Influence of Molecular Structure on Densities and Viscosities of Several Ionic Liquids

One of the potential applications of the ionic liquids (ILs) is their use as lubricants or lubricant additives. The choice of the cation and the anion of the ILs, as well as the side chain design, determine their fundamental properties, which permits us to create tailor-made lubricants and lubricant additives among other functional fluids. In this work we have studied 10 ILs for which we report densities and viscosities from 283.15 K (and for some ILs at lower temperatures) up to 373.15 K. The viscosities of the ILs fall in the range of the applications of hydraulic fluid or lubricants. Four of these ILs were composed by the anion tris(pentafluoroethyl)trifluorophosphate [(C2F5)3PF3]− and one of the following cations: 1-butyl-2,3-dimethylimidazolium [C4C1C1min]+, 1-butyl-1-methylpyrrolidinium [C4C1Pyrr]+, 1-(2-methoxyethyl)-1-methyl-pyrrolidinium [C1OC2C1Pyrr]+, and trihexyl(tetradecyl)phosphonium [P6,6,6,14]+. Furthermore, three of the ILs contain the anion bis(trifloromethylsulfonyl)imide [NTf2]− and one of the following cations [C4C1C1mim]+, [C4C1Pyrr]+, or [C1OC2C1Pyrr]+. Two other ILs contain the 1-ethyl-3-methylimidazolium cation [C2C1mim]+ and an alkyl sulfate anion (ethylsulfate [C2SO4]− or n-hexylsulfate [C6SO4]−). The last one is composed by [C4C1Pyrr]+ and trifluoromethanesulfonate [CF3SO3]−. The experimental values were used to determine the glass-transition temperature and fragility of these ILs and to analyze the capability of two group contribution methods recently proposed by Gardas and Coutinho for the density and viscosity prediction of ILs.