LETTER pubs.acs.org/JPCL
Peculiar Behavior of Azolium Azolate Energetic Ionic Liquids N. V. Pogodina,*,† E. Metwalli,‡ P. M€uller-Buschbaum,‡ K. Wendler,§ R. Lungwitz,|| S. Spange,|| J. L. Shamshina,^,# R. D. Rogers,^ and Ch. Friedrich*,† †
Material Research Center (FMF), The University of Freiburg, Stefan-Meier-str. 21, 79104 Freiburg, Germany Technische Universit€at M€unchen, Lehrstuhl f€ur Funktionelle Materialien, Physikdepartment E13, James-Franck-Str. 1, 85747 Garching, Germany § Max-Planck Institute for Polymer Research, Mainz, Germany Department of Polymer Chemistry, Chemnitz University of Technology, Chemnitz, Germany ^ Center for Green Manufacturing and Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama, United States
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bS Supporting Information 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 failure of time temperature superposition. We attribute these peculiarities to specific interactions in the nitrogen-rich planar rings of azolate ILs. X-ray scattering measurements reveal a “nanometer” ordered organization in azolium azolates. High values of KamletTaft polarity parameters indicate high probability and strength of hydrogen bond interactions in azolate ILs. This conclusion is also supported by ab initio calculations. Our next effort is to break/enhance existing interactions in nitrogenrich ILs by adding a “H-acceptor”. This will allow better understanding of the nature of interactions in azolium azolates and eventually their control. SECTION: Dynamics, Clusters, Excited States
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.14 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. Experimental5,6 and theoretical7 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]BF4 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 r 2011 American Chemical Society
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 Tg-scaled Arrhenius representation of IL viscosities. (Here we use calorimetric glass-transition temperature Tg.) The viscosity growth spans over 12 decades in the studied broad temperature range up to Tg. 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 ηexp = 5 Pa 3 s, which exceeds the η-value calculated from ion dimensions (COSMO program) by an order of magnitude ηcal = 0.45 Pa 3 s (table, Supporting Information). This indicates strong interactions between ions and possible structuration of [Bmim]5AT. Received: August 26, 2011 Accepted: September 26, 2011 Published: September 26, 2011 2571
dx.doi.org/10.1021/jz201175v | J. Phys. Chem. Lett. 2011, 2, 2571–2576
The Journal of Physical Chemistry Letters There are different definitions of the glass-transition temperature. According to one definition, Tg is the temperature at which the shear viscosity reaches 1012 Pa 3 s10 (in which case all curves in Figure 1 pass through the same point Tg/T = 1). Another definition of Tg states that it is the temperature at which the Scheme 1. Molecular Structures of Studied ILs: (1) 1-Butyl3-methylimidazolium Tetrafluoroborate ([Bmim]BF4), (2) 1-Butyl-3-methylimidazolium 4-Nitroimidazolate ([Bmim]4Ni-Im), (3) 1-Butyl-3-methylimidazolium 5-Aminotetrazolate ([Bmim]5AT), and (4) Triisobutylmethylphosphonium Tosylate ([TMP]Tos)
Figure 1. Viscosity versus Tg/T for the [Bmim]BF4 (squares), [TMP]Tos (triangles), [Bmim]5AT (circles), and [Bmim]4Ni-Im (diamonds). The dashed straight line represents the Arrhenius model, and the solid curved lines represent the VFT model. Only the fits for the [Bmim]5AT and [Bmim]BF4 are shown to visualize more easily how close the experimental data points for three ILs ([Bmim]BF4, [Bmim]4Ni-Im, and [TMP]Tos) are. A and B parameters of the VFT are given in the Table (SI) for all ILs.
LETTER
characteristic molecular relaxation time reaches τg = 100 s, and in this case the viscosity is not always 1012 Pa 3 s at Tg.11 Taking into account the relationship among viscosity, molecular relaxation time, and plateau modulus η = Gpτ as well as the weak temperature dependence of the plateau modulus Gp and the values of Gp and τ at the glassy state, we have the viscosity at the glass transition ηg = 109 Pa 3 102 s = 1011 Pa 3 s, which is close to the viscosities at approaching Tg in Figure 1. Near the glass transition the viscosity of ILs is extremely sensitive to temperature and shows a very steep increase. The super-Arrhenius temperature dependencies of IL viscosities were fitted to the VogelFulcherTammann (VFT) equation. The parameters determined from the VFT fit and fragility values m ≈ d(ln η)/ d(1/T) are given in the table (Supporting Information (SI)) for all ILs. The estimated values of fragility vary between intermediate m and high, which allows us to classify the ILs under study as fragile glass formers.12 The dynamic moduli G0 and G00 from small amplitude oscillatory shear (SAOS) experiments in a wide temperature range are plotted versus complex modulus |G*| in Figure 2ac for the [Bmim]BF4, [Bmim]4Ni-Im, and [Bmim]5AT. G0 curves show a drastically different shape of the dependencies. The [Bmim]BF4 and [Bmim]4Ni-Im exhibit scaling behavior with the limiting slopes of 1 and 2 for G00 and G0 in the low |G*| range (which also corresponds to the low-frequency ω range since |G*| ≈ ω). (The dependence G0 = f(|G*|) for [Bmim]BF4 (Figure 2a) shows the tendency for a slope of 95% purity. Additional information about purity of azolium azolates is given in the Supporting Information. [Bmim]BF4 and [TMP]Tos were obtained from Iolitec, Heilbronn, Germany. All ILs were used without further purification. Prior to measurements, the samples were dried under vacuum (
