Structural Microheterogeneity in Ionic Liquid/Solvent Mixtures Influenced by Solvent Polarity and Ion Concentration
Room temperature ionic liquid (RTIL) and organic solvent mixtures, in supercapacitors, have desirable properties in comparison to conventional electrolytes. We studied the nanostructural properties of mixtures of the RTIL: 1-butyl-3-methylimidazolium bis (trifluoromethyl sulfonyl) imide (abbreviated to BMIM+[TSI]-), with organic solvents: acetonitrile, dichloromethane, benzene, toluene, and tetrahydrofuran. The mass percentage at which macroscopic phase separation is visible in each RTIL and solvent mixture was determined by slowly increasing the solvent concentration. Small-angle x-ray scattering (SAXS) measurements, at RTIL mass percentage lower than the phase separation concentrations, were carried out to determine whether nanoheterogeneity is presently leading up to macroscopic phase separation. The x-ray scattering measurements provide us with statistically averaged structural information over length scales of 0.25 nm to 60 nm. We find an excess scattering at low-Q (large length scales) compared to the expected scattering from a simple mixture of two liquids, suggesting nanometer-scale composition fluctuations. We describe the system’s approximately 1 nm-size heterogeneity. A model composed of the Teubner-Strey function and a summation of Lorentzian functions is employed to analyze the mixtures’ scattering curves. The Teubner-Strey function’s periodicity, correlation length, and suggested Lifshitz line - and the Lorentzian functions peaks’ positions and widths- are studied. We propose a model based on the solvent’s physicochemical properties to explain the complex mixing mechanics of toluene, benzene, dichloromethane, acetonitrile, and tetrahydrofuran with BMIM+[TFSI]−. Results and analyses of the length scales of the heterogeneity and changes in intermolecular coordination in these systems will be discussed.
Physics|Atomic physics|Molecular physics
Falola, Babatunde Olukunle, "Structural Microheterogeneity in Ionic Liquid/Solvent Mixtures Influenced by Solvent Polarity and Ion Concentration" (2021). ETD Collection for University of Texas, El Paso. AAI28714983.