Hypergolic ionic liquids: to be or not to be?

Abstract

Hypergolic ionic liquids (HIL) – ionic liquids which ignite spontaneously upon contact with an oxidizer – emerged as green space propellants. Exploiting the previously marked hypergolic [EMIM][CBH] – WFNA (1-ethyl-3-methylimidazolium cyanoborohydride – white fuming nitric acid) system as a benchmark, through the utilization of a novel chirped-pulse droplet-merging technique in an ultrasonic levitation environment and electronic structure calculations, this work deeply questions the hypergolicity of the [EMIM][CBH]-WFNA system. Molecular oxygen is critically required for the [EMIM][CBH]-WFNA system to ignite spontaneously. State-of-the-art electronic structure calculations identified the resonantly stabilized N-boryl-N-oxo-formamide [(H₃B–N(O)–CHO)⁻; BOFA] radical anion as the key intermediate in driving the oxidation chemistry upon reaction with molecular oxygen of the ionic liquid. These findings challenge conventional wisdom of ‘well-established’ test protocols as indicators of the hypergolicity of ionic liquids thus necessitating truly oxygen-free experimental conditions to define the ignition delay upon mixing of the ionic liquid and the oxidizer and hence designating an ionic liquid as truly hypergolic at the molecular level.