Evaluating ionic liquids as hypergolic fuels: from reactive nanomaterials to trigger additives

International audienc

Nonaborane and Decaborane Cluster Anions Can Enhance the Ignition Delay in Hypergolic Ionic Liquids and Induce Hypergolicity in Molecular Solvents

The dissolution of <i>nido</i>-decaborane, B₁₀H₁₄, in ionic liquids that are hypergolic (fuels that spontaneously ignite upon contact with an appropriate oxidizer), 1-butyl-3-methylimidazolium dicyanamide, 1-methyl-4-amino-1,2,4-triazolium dicyanamide, and 1-allyl-3-methylimidazolium dicyanamide, led to the <i>in situ</i> generation of a nonaborane cluster anion, [B₉H₁₄]⁻, and reductions in ignition delays for the ionic liquids suggesting salts of borane anions could enhance hypergolic properties of ionic liquids. To explore these results, four salts based on [B₁₀H₁₃]⁻ and [B₉H₁₄]⁻, triethylammonium <i>nido</i>-decaborane, tetraethylammonium <i>nido</i>-decaborane, 1-ethyl-3-methylimidazolium <i>arachno</i>-nonaborane, and <i>N</i>-butyl-<i>N</i>-methyl-pyrrolidinium <i>arachano</i>-nonaborane were synthesized from <i>nido</i>-decaborane by reaction of triethylamine or tetraethylammonium hydroxide with nido-decaborane in the case of salts containing the decaborane anion or via metathesis reactions between sodium nonaborane (Na­[B₉H₁₄]) and the corresponding organic chloride in the case of the salts containing the nonaborane anion. These borane cluster anion salts form stable solutions in some combustible polar aprotic solvents such as tetrahydrofuran and ethyl acetate and trigger hypergolic reactivity of these solutions. Solutions of these salts in polar protic solvents are not hypergolic

Chemical characteristics and molecular interactions causing liquid clathrate formation

International audienc

A critical assessment of the mechanisms governing the formation of aqueous biphasic systems composed of protic ionic liquids and polyethylene glycol

An extensive study on the formation of aqueous biphasic systems (ABS) using aqueous solutions of protic ionic liquids (PILs) and polyethylene glycol (PEG) was performed in order to understand the mechanisms underlying the phase separation. Aqueous solutions of PEG polymers with different molecular weights (600, 1000, 2000, and 3400 g mol{\textminus}1) and several N-alkyl-, dialkyl-, and trialkyl-ammonium salts of acetate, propanoate, butanoate, hexanoate and octanoate were prepared and their ability to form ABS at several temperatures assessed. The ternary liquid{\textendash}liquid phase diagrams were determined at several temperatures, as well as binary PIL (or salt)-PEG-1000 and salt-water solubility data to better clarify the mechanisms responsible for the phase separation. All data gathered indicate that the formation of PEG{\textendash}PIL-based ABS is mainly governed by the PIL{\textendash}PEG mutual interactions, where PILs with a higher solubility in the polymer exhibit a lower aptitude to form ABS displaying thus a smaller biphasic region, for which a direct correlation was identified. The effects of the molecular weight and temperature of the polymer were also addressed. The increase of the PEG hydrophobicity or molecular weight favours the phase separation, whereas the effect of temperature was found to be more complex and dependent on the nature of the PIL, with an increase or decrease of the biphasic regime with an increase in temperature

Evaluating Ionic Liquids as Hypergolic Fuels: Exploring Reactivity from Molecular Structure

International audienceA total of 38 ionic liquids (19 of which are new) comprised of 13 cations, 1-propargyl-3-methyl-imidazolium, 1-allyl-3-methyl-imidazolium, 1,3-dimethyl-imidazolium, 1-ethyl-3-methyl-imidazolium, 1-butyl-3-methyl-imidazolium, 1-methoxyethyl-3-methyl-imidazolium, 1-methyl-4-butyl-1,2,4-triazolium, 1-methyl-4-allyl-1,2,4-triazolium, 1-methyl-4-amino-1,2,4-triazolium, N-butyl-N-methyl-pyrrolidinium, N-allyl-N-methyl-pyrrolidinium, N-allyl-pyridinium, and N-butyl-3-methyl-pyridinium, paired with three anions, dicyanamide, azide, and nitrocyanamide, have been prepared, characterized, and evaluated as potential hypergolic fuels by determining key physical, thermal, and ignition properties. The reactivity of these ionic liquids (32 liquids and 6 solids which melt below 100 degrees C) was strongly correlated to increased electron density in the cation, while small changes in physical properties had little effect beyond a certain threshold, suggesting that subtle changes in chemical reactivity can greatly influence the hypergolic ignition pathway

