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

1 Viscosity-pressure dependence for Nanostructured Ionic Liquids. Experimental values for butyltrimethylammonium and 1-butyl-3- methylpyridinium bis(trifluoromethylsulfonyl)imide Mohamed Dakkach1,2; Félix Μ. Gaciño1, María J. G. Guimarey1, Sofia K. Mylona3,4, Xavier Paredes1,5, María J. P. Comuñas1, Josefa Fernández1,*, Marc J. Assael3 1Laboratorio de Propiedades Termofísicas, Grupo NaFoMat, Departamento de Física Aplicada, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain 2ISPITS Tétouan/Tanger, Faculté des Sciences Tétouan, Université Abdelmalek Essâadi, 93030 Tétouan, Morocco 3Laboratory of Thermophysical Properties and Environmental Processes, Chemical Engineering Department, Aristotle University, Thessaloniki 54124, Greece 4Fluid Science & Resources Division, School of Mechanical & Chemical Engineering, The University of Western Australia, Perth, 6009, Australia 5Centro de Química Estrutural, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal * Corresponding author. E-mail address: [email protected] (Josefa Fernández) 2 ABSTRACT 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 pressureviscosity 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. Keywords: Vibrating Wire Viscometer; Falling Body Viscometer; Rotational Viscometer; High Pressure Viscosity; Ionic Liquids 3 1. Introduction The main requirement for a lubricant is to maintain a separation between moving parts to minimize wear and friction [1]. Depending on the presence or absence of an external pressure source, different lubrication regimes are identified, which can be classified by means of the Stribeck curve [2-4] that characterizes the different forms of contact between surfaces as well as lubrication conditions that can happen in tribosystems. Therefore, the suitability of a lubricant for systems as gears, or bearings, depends on the temperature and pressure of the lubricants, as well as the load, speed and mechanical properties of the materials of the tribological contact. These conditions identify the four different lubrication regimes: hydrodynamic, elastohydrodynamic (EHL), mixed and boundary lubrication. In the EHL regime, the film thickness of a lubricant depends strongly on its viscosity, , and to a minor extent, on its universal pressure-viscosity coefficient [5]. Oils in gears are in contact with most of the other machine elements and fulfill the tasks of lubrication, cooling and removing the friction heat generated in the sliding rolling contacts. Thus, these oils must have the following requirements: resistance to aging and oxidation, low foaming tendency, good load-carrying capacity, neutral towards the materials involved and good viscosity-temperature behaviour [6,7]. The most important property for the formulation of a gear lubricant is the viscosity and its temperature and pressure dependence. Therefore, a description in a wide range of pressure and temperature of the viscous behaviour of the oil is required. One of the most promising applications of ionic liquids (ILs) is their use as lubricants of gears or additive of lubrications because they can be synthetized to have negligible volatility, non-flammability, high thermal stability, low melting point and conductivity, high thermal stability and chemical inertness [8-11]. For this kind of application as well as their use as hydraulic and thermal fluids, among others, accurate viscosity data of ILs are required in order to study their suitability. Thus, several viscosity values for these fluids ranging at least from (6 to 7600) mPa·s at T = 298.15 K and p= 0.1 MPa have been published [12-14]. Viscosity depends strongly on their pressure and temperature conditions as well as their molecular structure. It is important 4 to remark that viscosity, as well as other thermophysical properties, is strongly dependent on the impurities, like water and halides [15]. The good results obtained by ILs as lubricants can be attributed to their dipolar structure, allowing easy physical or chemical adsorption in most of the surfaces, and leading to the development of boundary films between friction contact pairs and contributing significantly to the reduction in friction and wear under high pressures [1,16-18]. The low wear and friction obtained with ILs has also been attributed to the asymmetric