Hypergolic ionic liquids (HILs) have emerged as a green energy alternative in the space propulsion technologies replacing toxic and carcinogenic hydrazine and its derivatives. By definition, HILs are a class of ionic liquids which ignite spontaneously upon contact with an oxidizer. To understand the origin of hypergolicity, the complex reaction sequence has been unraveled by exploring a previously labelled hypergolic [EMIM][CBH]-WFNA (1-ethyl-3-methylimidazolium cyanoborohydride - white fuming nitric acid) system on the molecular level through the exploitation of a novel chirped-pulse droplet-merging technique in a controlled ultrasonic levitation environment along with state-of-the-art electronic structure calculations. The merging process of the reactant droplets was probed in-situ by an array of time-resolved resolved, synchronized spectroscopic detections (FTIR, Raman, UV-Vis emission); the visualization of the simultaneous physical changes was captured by high-speed optical and infrared cameras. Overall, the findings not only deciphered initial oxidation steps and intermediates, but also raised severe concerns against traditional drop test methodologies, while determining the hypergolicity as the given bipropellant system did not ignite in a molecular oxygen free surrounding with an oxidizer to ionic liquid molar ratio exceeding five. The study has been extended by replacing WFNA with a less corrosive oxidizer - hydrogen peroxide (H₂O₂) which revealed an extensive facile oxidation of the boron center in the CBH anion. This set of studies demonstrates a very first step toward a systematic elucidation of the hypergolicity and the intricate reactivity of the ionic liquids. Further studies aiming at different categories of anionic and cationic components would provide us a broader understanding and overview of the efficiencies, such as improving the ignition delay (ID) and extent of energy release.

Chirped pulse triggered merging stages of ionic liquid and oxidizer droplets in an ultrasonic levitator.

Dababrata Paul

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