Published data on the thermophysical properties of ionic liquids are normally in disagreement if results from different laboratories, using different samples and different measurement protocols, are compared. This fact was recognized years ago at the level of the International Union of Pure and Applied Chemistry (IUPAC), which established IUPAC Project 2002-005-1-100 (Thermodynamics of ionic liquids, ionic liquid mixtures, and the development of standardized systems), with the main objective of recommending a reference ionic liquid, making reference-quality measurements on selected thermophysical properties of both the pure ionic liquid and its mixtures, establishing recommended values for the properties measured, and providing recommendations on measurement methods. The ionic liquid chosen was 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C6mim][(CF3SO2)2N], because of its stability, low viscosity compared with that of most common ionic liquids, low water solubility, ease of preparation and purification, and commercial availability. Due to its hydrophobicity, it is capable of being obtained very pure, with water amounts as small as 20 ppm. This paper reports new results obtained with the sample of [C6mim][(CF3SO2)2N] synthesized in the IUPAC project, namely on density, speed of sound, surface tension, and refractive index, as well as thermal conductivity of a commercial sample at P = 0.1 MPa, as a function of temperature, and proposes reference data and reference data correlations for the density, speed of sound, heat capacity, surface tension, viscosity, electrical conductivity, thermal conductivity, refractive index, ion self-diffusion coefficient, and melting temperature of this ionic liquid at 0.1 MPa, as a function of temperature, using these and other data reported up to July 2020. Rheological measurements demonstrated that this ionic liquid is Newtonian.

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