Studies on the translational and rotational motions of ionic liquids composed of N-methyl-N-propyl-pyrrolidinium (P13) cation and bis(trifluoromethanesulfonyl)amide and bis(fluorosulfonyl)amide anions and their binary systems including lithium salts

Room-temperature ionic liquids (RTIL, IL) are stable liquids composed of anions and cations. N-methyl-N-propyl-pyrrolidinium (P-13, Py-13, PYR13, or mppy) is an important cation and produces stable ILs with various anions. In this study two amide-type anions, bis(trifluoromethanesulfonyl)amide [N(SO2CF3)(2), TFSA, TFSI, NTf2, or Tf2N] and bis(fluorosulfonyl)amide [N(SO2F)(2), FSA, or FSI], were investigated. In addition to P-13-TFSA and P-13-FSA, lithium salt doped samples were prepared (P-13-TFSA-Li and P-13-FSA-Li). The individual ion diffusion coefficients (D) and spin-lattice relaxation times (T-1) were measured by H-1, F-19, and Li-7 NMR. At the same time, the ionic conductivity (sigma), viscosity (eta), and density (rho) were measured over a wide temperature range. The van der Waals volumes of P-13, TFSA, FSA, Li(TFSA)(2), and Li(FSA)(3) were estimated by molecular orbital calculations. The experimental values obtained in this study were analyzed by the classical Stokes-Einstein, Nernst-Einstein (NE), and Stokes-Einstein-Debye equations and Walden plots were also made for the neat and binary ILs to clarify physical and mobile properties of individual ions. From the temperature-dependent velocity correlation coefficients for neat P-13-TFSA and P-13-FSA, the NE parameter 1-xi was evaluated. The ionicity (electrochemical molar conductivity divided by the NE conductivity from NMR) and the 1-xi had exactly the same values. The rotational and translational motions of P-13 and jump of a lithium ion are also discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3505307]