Viscosity measurements of three ionic liquids using the vibrating wire technique

The paper reports measurements of the viscosity of the ionic liquids 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C6mim][NTf2]), 1-ethyl-3-methylimidazolium ethyl sulfate ([C2mim][EtSO4]) and 1-ethyl-3-methylpyridinium ethyl sulfate [C2mpy][EtSO4], by means of a vibrating wire viscometer. Density measurements on the same samples have been carried out using an Anton Paar U-tube density meter. Recently, we have studied the applicability of the vibrating-wire technique to the measurement of the viscosity of ionic liquids. The fact that these liquids are electrical conductors represents a source of additional experimental uncertainty, if a bare metallic wire is used as a sensor. The previous study involved the measurement of the viscosity of trihexyl(tetradecyl)phosphoniumdicyanamide [P6,6,6,14][dca], and it was shown that the effect of the electrical conductivity was essentially insignificant. The current study extends our work to liquids 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C6mim][NTf2]), 1-ethyl-3-methylimidazolium ethyl sulfate ([C2mim][EtSO4]) and 1-ethyl-3-methylpyridinium ethyl sulfate ([C2mpy][EtSO4]) which have an order of magnitude higher electrical conductivity than [P6,6,6,14][dca]. The main aim of the present work remains the study of the effect of the electrical conductivity on the performance of the vibrating wire instrument used. One important difference from our previous work, however, is the fact that [C6mim][NTf2] has been proposed to be a reference fluid for several thermophysical properties by IUPAC, for which a round-robin set of thermophysical property measurements has recently been carried out. Therefore another of the main goals of this work is to compare our vibrating wire results for the viscosity of the reference liquid [C6mim][NTf2] with those data. The present results agree within ±2% with a correlation of those results obtained with the IUPAC sample that report water contamination below 120 mg kg−1.

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