Evolution of Ion-Ion Interactions and Structures in Smectic Ionic Liquid Crystals

A mixture of ionic liquids, 1,3-dimethylimidazolium 2-methoxy(2-ethoxy(2-ethoxy(2-ethoxy)))ethylphosphite ([DMIm][MPEGP]) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), which forms an ionic liquid crystal with a superior ionic conductivity, evolves a smectic structure through ion-ion interactions as a function of LiTFSI concentration. Nuclear magnetic resonance (NMR) spectroscopy and pulsed-field-gradient (PFG) NMR examinations showed that the morphology of the structures and the strength of ion-ion interactions are closely related to the ratio of Li+/[MPEGP](-). The results revealed structures composed of Li+/[MPEGP](-)/TFSI- (approximate to 3:4:1)mainly by the Coulombic interactions between Li+ cations and phosphite head groups in [MPEGP](-) anions. NMR diffraction on PFG echo profiles revealed a cluster size of similar to 2 mu m, inversely proportional to the number of mobile [MPEGP](-) anions, and an ion diffusion on the cluster surfaces that is 1000 times faster than that in the bulk liquid. These observations suggest that the superior ionic conductivity in the mixture is mainly due to the fast ion transport on the cluster surfaces of smectic ionic liquid crystals. The variations of diffusion ratios between mobile Li+, TFSI-, [DMIm](+), and [MPEGP](-) ions indicate that these mobile ions remaining in the voids of structures preserve the bulk ionic liquid properties.