Ion diffusion in molten salt mixtures

The molten salts, 1-methyl,3-ethylimidazolium trifluoramethanesulfonate (tr iflate salt, MeEtImTf) and 1-methyl,3-ethylimidazolium bis(trifluoromethane sulfonimide) (imide salt, MeEtImNTf(2)) are colourless ionic liquids with c onductivities of the order of 10(-2) S cm(-1) at room temperature. DSC meas urements revealed subambient melting and glass transition temperatures. Ana lysis of the anion and cation diffusion coefficients suggested that the cat ion was the dominant charge carrier and that the motion was largely indepen dent of the anion. Haven ratios (H(R)s) of 1 and 1.6 were determined for th e imide and triflate salts, respectively, at 30 degrees C (303 K). Values g reater than one imply some degree of ionic association, suggesting that agg regation is present in the triflate salt. Mixing of the salts to form binar y systems resulted in enhanced conductivities which deviated from a simple law of mixtures. Thermal analysis showed no evidence of a melting point wit h only a glass transition observed. Corresponding diffusion measurements fo r the binaries appeared to show a weighted average of the diffusion coeffic ients of the pure components. The increased conductivity can be attributed to an increase in the number of charge carriers as a result of decreased io n association in the binary. (C) 2000 Elsevier Science Ltd. All rights rese rved.