structure of their ions, which can resist the solidification that occurs when liquids are confined to thin films under pressure [11]. However, the number of ILs under study remains scarce, moreover if we should analyse their suitability as lubricants in the elastohydrodynamic regime of lubrication (EHL) [19-22]. Thus, the studies of the universal pressure-viscosity coefficients and the film thickness for this kind of fluids are quite scarce, as well as studies of their viscosity at high pressures [20-25]. Viscosity measurements for these fluids have been carried out by means of several techniques, such as capillary viscometer, rotational viscometer, falling or rolling body viscometers, as well as vibrating wire viscometer [13,24,26-28]. Nieto de Castro et al. [29] have recommended the use only of primary viscometric techniques like the vibrating-wire viscometer [26,29]. However, it is important to point out that this kind of device is expected to present several troubles when the wire is immersed in a conducting fluid. The conductivity of these fluids can provide alternative paths for the electrical current applied thereby affecting the performance of the instrument [30,31]. To our knowledge there are only four works that report viscosity values for ILs obtained by using the vibrating wire method [26,27,30,31]. Kandil et al. [27] and Diogo et al. [26,30] have measured and analysed the viscosities of 1-hexyl-3- methylimidazolium bis(trifluorosulfonyl)imide. This fluid has been chosen in an IUPAC project to analyse the reasons of the uncertainties of several properties of ILs. This IL has an electrical conductivity at T = 323.15 K of 0.54 S·m−1. The viscosities obtained with the vibrating-wire technique (VW) agree with the results obtained with other techniques [26,27,30]. Diogo et al. [26,30] have also measured and analysed using different techniques the viscosities of two ILs among them 1-ethyl-3- methylimidazolium ethyl sulfate, [C2C1Im][C2SO4], whose electrical conductivity at T 5 = 323.15 K is 1.01 S·m−1 [32-34]. These authors also concluded that the viscosities obtained with VW techniques are coherent with those obtained with other techniques. As Diogo et al. [26] explain, in presence of an electrically conducting fluid one would expect that the electromotive force at the wire terminals should depend on the frequency. These authors analyse the variation of the impedance with the frequency when the cell of the vibrating wire is full of [C2C1Im][C2SO4]. At the frequencies of interest to measurement (around 1 kHz), the effect of frequency in the impedance is negligible. Therefore, Diogo et al. [26] showed that the electrical conductivity up to about 1 S·m-1 does not imply any significant error on the viscosity measurements with the VW technique. The effect of pressure on the viscosity of ILs has been mostly studied by means of falling body viscometers [23,24,35-40], although techniques based on rolling body [41-43], moving piston [44,45], rheometry [24] or vibrating wire [27] have been also employed. Harris et al. [35-38] reported viscosity values at pressures up to 300 MPa with an overall relative uncertainty of (2 to 3) % of six ILs composed by an alkyl methyl imidazolium, [CnC1Im]+ and one of the following anions: tetrafluoroborate [BF4]-, hexafluorophosphate [PF6]- and bis(trifluoromethylsulfonyl)imide, [NTf2]- namely: [C1C8Im][BF4], [C4C1Im][BF4], [C1C8Im][PF6], [C4C1Im][PF6], [C6C1Im][PF6] and [C4C1Im][NTf2]. Gaciño et al. [24,39,40] reported the viscosities up to p = 150 MPa with a relative expanded uncertainty (k = 2) of (3.5 to 5) % of eight ILs based on the cations1-(2-methoxyethyl)-1-methyl-pyrrolidinium , ([C1OC2C1Pyrr]+, 1-butyl-1- methylpyrrolidinium, [C4C1Pyrr]+ or 1-butyl-2,3-dimethylimidazolium, [C4C1C1Im]+, and the anions alkyl sulfate [CnSO4], tris(pentafluoroethyl)trifluorophosphate, [FAP]- or trifluoromethanesulfonate, [CF3SO3]- namely: [C1OC2C1Pyrr][NTf2], [C1OC2C1Pyrr][FAP], [C2C1Im][C2SO4], [C4C1Pyrr][NTf2], [C4C1C1Im][NTf2], [C4C1C1Im][FAP], [C4C1Pyrr][FAP], [C4C1Pyrr][CF3SO3]. Tomida et al. [41-43] reported values for [C4C1Im][BF4], [C4C1Im][PF6], [C6C1Im][PF6] and [C8C1Im][PF6] at pressures up to 20 MPa with uncertainties between 1.6 % and 3.0 %. Ahosseini and Scurto [44] have reported viscosity values (estimated relative uncertainty of 3 %) for ILs