Peculiar Behavior of Azolium Azolate Energetic Ionic Liquids

Abstract

b S Supporting Information I onic liquids (ILs) are liquids formed by ions whose properties are tuned by the cation/anion combination. A great variety of possible cation/anion combinations drives the development of novel ILs and allows flexibility in tailoring ILs for specific applications. Azolium azolates possess superior energy density and therefore are targeted for propulsion and explosives applications. 1À4 However, the fundamental physical properties of these ILs and their full potential are not yet explored.Common ILs, which have already been widely studied, possess an organic cation and an inorganic anion. Azolium azolates consist of an organic azolium cation and an organic azolate anion; that is, the anion has the structure of an aromatic planar rich in nitrogen ring, which may lead to specific behavior of ILs. It is already accepted that ILs are complex fluids and show structuring in their liquid phases. Experimental 5,6 and theoretical 7 evidence suggests that ILs are heterogeneous on the nanoscale with polar and apolar domains in the bulk liquid, which makes them similar to bicontinuous microemulsion but with orders of magnitude lower length scales. 7,8 Our first thermorheological studies in a wide temperature range of the two ILs with inorganic anions (Scheme 1, [Bmim]BF 4 and [TMP]Tos) revealed that both ILs are characterized by broad multimodal spectra of relaxation times and show cooperative dynamics and structural heterogeneity. 9 However, the degree of this structuration depends strongly on the molecular structure of the ILs and the strength and types of interand intra-ion interactions. Present research has been undertaken to explore the effect of azolate anion structure on the physical properties of ILs, specifically on their glassy dynamics and ion interactions, utilizing rheology, X-ray scattering, UV/vis absorption, and ab initio calculations. We have studied four ILs (Scheme 1), three of which shared the same 1-butyl-3-methylimidazolium ([Bmim]) cation. Figure 1 shows a T g -scaled Arrhenius representation of IL viscosities. (Here we use calorimetric glass-transition temperature T g .) The viscosity growth spans over 12 decades in the studied broad temperature range up to T g . The [Bmim]5AT exhibits higher viscosities in comparison with the other three ILs in the accessed temperature range. Note that the experimental viscosity of [Bmim]5AT at room temperature is η exp = 5 Pa 3 s, which exceeds the η-value calculated from ion dimensions (COSMO program) by an order of magnitude η cal = 0.45 Pa 3 s (table, Supporting Information). This indicates strong interactions between ions and possible structuration of [Bmim]5AT. ABSTRACT:We present studies of molecular dynamics and interactions in azolium azolate energetic ionic liquids (ILs) by utilizing a variety of physical methods. We observe peculiar rheological behavior for these ILs, which deviates from the one expected for molecular glass-formers. Major peculiarities include high elasticity in the low-frequency zone, peculiar van Gurp plots, and...

Rheology of Pure Ionic Liquids and Their Complex Fluids: A Review

“…The higher fragility of ILs with aromatic groups (ranging from 117 to 130) as compared to the aliphatic groups was noted. A unique rheological behavior of azolium azolate based ILs was described by Pogodina et al This uniqueness resulted from the high elasticity of ILs at low frequency and the failure of time–temperature superposition principle. This peculiar behavior was attributed to the particular interactions in nitrogen rings of azolate ILs.…”

Exploring the Structure of Nitrogen-Rich Ionic Liquids and Their Binding to the Surface of Oxide-Free Boron Nanoparticles

“…Also, such propellants can potentially have significantly higher energy density than those of hydrazine and its derivatives. Shreeve et al synthesized a number of highly energetic ionic liquids with multiple nitrogen atoms in their structures, − in the hope of increasing volumetric and gravimetric energy density by introducing multiple nitrogen atoms and more nitrogen–nitrogen bonds. − In lieu of this, azolium azolate ILs, salts with both cation and anion composed of heterocyclic organic ions, are now being considered as the next stage in IL propellant development, although their hypergolic property is yet to be demonstrated. − A more detailed review of some of the considerations involved in developing IL propellants is given in the Supporting Information (SI).…”

Triborheology and orientational dynamics of ionic liquid crystals

“…Our own studies of ILs confirmed that ILs are complex fluids formed by heterogeneous polar and nonpolar domains, which show structure formation in their liquid phase [7], [8].The degree of structuring depends on chemical structure and type and strength of interactions (ionic, H-bonding, dipole-dipole, pi-pi). At high level of structuring we monitored even temporal network formation [8].…”

“…Our own studies of ILs confirmed that ILs are complex fluids formed by heterogeneous polar and nonpolar domains, which show structure formation in their liquid phase [7], [8].The degree of structuring depends on chemical structure and type and strength of interactions (ionic, H-bonding, dipole-dipole, pi-pi). At high level of structuring we monitored even temporal network formation [8]. These different types of interactions are the dominant factors which determine the structuring process and physical properties of ILs.Our investigations also showed [9], [10], [11] [12], [13]that ILs exhibit improved tribological properties.…”