Controlling the properties of energetic ionic liquids by stabilizing reactive nanomaterials

Abstract

Unique, accessible properties, such as high thermal stability, large liquid ranges, high heats of formation, and low to negligible volatility, have led to increased efforts to utilize ionic liquids (ILs; salts with melting points below 100 °C) to replace currently used energetic materials, such as hydrazine. Initial strategies focused on the independent design of either ion to tune the physical and chemical properties of ILs; however, often the prospective energetic ionic liquids (EILs) still suffer from low energetic densities and heat of combustion. Thus, a complementary strategy must be developed to improve deficient properties of EILs without interfering with beneficial IL properties. A correctly chosen nano-additive can be incorporated utilizing the unique solvent capability of ILs to stabilize a variety of nanomaterials, such as unoxidized nanoparticles or graphene. However, due in part to their high surface reactivity, freshly synthesized nanoparticles are typically kinetically unstable in solution. There is a constant requirement for a stabilizing force in order to keep the nanoparticles the intended size. In addition to the stereoelectronic stabilizing forces provided inherently by the IL, specific functionality can be incorporated to the EIL structure provide additional ligand-assisted stabilization of the nanomaterial in suspension. Here, it will be demonstrated that this approach can lead to stable suspensions of boron, titanium, and graphene in EILs, which were further stabilized by designed reactivity guided by traditional metal-ligand theory. One key challenge with these nanoparticulate systems has been colloidal stability, which was only on the order of less than 2-3 days in many cases. It was hypothesized that the stability of energetic additives could be improved by reducing their size from suspended nanoparticles to solutions of molecular clusters on the order of angstroms rather than nanometers. However, in the case of the incorporation of neutral borane clusters into EILs, a different set of chemical reactivity was observed. While the composite nanoparticle-IL systems were guided and stabilized by weak metal-ligand surface reactivity, the addition of nido-decaborane (B10H14) to EILs led to direct acid base chemical reactions. The initial deprotonation of B10H14 led to a cascade of reactions ultimately generating a negatively charged boronate cluster anion fully solubilized in the EIL. These fully solubilized clusters bypass the previous colloidal stability limitations of the nanomaterial-IL suspensions while still providing an enhanced energetic effect. Both strategies were successful in producing composite EIL suspensions or solutions, all containing an energetic additive incorporated for a specific effect, such as increased heat of combustion, decreased viscosity, decreased ignition delay, etc. Each neat EIL composite was evaluated to determine the effect of the addition of the selected nanomaterial or molecular cluster on the specific EIL properties, such as thermal stability, melting point, density, and viscosity. Additionally, the impacts on their hypergolic ignition properties were determined. Overall, the obtained results signify that nanomaterials and molecular clusters can be incorporated as energetic additives into EILs in order to improve upon their previously deficient properties, thereby thrusting EILs as practical energetic materials of the future.