In many room temperature ionic liquids (RTILs), cations are larger than anions and also more topologically rich. It is likely that significant aspects of the liquid landscape are dominated by the different structural features of the cations. However, we have demonstrated1 that for most common RTILs it is the anionic component and not the cations that dominate the experimentally observed X-ray scattering intensity and corresponding structure function S(q). Hence, we necessarily learn about liquid landscape mainly from the scattering of the anions and any structural change in this component strongly affects S(q). Recently, the groups of Castner and Margulis studied the temperature dependent structure of tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl) amide containing RTILs.2 One of the findings of this article was that the first sharp diffraction peak (FSDP) that denotes the characteristic inverse length scale for polar-apolar alternations has anomalous temperature dependence. This is, the FSDP significantly increases when temperature increases hinting at better polar-apolar organization at higher temperatures. This unexpected phenomenon was recently investigated by Hettige and coworkers3 in the context of competing polar and apolar packing interactions that have opposite temperature dependence. Even though apolar components appear to be more disorganized at higher temperatures, polar components have the opposite behavior. Once again because anions are the main reporters of structure this polar organization is strongly highlighted in X-ray experiments.                   (1)          Hettige, J. J.; Kashyap, H. K.; Annapureddy, H. V. R.; Margulis, C. J. Anions, the Reporters of Structure in Ionic Liquids. Journal of Physical Chemistry Letters 2013 , 4, 105.                (2)          Kashyap, H. K.; Santos, C. S.; Annapureddy, H. V. R.; Murthy, N. S.; Margulis, C. J.; Castner, E. W. Temperature-dependent structure of ionic liquids: X-ray scattering and simulations. Faraday Discussions 2012 , 154, 133.                (3)          Hettige, J. J.; Kashyap, H. K.; Margulis, C. J. Communication: Anomalous temperature dependence of the intermediate range order in phosphonium ionic liquids. The Journal of Chemical Physics 2014 , 140.

Anions as fundamental descriptors of structure and their role in the anomalous temperature dependence of the first sharp diffraction peak in phosphonium ILs

248th National Meeting (2014)

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