Ion diffusion in molten salt mixtures

The molten salts, 1-methyl,3-ethylimidazolium trifluoramethanesulfonate (triflate salt, MeEtImTf) and 1-methyl,3-ethylimidazolium bis(trifluoromethanesulfonimide) (imide salt, MeEtImNTf(2)) are colourless ionic liquids with conductivities of the order of 10(-2) S cm(-1) at room temperature. DSC measurements revealed subambient melting and glass transition temperatures. Analysis of the anion and cation diffusion coefficients suggested that the cation was the dominant charge carrier and that the motion was largely independent of the anion. Haven ratios (H(R)s) of 1 and 1.6 were determined for the imide and triflate salts, respectively, at 30 degrees C (303 K). Values greater than one imply some degree of ionic association, suggesting that aggregation is present in the triflate salt. Mixing of the salts to form binary systems resulted in enhanced conductivities which deviated from a simple law of mixtures. Thermal analysis showed no evidence of a melting point with only a glass transition observed. Corresponding diffusion measurements for the binaries appeared to show a weighted average of the diffusion coefficients of the pure components. The increased conductivity can be attributed to an increase in the number of charge carriers as a result of decreased ion association in the binary. (C) 2000 Elsevier Science Ltd. All rights reserved.