acoustic levitator, ultrasonic sound waves with a frequency of 58 kHz are produced by a piezoelectric transducer and reflect from a concave plate to generate a standing wave . The sound waves produce acoustic radiation pressure, , which levitate a sample slightly below one of the pressure minima of the standing wave.…”

“…The reaction rate constants can be determined by analyzing the growth curves for formation of the new peaks or the decay curves for the reduction of the unreacted peaks in a sequence of the Raman spectra recorded as a function of time. Such an approach has previously been applied to deduce the reaction rates for the oxidation of a levitated droplet of an energetic ionic liquid by nitrogen dioxide . The fastest reaction rates that can be followed are ultimately limited by the minimum times to collect a single Raman, FTIR, or UV–vis spectrum of 0.25 s, 1 s, and 0.001 s, respectively.…”

“…Such an approach has previously been applied to deduce the reaction rates for the oxidation of a levitated droplet of an energetic ionic liquid by nitrogen dioxide. 16 The fastest reaction rates that can be followed are ultimately limited by the minimum times to collect a single Raman, FTIR, or UV−vis spectrum of 0.25 s, 1 s, and 0.001 s, respectively.…”

“…Levitation has the advantage of avoiding the complicating effects of a contacting surface and so permits the so-called “containerless processing” of a single, levitated particle. Acoustic levitation has been used to process containerlessly individual particles and single droplets. ,− To extend the acoustic technique to the transport and coalescence of levitated droplets, ultrasonic phased arrays consisting of a chessboard-type arrangement of piezoelectric transducers have been developed that horizontally transport and merge matter by the spatial and temporal modulation of the acoustic field. ,, However, the complex design and size make these acoustophoretic devices less suitable for operation within a chemical process chamber at controllable temperatures, and reduces the accessibility for the multiple, complementary spectroscopic probes required to trace the chemical reactions after merging the two droplets. Within a single-axis acoustic levitator, Bjelobrk et al united two droplets in air by merging two pressure nodes into one by varying the distance between the transducer and reflector, but the change in radiation pressure during the droplet movement adversely affects the acoustic resonance condition which can lead to disintegration of the droplets.…”