SAXS Studies of Nanostructure of Ionic Liquid - Organic Solvent Mixtures: Concentration, Temperature, and Polarity Effects

Room-temperature ionic liquid (RTIL) mixtures, as electrolytes in supercapacitors, have desirable properties in comparison to conventional electrolytes. We studied the nanostructural properties of mixtures of RTILs (e.g., BMIM+[TFSI]−) with organic solvents: acetonitrile, dichloromethane, benzene, toluene, and tetrahydrofuran. The mass % at which macroscopic phase separation is visible, in the BMIM+[TFSI]−/ solvent mixtures, was determined by slowing increasing the solvent concentration. SAXS measurements at RTIL mass % lower than the phase separation concentration were carried out to determine whether nano-heterogeneity is present leading up to macroscopic phase separation, and to characterize its structural properties; this same measurements were then replicated at different temperatures ranging from -10°C to 60°C. We used the range of .01Å^-1 to 2.6Å^-1 for our scattering measurements. We find an excess scattering at low-Q compared to the expected scattering from a simple mixture of two liquids, suggesting nanometer-scale composition fluctuations. We describe the system’s heterogeneity, in a 11Å length scale, dependence on temperature. Results and analysis of the length scales of the heterogeneity, and changes in intermolecular coordination in these systems will be discussed.