Low-melting, low-viscous, hydrophobic ionic liquids: Aliphatic quaternary ammonium salts with perfluoroalkyltrifluoroborates

A novel class of low-melting, hydrophobic ionic liquids based on relatively small aliphatic quaternary ammonium cations ([(RRRNR)-R-1-R-2-N-3](+), wherein R-1,R-2,R-3=CH3 or C2H5, R=n-C3H7, n-C4H9, CH2CH2OCH3) and perfluoroalkyltrifluoroborate anions ([RFBF3](-), R-F=CF3, C2F5, n-C3F7, n-C4F9) have been prepared and characterized. The important physicochemical and electrochemical properties of these salts, including melting point, glass transition, viscosity, density, ionic conductivity, thermal and electrochemical stability, have been determined and comparatively studied with those based on the corresponding [BF4](-) and [(CF3SO2)(2)N](-) salts. The influence of the structure variation in the quaternary ammonium cation and perfluoroalkyltrifluoroborate ([RFBF3](-)) anion on the above physicochemical properties is discussed. Most of these salts are liquids at 25degreesC and exhibit low viscosities (58-210 cP at 25degreesC) and moderate conductivities (1.1-3.8 mS cm(-1)). The electrochemical windows of these salts are much larger than those of the corresponding 1,3-dialkyimidazolium salts. Additionally, a number of [RFBF3](-) salts exhibit plastic crystal behavior.