Viscosity-pressure dependence for nanostructured ionic liquids. Experimental values for butyltrimethylammonium and 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide

Highlights: • Viscous behavior of two ILs ([C₄C₁Pyr][NTf₂]) ([N_4,1,1,1][NTf₂]) is studied from (278.15 to 373.15) K up to 150 MPa. • The IL with [N_4,1,1,1]⁺ presents a higher viscosity than the IL containing [C₄C₁Pyr]⁺ cation. • The pressure-viscosity coefficient, α_film, and the factor ${\eta }_0^{0.69}{\alpha }_{\mathrm{film}}^{0.56}$ due to the lubricant of the film thickness were determined. • From the analysis of 30 ILs it has been concluded that the highest α_film values are obtained for [FAP]⁻ ILs. • The highest ${\eta }_0^{0.69}{\alpha }_{\mathrm{film}}^{0.56}$ values were obtained for [C₈C₁Im][PF₆] and [P_6,6,6,14][FAP]. New measurements of the viscosity of three ionic liquids (ILs), 1-ethyl-3-methylimidazoliumethylsulfate, 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide and butyltrimethylammonium bis(trifluoromethylsulfonyl)imide were carried out in two laboratories by using three different devices. The viscosity of these ionic liquids was measured at atmospheric pressure by means of a vibrating wire viscometer and a rotational viscometer with relative expanded uncertainties (k = 2) of 1.5% and 1%, respectively. Viscosity behaviour of the last two ILs at high pressure was studied with a falling body viscometer with a relative expanded uncertainty (k = 2) of 3.5%. In addition, the universal pressure-viscosity coefficient was calculated for these fluids, as well as for 14 ionic liquids studied in the literature. Taking into account the high pressure viscosity behaviour of 30 ILs it have been concluded that for similar viscosity grade ILs based on tris(pentafluoroethyl)trifluorophosphate anion present the highest film thickness capacity.