The influence of lithium cations on dynamics and structure of room temperature ionic liquids

The orientational relaxation dynamics of perylene in the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonyl imide were studied as a function of lithium ion concentration. Perylene is nonpolar and locates in the alkyl regions of the RTIL. The lithium cation was added as lithium bis(trifluoromethyl)sulfonyl imide, so the addition of Li(+) did not change the anion. The Li(+) concentration ranged from 0 to 0.4 mol fraction. The dynamics were measured by observing the fluorescence anisotropy decay using time correlated single photon counting. The anisotropy experiments and viscosity measurements were performed as a function of temperature for each Li(+) concentration sample. Because perylene has high symmetry, it was possible to independently determine the in-plane and out-of-plane diffusion constants and friction coefficients. With increasing concentration of lithium salt the viscosity increases and both the in-plane and out-of-plane orientational relaxations of perylene become slower. However, the corresponding molecular friction coefficients decreased, with the in-plane coefficient decreasing to a greater extent than the out-of-plane coefficient. The decrease in the friction coefficients demonstrates that lithium ions, which are located in the ionic regions of the RTILs, change the structure of the alkyl regions of the RTIL.