pubs.acs.org/JPCL
Translational and Reorientational Dynamics of an Imidazolium-Based Ionic Liquid Bachir Aoun,†,‡ Miguel A. Gonz alez,‡ Jacques Ollivier,‡ Margarita Russina,§ Zunbeltz Izaola,§ # David L. Price, and Marie-Louise Saboungi*,† †
Centre de Recherche sur la Mati ere Divis ee, CNRS-Universit e d'Orl eans, 1b rue de la F erollerie, 45071 Orl eans Cedex 2, France, Institut Laue Langevin, 6 rue Jules Horowitz, 38042 Grenoble Cedex 9, France, §Berlin Neutron Scattering Center, Helmholtzemes et Zentrum Berlin f€ ur Materialen und Energie GmbH, Glienicker Strasse 100, 14109 Berlin, Germany, and #Conditions Extr^ Materiaux: Haute Temp erature et Irradiation, 1d avenue de la Recherche Scientifique, 45071 Orl eans Cedex 2, France ‡
ABSTRACT We present results of parallel quasielastic neutron scattering (QENS) experiments and molecular dynamics numerical simulations for the dynamics of a prototype ionic liquid, 1-ethyl-3-methyl-imidazolium bromide. Differences and similarities with those from the crystal phase are also discussed. Both experiment and simulation demonstrate that, in the length and time scales being probed here (fractions of a nm and a few ps), the dynamics are dominated by activated translational diffusion in the liquid phase and reorientations of the ethyl groups in both solid and liquid. SECTION Dynamics, Clusters, Excited States
oom-temperature ionic liquids (RTILs) have been considered for some time as ideal solvents for many reaction and extraction processes1-3 due to a remarkable combination of properties: low melting point, negligible vapor pressure, nonvolatility, and high degree of thermal and chemical stability.4 However, the range of their possible applications is rapidly being extended in several other directions such as sensors, dye-sensitized solar cells, batteries, fuel cells, capacitors, catalysis, hydrogen storage, and energetic materials.5-12 The optimization of their performance over this broad field of applications requires a detailed knowledge of their fundamental properties and behavior. For example, their use in binary systems for lithium batteries demands a reduction in viscosity in order to increase the lithium ion mobility. Even for the more traditional uses as solvents, their complex dielectric response ; partly dipolar, partly ionic ; needs to be better understood.13 Therefore, the final goal of being able to design a specific RTIL for a particular application requires a much better understanding of the molecular properties of RTILs. While considerable progress has been achieved during the past decade,14-16 many problems remain unresolved. In particular, the complex dynamics exhibited by these systems, extending over a wide range of time scales, are not yet well understood. A thorough investigation of the dynamics of RTILs involves a wide range of energy and length scales and hence, eventually, a variety of experimental and numerical approaches. In the present work, we have chosen to use quasielastic neutron scattering (QENS), a powerful technique for studying liquid dynamics on the length scale of fractions of a nm and time scales of a few ps.17 However the number of QENS studies is still small.18-22 Results in this domain can be connected with important macroscopic parameters such as diffusion
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r 2010 American Chemical Society
constants and viscosities and furthermore lend themselves to parallel numerical simulations with classical molecular dynamics (MD), which turns out to be essential for interpreting experimental results on these complex systems. We have chosen cations based on the widely studied imidazolium ion and a simple monatomic anion. In this Letter, we present our experimental and simulation results for the dynamics of a prototype system, 1-ethyl-3-methyl-imidazolium bromide (EmimBr, mp ≈ 353 K) in both solid and liquid phases. A parallel investigation of the structure of liquid EmimBr with high-energy X-ray diffraction and MD simulation has been reported separately.23 Neutron scattering from such a system is predominantly incoherent since it is dominated by the large incoherent cross section of hydrogen that contributes to 92% of the total scattering. The scattering function in the solid phase could be fitted satisfactorily with the combination of an elastic line and a Lorentzian function SðQ, EÞ ¼ expð - Q2 Æu2 æÞfA0 ðQÞδðEÞ þ ½1 - A0 ðQÞLðWr , EÞg ð1Þ where Q is the amplitude of the scattering vector, E is the energy transfer, Æu2æ is the mean-square amplitude of the vibrations (with energies assumed to be outside of the QENS window), A0 is the elastic incoherent structure factor (EISF), δ is the Kronecker delta function, L(W,E) is the Lorentzian function of unit area and full-width at half-maximum (fwhm) W, and Wr is the line width of the reorientational Received Date: June 23, 2010 Accepted Date: August 2, 2010 Published on Web Date: August 06, 2010
2503
DOI: 10.1021/jz100856t |J. Phys. Chem. Lett. 2010, 1, 2503–2507
pubs.acs.org/JPCL
Figure 2. (a) Energy width Wt for translational diffusion, together with the fits of eq 3; (b) energy width Wr for reorientational relaxation.
Figure 1. Scattering function for selected Q values in (a) the solid phase at 300 K (showing also the resolution function measured with a vanadium sample) and (b) the liquid phase at 373 K. The solid lines show the fits of eq 1 and 2 convoluted with the instrumental resolution in (a) and (b), respectively.
Table 1. Values of the Translational Diffusion Constant D and Residence Time τ0 in the Liquid
relaxation. Equation 1 corresponds to the first two partial waves in the scattering function for freely rotating molecules.24 Truncation at the second-order term is a reasonable approximation in view of the relatively low values of Q (e2.2 Å-1) and jump distances (
