The physicochemical properties of some imidazolium-based ionic liquids and their binary mixtures

The density, viscosity and conductivity of ionic liquids (ILs), 1-octyl-3-methylimidazolium tetrafluoroborate ([omim][BF₄]), 1-octyl-3-methylimidazolium chloride ([omim][Cl]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim] BF₄]), 1-hexyl-3-methylimidazolium chloride ([hmim][Cl]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF₆]), and the [omim][BF₄] + [omim][Cl], [hmim][BF₄] + [hmim][Cl], and [hmim][PF₆] + [hmim][Cl] binary mixtures were studied at different temperatures. It was demonstrated that the densities of both the neat ILs and their mixtures varied linearly with temperature. The density sensitivity of a binary mixture is between those of the two components. The excess molar volumes (V ^E) of [hmim][BF₄] + [hmim][Cl] and [hmim][PF₆] + [hmim][Cl] mixtures are positive in the whole composition range. For [omim][BF₄] + [omim][Cl], the V ^E is also positive in the [omim][Cl]-rich region, but is negative in the [omim][BF₄]-rich region. The viscosity or conductivity of a mixture is in the intermediate of those of the two neat ILs. For all the neat ILs and the binary mixtures studied, the order of conductivity is opposite to that of the viscosity. The Vogel-Tammann-Fulcher (VTF) equations can be used to fit the viscosity and conductivity of all the neat ILs and the binary mixtures. The neat ILs and their mixtures obey the Fractional Walden Rule very well, and the values of the Walden slopes are all smaller than unit, indicating obvious ion associations in the neat ILs and the binary mixtures.