Mesoscopic organization in ionic liquids: structural and dynamic implications.

Room temperature ionic liquids are an exciting novel class of materials1 that are composed solely of ionic species; nevertheless, due to their asymmetric chemical structure (see the figure), they show a low (<100°C) melting point. They show negligible vapor pressure and a wide tunability of the chemical-physical properties upon slight changes of the chemical details (e.g. anion and cation nature). One of the most striking features emerging among the properties of these materials is the existence of high degree of structural heterogeneity over the nm spatial scale. Small-Angle X-ray Scattering (SAXS) patterns have been collected on a series of ILs that are characterized by a varying alkyl chain length2. Remarkably, these materials are characterized by a low Q amorphous, whose amplitude as well as position depends on the chain length. These findings have been rationalized in terms of a nanoscale segregated morphology, where the alkyl chains self-assemble into domains embedded into the charged matrix. Such a structural scenario can be expected to play a role in the relaxation dynamics in ILs. We collected Neutron Spin Echo measurements on a series of deuterated ILs, in order to access specific information on the collective dynamics in these salts3. These experiments highlighted evidences of deviations from the commonly observed viscosity-driven collective dynamics. 1. E. W. Castner et al., J. Chem. Phys., 2010, 132, 120901; J. Wishart, J. Phys. Chem. Lett., 2010, 1, 1629 2. A. Triolo et al., J. Phys. Chem. B, 2007, 111, 4641; Chem. Phys. Lett., 2008, 457, 362; J. Chem. Phys., 2009, 130, 164521 3. A. Triolo et al., J. Phys. Chem. B, 2009, 113, 8469