Fluid Phase Equilibria 353 (2013) 76–86 Contents lists available at SciVerse ScienceDirect Fluid Phase Equilibria j our na l ho me pa ge: www.elsevier.com/locate/fluid Viscosity measurements of three ionic liquids using the vibrating wire technique João C.F. Diogo, Fernando J.P. Caetano 1 , João M.N.A. Fareleira ∗ , William A. Wakeham ∗∗ Centro de Química Estrutural, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal a r t i c l e i n f o Article history: Received 9 January 2013 Received in revised form 22 February 2013 Accepted 20 May 2013 Available online 28 May 2013 Keywords: Vibrating wire Reference ionic liquid Viscosity 1-Hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C6mim][NTf2]) 1-Ethyl-3-methylimidazolium ethyl sulfate ([C2mim][EtSO4]) 1-Ethyl-3-methylpyridinium ethyl sulfate [C2mpy][EtSO4] a b s t r a c t The paper reports measurements of the viscosity of the ionic liquids 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C 6 mim][NTf 2 ]), 1-ethyl-3-methylimidazolium ethyl sulfate ([C 2 mim][EtSO 4 ]) and 1-ethyl-3-methylpyridinium ethyl sulfate [C 2 mpy][EtSO 4 ], 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 [P 6,6,6,14 ][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 ([C 6 mim][NTf 2 ]), 1-ethyl- 3-methylimidazolium ethyl sulfate ([C 2 mim][EtSO 4 ]) and 1-ethyl-3-methylpyridinium ethyl sulfate ([C 2 mpy][EtSO 4 ]) which have an order of magnitude higher electrical conductivity than [P 6,6,6,14 ][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 [C 6 mim][NTf 2 ] has been proposed to be a reference fluid for several thermo- physical 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 [C 6 mim][NTf 2 ] 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 . © 2013 Elsevier B.V. All rights reserved. 1. Introduction Recently, we have studied the applicability of the vibrating-wire technique to the measurement of the viscosity of ionic liquids [1]. 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 viscosity Abbreviations: Aft, after; Be, before; FP, free preparation; IC, ionic chro- matography; IUPAC, International Union of Pure and Applied Chemistry; KF, Karl–Fischer; NMR, nuclear magnetic resonance; ND, not detected; NR, not reported; NS, not specified; ppm, parts per million; PTFE, polytetrafluoroethylene; VFT, Vogel–Fulcher–Tammann equation. ∗ Corresponding author. Tel.: +35 1218419265. ∗∗ Corresponding author. E-mail addresses: j.fareleira@ist.utl.pt (J.M.N.A. Fareleira), w.a.wakeham@soton.ac.uk (W.A. Wakeham). 1 Universidade Aberta, R. da Escola Politécnica, 147, 1269-001 Lisbon, Portugal. measurement of trihexyl(tetradecyl)phosphoniumdicyanamide, [P 6,6,6,14 ][dca], [1] and it was shown that the uncertainty owing to the electrical conductivity of the liquid was insignificant. However, that liquid has a low electrolytic conductivity, when compared to 1- hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, [C 6 mim][NTf 2 ], which has recently been proposed as a reference fluid for several thermophysical properties [2,3] (IUPAC Project 2002-005-1-100: Thermodynamics of ionic liquids, ionic liquid mixtures, and the development of standardized systems). In the present work, we have measured the viscos- ity of the ionic liquids 1-hexyl-3- methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [C 6 mim][NTf 2 ], 1-ethyl- 3-methylimidazolium ethyl sulfate [C 2 mim][EtSO 4 ] and 1-ethyl-3-methylpyridinium ethyl sulfate [C 2 mpy][EtSO 4 ], at atmospheric pressure and at several temperatures in the range (293 ≤ T ≤ 323) K, using the vibrating wire technique. The results were compared with measurements made with other techniques of high accuracy. In our view the validation of the technique for 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.05.012