containing [CnC1Im]+ cation, and [BF4]-, [PF6]- and [NTf2]- anions. Aparicio et al. [45] reported viscosity values for ILs containing the [C2C1Im]+ cation and tosylate, [TOS]-, and [C1SO4]- anions at pressures up 150 MPa with an estimated relative uncertainty of 2 %; and Kandil et al. [27] measured the viscosity of [C6C1Im][NTf2] at 6 temperatures between 288 K and 433 K up to p = 50 MPa with a relative expanded uncertainty (k = 2) of 2 %. Bandrés et al. [46] reported viscosity values (estimated relative uncertainty of 2 %) of eight ILs at to p = 65 MPa based on alkyl pyridinium [CnPy]+, 1-alkyl-2-methylpyridinium [Cn2C1Py]+, 1-alkyl-3-methylpyridinium [Cn3C1Py]+ or 1-alkyl-4-methylpyridinium [Cn4C1Py]+ cations with dicyanamide, [N(CN)2]−, [BF4]-, or [CF3SO3]- anions. Mordukhovich et al. [22] also reported the viscosity values of [C2C1Im][NTf2] at pressures up to 350 MPa from T = 273.15 K to T = 373.15 K. Recently, Regueira et al. [25] characterized the rheological behaviour of [C2C1Im][C6SO4] and an IL composed by the cation trihexyl(tetradecyl)phosphonium, [P6,6,6,14] and the anion [(C2F5)3PF3]- up to p = 75 MPa and they determined the viscosity values by means of a Reologica StressTech Couette HTHP rheometer with an estimated relative expanded uncertainty (k = 2) of 7 %. Voeltzel et al. [47] measured the viscosities of [C1C1Im][NTf2] in a falling-body viscometer for pressures up to 800 MPa and for T = (303, 353 and 403) K with a relative uncertainty of 3 %. In 2008, Pensado et al. [20] published the viscosity pressure coefficients for seven ILs, based on imidazolium cations whose viscosity data at high pressures were previously published [27,28,35-37,41,42]. These coefficients were determined between the temperatures 383.15 K and 353.15 K using two definitions: the reciprocal asymptotic isoviscous pressure coefficient, α*, which is given by [48] the following integral: ( ) ( ) 1 0 0 −           = = ` p dpp *    (1) and the universal pressure–viscosity coefficient, αfilm (also named film pressureviscosity coefficient) [49]: *)/(p )exp( iv film   3 31 −− = (2) where the isoviscous pressure coefficient, piv, is a function of the pressure p given by ( ) ( ) = = p` iv 'p 'dp'p )p(p 0 0   (3) 7 In 2012, Paredes et al. [21] reported the αfilm values for other five imidazolium based on experimental viscosity values reported by Ahosseini et al. [44] and Aparicio et al. [45]. Fernández et al. [23] published graphical information of the αfilm at T = 313.15 K for six ILs but numerical values were not given. In 2013, Mordukhovich et al. [22] determined the α* values at T = (295.15, 373.15 and 423.15) K for [C4C1Im][NTf2] from viscosity measurements up to pressures of 350 MPa. The authors conclude that this IL, which shows low α* values, delivers low friction in mixed lubrication (ML) and provides betters cuffing protection in boundary lubrication (BL) than mineral lubricants with similar viscosities. Recently, Gaciño et al. [24] have published both α* and αfilm values between the temperatures 303.15 K and 353.15 K for two ILs with the cation 1-butyl-1-methylpyrrolidinium from their own viscosities values for pressures up to 150 MPa. From these studies, it can be concluded that αfilm values decrease strongly with the temperature. Thus, for [C4C1Im][BF4] at T = 393.15 K the αfilm takes the value of 11.7 GPa-1, whereas at T = 353.15 K αfilm is 6.3 GPa-1. Pressure viscosity coefficients can be also determined from film thickness measurements [50]. Arora and Cann [51] indicate that pressure viscosity coefficients determined indirectly from their measurements of the central film thickness at elastohydrodynamic regime for [C4C1Im][BF4], [C6C1Im][BF4], [C6C1Im][PF6] and [C6C1Im][PF6] agree with the values reported by Pensado et al. [20]. Voeltzel et al. [47] have determined  * values between the temperatures 298 K and 450 K using modified WLF-Yasutomi correlations of their experimental and NEMD viscosities for [C1C1Im][NTf2]. The main use of the viscosity pressure coefficients of ILs is to determine the film-thickness at EHL regime and to know the regime at specific conditions (temperature and pressure of the lubricants, load, speed and mechanical properties of the materials of the tribological contact). Xie et al. [52] have determined the film thickness using the Hamrock–Dowson equation with α* values of Pensado et al. [20] for three ILs in order to analyse the enhancement of the film thickness in nanoconfined conditions due to an external electrical field. Zhang et al. [53] in a recent study about the synergistic effect of a hybrid carbon nanotube−graphene oxide as nanoadditive enhancing the frictional properties of [C4C1Im][BF4], use its α* value determined by Pensado et al. [20]. 