Wide Electrochemical Window at the Interface between Water and a Hydrophobic Room-Temperature Ionic Liquid of Tetrakis[3,5-bis(Trifluoromethyl)phenyl]borate

Ionic liquids composed of a hydrophobic anion, tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB-), and various aromatic and aliphatic ammonium have been prepared. TFPB salts of N-octadecylisoquinolinium (C18Iq+) and trioctylmethylammonium (TOMA+) show low melting points of 31 and 36 °C, respectively, whereas TFPB-based ionic liquids of 1-alkyl-3-methylimidazolium with the alkyl chain length of 5−12 show melting points lower than 90 °C. The [C18Iq][TFPB]|water (W) and [TOMA][TFPB]|W interfaces have the polarized potential window (ppw) of 0.8 V at 56 °C, which is twice as wide as the ppw at the interface between W and room-temperature ionic liquids of bis(perfluoroalkylsulfonyl)imide so far reported. The extension of the ppw by more than 0.4 V to the positive direction enables voltammetric measurements of the ion transfer of relatively hydrophobic anions such as bis(trifluoromethylsulfonyl)imide and bis(pentafluoroethylsulfonyl)imide and of hydrophilic cations such as tetraethylammonium and acetylcholine ions.