Structural Origins of Viscosity in Imidazolium and Pyrrolidinium Ionic Liquids Coupled with the NTf2- Anion

Abstract

Ionic liquid viscosity is one of the most important properties to consider for practical applications. Yet, the connection between local structure and viscosity remains an open question. This article explores the structural origin of differences in the viscosity and viscoelastic relaxation across several ionic liquids, including cations with alkyl, ether, and thioether tails, of the imidazolium and pyrrolidinium families coupled with the NTf₂^- anion. In all cases, for the systems studied here, we find that pyrrolidinium-based ions are "harder" than their imidazolium-based counterparts. We make a connection between the chemical concept of hardness vs softness and specific structural and structural dynamic quantities that can be derived from scattering experiments and simulations.

Figure 1

Ionic liquids comprising the imidazolium and pyrrolidinium cations paired with the NTf₂^– anion. The color scheme highlights the definition used in subsequent sections for cationic-head, cationic-tail, and anion; the figure also defines our naming convention for the different species.

Figure 2

Ionic structures (left) corresponding to ILs for which the viscosities at 25 °C (calculated from VTF fits) vs chain length are plotted on the right. Colors on the left match those of the symbols on the right. Alkylimidazolium NTf₂^– viscosities are from Tariq et al. Alkylpyrrolidinium NTf₂^– viscosities for n = 3, 6, and 10 are from Jin et al., the one for n = 4 is from Funston et al., and those for n = 5, 7, and 8 are from Lall-Ramnarine et al. Ether imidazolium and pyrrolidinium NTf₂^– viscosities with an ethylene bridge for n = 7 and 10, as well as for n = 4 for imidazolium, are from Lall-Ramnarine et al. while n = 4 for pyrrolidinium is from Funston et al. Ether imidazolium and pyrrolidinium NTf₂^– viscosities with a methylene bridge are from Chen et al.

Figure 3

(Left) For the family of imidazolium-based ILs coupled with NTf₂^–, experimentally determined viscosity values, VTF fits of the experimental data that are extrapolated out to the temperature of the MD simulations, and the simulated viscosities. Measurements for Bmim are from Tariq et al. (Right) Same as left but for the pyrrolidinium-based ILs coupled with NTf₂^–.

Figure 4

(Right) Experimental small-angle X-ray scattering total structure function S(q) at 300 K. (Left) Simulated S(q) at 400 K. Both figures show in the region above 1 Å^–1 and below 1.8 Å^–1 what we call an adjacency peak; they also show at around 0.8 Å^–1 the charge alternation peak associated with the charge trio discussed in Figure 5.

Figure 5

For all ILs, charge trio subcomponents of S(q) in the relevant charge alternation q-regime at 400 K.

Figure 6

Spatial distribution functions computed with the TRAVIS software for atoms in NTf₂^– around Bmim⁺ and BmPyrr⁺ at 400 K. Left and right panels are at the same isovalue. We see from these figures that there is a large and directional probability of finding anions on one side of the Bmim⁺ ring, but the distribution around BmPyrr⁺ is much more symmetrical and multidirectional. When the isovalue is increased in the inset, the figure for Bmim⁺ almost does not change, whereas BmPyrr⁺ has almost no contributions at this level highlighting the difference in the angular accumulation of the anions around each of the cations.

Figure 7

For NTf₂^– coupled with Bmim⁺, BmPyrr⁺, EOMmim⁺, and EOMmPyrr⁺, the S^H–A(q, t) subcomponent of S(q, t) at 400 K.

Figure 8

For ILs of Bmim⁺, BmPyrr⁺, EOMmim⁺, and EOMmPyrr⁺ coupled with NTf₂^–, the running integral at 400 K of S^H–A(q, t)² (∫₀^tS^H–A(q, t′)² dt′) at q values corresponding to the adjacency and the charge alternation structural motif as defined in Figure 7.

Figure 9

For NTf₂^–-based ILs coupled with different cations of the imidazolium and pyrrolidinium families, running integrals of the Green–Kubo expression, ⟨ζ(t)⟩_Fit, for the viscosity using the Maginn et al. method of calculation at 400 K. The most important finding, besides the actual viscosity value derived at a long time in each case, is the time it takes for each of these functions to converge to a flat value. In all cases, not only is the viscosity higher for the pyrrolidinium-based ILs but also the time it takes them to approach asymptotic values is always longer than for the imidazolium ILs.

Figure 10

α^H–A(q, t) (the normalized integral of S^H–A(q, t)² as defined in the Methods section), compared to the normalized viscoelastic relaxation ⟨ζ(t)⟩_Fit/η at 400 K. In all cases, the relaxation of the viscosity falls in between that of adjacency and charge alternation correlations indicating that its origin is a combination of processes associated with both.