Ordering Structure of Binary Aliphatic Quaternary Ammonium Ionic Liquids: AQA-TFSA Systems

The effects of secondary cationic species on the ordering structure in room temperature ionic liquids (RTILs) are discussed. Binary RTILs refer to mixture of trimetyl-n-octylammonium bis(trifluoromethanesulfonyl)imide (N-1118[TFSA]) and alkali or alkali-earth metal bis(trifluoromethanesulfonyl)imide (M[TFSA](y); M = Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba) in an appropriate molar ratio (x = 0.1, 0.2, 0.3, 0.5), denoted as M-x[N-1118](1-x)[TFSA](1-x+xy). From SAXS and WAXS measurements of M-x[N-1118](1-x)[TFSA](1-x+xy), the intensity of First peak derived from the ordering structure increases with addition of M[TFSA](y). It is suggested that the metal cation has an intensive interaction with TFSA anion and cationic structural ordering are enhanced. We discussed the relationship between ionic size and the formation mechanism of the local ordering structure in binary ionic liquids containing alkali metal and alkali earth metal based on aliphatic quaternary ammonium (AQA) cations is clarified.