SA002-1 Proc. QENS/WINS 2012 J. Phys. Soc. Jpn. 82 (2013) SA002 ©2013 The Physical Society of Japan Dynamics of butyl- and hexyl-methylimidazolium bromide ionic liquids Bachir Aoun 1 , Miguel A. González 1 , Margarita Russina 2 , David L. Price 3 , and Marie-Louise Saboungi 4 1 Institut Laue Langevin, BP 156, 38042 Grenoble Cedex 9, France 2 Berlin Neutron Scattering Center, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany 3 CEMHTI, 45071 Orléans Cedex 2, France 4 IMPMC, Université Pierre et Marie Curie, 75005 Paris & Université d’Orléans, Orléans, France E-mail: ml.saboungi@gmail.com (Received December 25, 2012) We have carried out quasi-elastic neutron scattering measurements, supported by classical molecular dynamics simulations, to determine the liquid state dynamics of 1-butyl-3- methylimidazolium bromide and 1-hexyl-3-methylimidazolium bromide. The spectra are fitted with two- and three-Lorentzian models and indicate the existence of a localized dynamics of the alkyl chains in the sub-picosecond range. The spatial dependence of the dynamics is consistent with a confined translational diffusion model with parameters consistent with the molecular dynamics simulations. 1. Introduction Room-temperature ionic liquids (RTILs) are receiving increased attention due to their unique properties, including non-volatility and solvating capability, leading to a wide range of potential uses in catalysis, separation technology, photovoltaics and fuel cells [1-12]. High freezing temperatures are generally achieved by combining large, asymmetrically substituted organic cations with either small or large anions. The length of the nonpolar side chains in the cations provides a sensitive handle for tuning the structure and properties of an ionic liquid. Many studies in the past decade have focused on the effects of changing the lengths of the alkyl chains in imidazolium-based liquids [13]. Regarding physical properties, their general trend is for a decreasing mass density, molar atomic number density, self-diffusion coefficient and ionic conductivity, and increasing viscosity with increasing chain length [14]. We have carried out a systematic investigation of the ionic liquid series 1-alkyl-3- methylimidazolium bromide ionic liquids, C n mimBr, with n = 2, 4 and 6. We have previously reported high-energy x-ray diffraction (HEXRD) measurements and atomistic molecular dynamics (AMD) numerical simulations on all three liquids. For C 2 mimBr, the liquid structure factor S(Q) is dominated by an intense peak at a scattering vector Q = 1.7 Å -1 , associated mainly with the packing of the anions around the large cations[15], whereas in C 4 mimBr and more strikingly in C 6 mimBr a low-Q peak rises and moves to lower Q with increasing chain length, reaching 0.33 Å -1 in C 6 mimBr [16]. This feature is consistent with the concept of nanoscale heterogeneity with small, crystal-like moieties [17]. For all three liquids, excellent agreement between the HEXRD experiments and AMD simulations is obtained, including the region of the low-Q peak that has proved problematic in previous work in the J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by 54.161.69.107 on 06/18/20