Effects of the polyfluoroalkyl side-chain on the properties of 1-methyl-3-polyfluoroalkylimidazolium fluorohydrogenate ionic liquids

A series of 1-methyl-3-polyfluoroalkylimidazolium fluorohydrogenate salts (C3F3MIm(FH)1.7F, C4F5MIm(FH)1.7F, C6F9MIm(FH)1.8F, C8F13MIm(FH)2.0F, and C10F17MIm(FH)2.0F; the subscript numbers denote the number of carbon and fluorine atoms in the side-chain, respectively, with which the two carbon atoms from the root of the side-chain are not fluorinated) were synthesized and their physicochemical properties, including decomposition temperature, phase transition temperature, density, viscosity, ionic conductivity, and electrochemical stability, were measured. The three salts with relatively short side-chain have melting points below room temperature: C3F3MIm(FH)1.7F (Tm: 274 K), C4F5MIm(FH)1.7F (Tg: 186 K), and C6F9MIm(FH)1.8F (Tm: 276 K). The other salts, C8F13MIm(FH)2.0F and C10F17MIm(FH)2.0F, are in the crystal phase at room temperature. The obtained salts are miscible with water and hydrophobic fluorohydrogenate ionic liquids were not obtained in the present series. Introduction of fluorine atoms results in the increase in the density and viscosity, decrease in the ionic conductivity, and decrease in electrochemical stability against reduction. The liquid crystal phase was observed for C8F13MIm(FH)2.0F and C10F17MIm(FH)2.0F according to polarized optical microscopy and X-ray diffraction analysis.