Ionic liquids are substances that are molten salts with a low boiling point (below 100 ° C). They consist of a large and asymmetrical organic cation and an organic or inorganic anion. Their advantages are low vapor pressure, wide temperature range in which they occur in liquid form, high stability, ability to dissolve many compounds and high conductivity. In recent years, interest in ionic liquids has been growing as an ecological alternative to organic solvents and in areas such as organic synthesis, electrochemistry, separation and analytical techniques, material chemistry and others.
By choosing the right compound structure, you can get high-energy ionic liquids that can be used as explosives or rocket fuels. Thus, they can successfully replace the currently used hydrazine, which is a volatile, toxic and carcinogenic substance. The advantage of ionic liquids is also the ease of working with them due to low toxicity and very low volatility. Many ionic liquids show spontaneous (hypergolytic) ignition in contact with some oxidants. This feature is very desirable in rocket propulsion, because the ignition of both components occurs spontaneously and no additional ignition system or catalyst is required. An important parameter characterizing the hypergolic fuel is its ignition delay. It is the time it takes for initiation that elapses from the moment the fuel (e.g. ionic liquid) and oxidant touch together. It is preferred that this parameter has the lowest value possible. Properly selected substances and oxidant can achieve a very low ignition delay time. Currently, the typical hyperergolic system used is dimethylhydrazine and nitrate tetroxide or fuming red nitric acid. All these compounds are strongly irritating, poisonous and dangerous. Ionic liquids are a much more advantageous alternative, the more so because some compounds show ignition with highly concentrated hydrogen peroxide, which is a substance that is quite safe, but above all non-toxic.
The table presents frequently used groups of cations and anions to create high-energy ionic liquids. The cations used are often suitable derivatives of heterocyclic compounds (such as imidazole), for this purpose by chemical synthesis products containing substituents such as 1-butyl-3-methylimidazole, 1-ethyl-3-methylimidazole, 1-allyl-3-ethylimidazole etc. are obtained. Similarly, the anions used can be appropriately modified from the borohydride to obtain cyanoborohydride or dicyandohydride. This means that the amount of ionic liquids that can be formed is basically unlimited.