Energetic Ionic Liquids (EILs)

The beginning of the xxi ^e century is particularly interesting in the field of energy. Indeed, the pollution and additional greenhouse effect caused by the combustion of fossil fuels are driving the search for new fuels that are more attractive from an environmental point of view and, if possible, from an energy point of view (energy efficiency). These new fuels are not totally "green", but they do at least partially overcome some of the drawbacks of current fuels. This ecological concern is not limited to automotive and aviation fuels, but extends to all fuels, including space fuels. The prospects of the Americans, Russians, Chinese, Indians and Europeans "reconquering" the Moon in the medium term, and the conquest of Mars in the long term, make the potential developments described in this article extremely interesting. One of the most widely used bipropellant (diergolic) combinations for satellite propulsion and interplanetary missions, monomethylhydrazine (MMH)/nitrogen tetroxide (NTO), also falls within this theme. These fuels (MMH) and oxidizers (NTO) are shipped in a liquid state (known as storable liquid propellants), but their vapour pressures are high, and they are toxic and carcinogenic. What's more, because MMH has a positive enthalpy of formation, it is capable of sustaining a so-called simple decomposition flame, making this fuel potentially flammable, even "explosive", even in the absence of an oxidizer. In addition, MMH has poor compatibility with many materials, on which it decomposes exothermically fairly rapidly, while NTO is corrosive to some materials. It is clear from the above facts that there are many arguments to support the search for a fuel capable of replacing MMH. The replacement of nitrogen tetroxide NTO is also probably of interest, but this question is less crucial. NTO itself is not flammable and can be replaced by nitric acid or other liquid oxidizers if required. Among the alternatives proposed to replace MMH are ionic liquids, which have a low vapor pressure at room temperature, an advantage over MMH. However, the main interesting feature of the bipropulsive MMH/NTO combination is that MMH and NTO form so-called hypergolic mixtures, i.e. capable of self-ignition by simple contact at room temperature. This is an interesting feature for satellite propulsion, since no external device (such as an electric spark) is required to ignite the MMH/NTO mixture. The prospect of hypergolic ignition of ionic liquids (ILs) requires a detailed understanding of the underlying chemical pathways and reaction mechanisms involved when ionic liquids are used with NTO or any suitable oxidant (Red Fuming Nitric Acid (RFNA), White Fuming Nitric Acid (WFNA) among the most frequently used). The prospect of developing hypergolic ionic liquids...