X-ray scattering investigation of ion aggregation in imidazolium-based ionic liquids upon doping with lithium, sodium, potassium, rubidium and cesium salts

X-ray scattering measurements were utilized to probe the effects of doping the neat 1-methyl-3-octylimidazolium bis (trifluoromethylsulfonyl) imide [C8mim][TFSI] room temperature ionic liquid with alkali-TFSI salts of varying concentrations. The introduction of lithium TFSI, sodium TFSI, potassium TFSI, rubidium TFSI and cesium TFSI has the universal effect on the scattering intensity of depressing the charge alternation peak and enhancing the aggregation peak as the relative concentration is increased. The changes in the liquid structure are interpreted as a result of the formation of long-lived alkali-TFSI complexes. The depression of the charge alternation peak is due to a more disordered structure at that length scale as the doping concentration increases. The aggregation peak is demonstrated to increase in height for all alkali complexes due to an increase in the scattering strength of the complexes and an increase in their prevalence as the concentration of each alkali salt is increased. •The liquid structure of an alkyl-imidizolium ionic liquid was investigated.•The liquid was doped with lithium, sodium, potassium, rubidium and cesium TFSI.•High energy synchrotron x-ray diffraction was utilized in SAXS and WAXS.•The changes in the liquid structure result from long-lived alkali-TFSI complexes.•A more disordered structure at the charge alternation length scale is observed.