Controlling the properties of energetic ionic liquids by stabilizing reactive nanomaterials

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. (Published By University of Alabama Libraries

Evaluating Ionic Liquids as Hypergolic Fuels: Exploring Reactivity from Molecular Structure

A total of 38 ionic liquids (19 of which are new) comprised of 13 cations, 1-propargyl-3-methyl-imidazolium, 1-allyl-3-methyl-imidazolium, 1,3-dimethyl-imidazolium, 1-ethyl-3-methyl-imidazolium, 1-butyl-3-methyl-imidazolium, 1-meth-oxyethyl-3-methyl-imidazolium, 1-methyl-4-butyl-1,2,4-triazolium, 1-methyl-4-allyl-1,2,4-triazolium, 1-methyl-4-amino-1,2,4-tri-azolium, <i>N</i>-butyl-<i>N</i>-methyl-pyrrolidinium, <i>N</i>-allyl-<i>N</i>-methyl-pyrrolidinium, <i>N</i>-allyl-pyridinium, and <i>N</i>-butyl-3-methyl-pyridinium, paired with three anions, dicyanamide, azide, and nitrocyanamide, have been prepared, characterized, and evaluated as potential hypergolic fuels by determining key physical, thermal, and ignition properties. The reactivity of these ionic liquids (32 liquids and 6 solids which melt below 100 °C) was strongly correlated to increased electron density in the cation, while small changes in physical properties had little effect beyond a certain threshold, suggesting that subtle changes in chemical reactivity can greatly influence the hypergolic ignition pathway

Evaluating Ionic Liquids as Hypergolic Fuels: Exploring Reactivity from Molecular Structure

A total of 38 ionic liquids (19 of which are new) comprised of 13 cations, 1-propargyl-3-methyl-imidazolium, 1-allyl-3-methyl-imidazolium, 1,3-dimethyl-imidazolium, 1-ethyl-3-methyl-imidazolium, 1-butyl-3-methyl-imidazolium, 1-meth-oxyethyl-3-methyl-imidazolium, 1-methyl-4-butyl-1,2,4-triazolium, 1-methyl-4-allyl-1,2,4-triazolium, 1-methyl-4-amino-1,2,4-tri-azolium, <i>N</i>-butyl-<i>N</i>-methyl-pyrrolidinium, <i>N</i>-allyl-<i>N</i>-methyl-pyrrolidinium, <i>N</i>-allyl-pyridinium, and <i>N</i>-butyl-3-methyl-pyridinium, paired with three anions, dicyanamide, azide, and nitrocyanamide, have been prepared, characterized, and evaluated as potential hypergolic fuels by determining key physical, thermal, and ignition properties. The reactivity of these ionic liquids (32 liquids and 6 solids which melt below 100 °C) was strongly correlated to increased electron density in the cation, while small changes in physical properties had little effect beyond a certain threshold, suggesting that subtle changes in chemical reactivity can greatly influence the hypergolic ignition pathway

Evaluating Ionic Liquids as Hypergolic Fuels: Exploring Reactivity from Molecular Structure

A total of 38 ionic liquids (19 of which are new) comprised of 13 cations, 1-propargyl-3-methyl-imidazolium, 1-allyl-3-methyl-imidazolium, 1,3-dimethyl-imidazolium, 1-ethyl-3-methyl-imidazolium, 1-butyl-3-methyl-imidazolium, 1-meth-oxyethyl-3-methyl-imidazolium, 1-methyl-4-butyl-1,2,4-triazolium, 1-methyl-4-allyl-1,2,4-triazolium, 1-methyl-4-amino-1,2,4-tri-azolium, <i>N</i>-butyl-<i>N</i>-methyl-pyrrolidinium, <i>N</i>-allyl-<i>N</i>-methyl-pyrrolidinium, <i>N</i>-allyl-pyridinium, and <i>N</i>-butyl-3-methyl-pyridinium, paired with three anions, dicyanamide, azide, and nitrocyanamide, have been prepared, characterized, and evaluated as potential hypergolic fuels by determining key physical, thermal, and ignition properties. The reactivity of these ionic liquids (32 liquids and 6 solids which melt below 100 °C) was strongly correlated to increased electron density in the cation, while small changes in physical properties had little effect beyond a certain threshold, suggesting that subtle changes in chemical reactivity can greatly influence the hypergolic ignition pathway