Novel Hydrophobic Molten Salts Based on Tetrakis[3,5-bis(Trifluoromethyl)Phenyl]Borate Anion for Electrochemistry of the Molten Salt|Water Interface

Room-temperature molten salts (RTMSs) that form a polarized RTMS|W interface, so far proposed, are composed of hydrophobic cations, such as tetraalkylammonium ions, and the hydrophobic anions, such as PF6 or bis(perfluoroalkylsulfonyl)imide anion (CnCnN ). To extend the polarized potential window so that the transfer of more hydrophobic cations or of more hydrophobic anion from W to RTMS. RTMSs composed of more hydrophobic anions are required. In this study, we will show that the potential window can be extended by using the hydrophobic anion, tetrakis[3,5bis(trifluoromethyl)phenyl]borate (TFPB). The cations used are 1-alkyl-3-methylimidazolium (Cnmim , n=2, 4, 5, 6, 7, 8, 10, 12), 1-dodecylpyridinium (C12Py ), tri-n-octylmethylammonium (TOMA), and 2octadecylisoquinolinium (C18Iq ). The electrochemical voltammetric measurements of the RTMS|W interfaces were made by using capillary electrodes. The electrochemical cell is represented by Ag /AgCl 0.1 M Li2SO4 CA 0.1 M LiCl Ag/AgCl x M CCl W2 RTMS W1 where C and A denote the cation and anion comprising RTMS. The potential of the right-hand side terminal with respect to the left will be referred to as E. The melting points of the TFPB-based molten salts are shown in Table. 1. All Cnmim TFPB used and C12PyTFPB show melting points higher than room temperature. TOMATFPB and C18IqTFPB are found to be liquid at room temperature. The potential window of the RTMS|W interfaces are estimated by using cyclic voltammetry. The width of the potential window of TOMATFPB|W and C18IqTFPB|W interfaces were measured to be 150 mV and 300 mV respectively. Compared with our previous results of the potential window of C18IqC2C2N|W interface 250 mV, the potential window was 50 mV extended by using TFPB anion. Fig 1. Cyclic voltammogram for the ion transfer across C18IqC2C2N|W interface at 56 . Scan rate: 50 mV/sec.