8 In this work we present viscosity data for three ILs: 1-ethyl-3- methylimidazolium ethylsulfate ([C2C1Im][C2SO4]), 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([C4C1Py][NTf2]) and butyltrimethylammonium bis(trifluoromethylsulfonyl)imide ([N4,1,1,1][NTf2]). The viscosity of the three ILs was measured by using three different devices. The first one is a vibrating-wire viscometer which has been implemented at the Laboratory of Thermophysical Properties & Environmental Processes in Greece [54]. This device is designed in such a way that allows absolute measurements of high-viscosity liquids in the temperature range (250 – 400) K and at pressures up to 30 MPa. An Anton Paar Stabinger SVM 3000 rotating cylinder viscometer located at the Laboratory of Thermophysical Properties in Spain [55] was used to measure both the viscosity and density of the three ILs. The electrical conductivity of the three ILs in the measured range is lower than 1.01 S·m−1 [32- 34,56,57]. The behaviour of the viscosity as a function of pressure (and temperature) for [C4C1Py][NTf2] and [N4,1,1,1][NTf2] was also determined experimentally by means of a falling body viscometer implemented at the Laboratory of Thermophysical Properties [24]. The effect of pressure and temperature on the viscosity is analysed for these fluids, and, from the correlation of viscosity values we obtain the αfilm values as well as the film thickness. In addition, we present the viscosity pressure coefficients for other 12 ILs whose viscosity data at high pressures are available in the literature. 2. Experimental 2.1. Materials The ILs studied in this work are presented in Table 1. Before the density and viscosity measurements, samples were treated under vacuum for at least 48 h in order to remove any volatile compounds, like water and other impurities. Water content of samples was measured by means of a Karl Fisher titrator. In the case of viscosity measurements of [C4C1Py][NTf2] and [N4,1,1,1][NTf2] by means of the vibrating wire, the reported water content was the specified by the supplier. Samples were transferred to the different apparatus under a nitrogen atmosphere in order to avoid contact with air. The water content of each sample is shown in Table 1. 2.2. Experimental techniques 9 2.2.1. Vibrating Wire Viscometer A recently implemented vibrating wire viscometer at the Laboratory of Thermophysical Properties & Environmental Processes was used to study the viscosity behaviour of the ILs as a function of temperature and at atmospheric pressure. The vibrating wire is immersed in the fluid whose viscosity is to be measured. The wire, made of tungsten, has a diameter of 300 μm and 50 mm length vibrating with a frequency in vacuum of about 1 kHz. The wire is squeezed tight between two isolated plates at either end, and two tungsten parallel rods secure the constant tensioning of the wire at different temperatures. The viscometer is located inside a pressure vessel and two platinum resistance thermometers located at the top and the bottom of the vessel measure the experimental temperature with an expanded accuracy (k = 2) better than 20 mK. The wire is surrounded by two magnets made from samarium-cobalt that produce a field of about 1 T at the wire, in which oscillations are produced by applying two pulses of current of opposite sign to the wire. The passing of current through the wire under magnetic field generates a Lorentz force displacing the wire from its rest position. The instrument operates in the free decay mode, i.e., the wire oscillation is left to decay freely. The complete review theory of this kind of device has been further described [58-60]. An analysis of the errors for this device leads to an relative uncertainty of 1.5 % at the 95 % confidence level [54]. Although the vibrating wire allows viscosity measurements in the temperature range (250 – 400) K up to p = 30 MPa, in this work we only present data at atmospheric pressure in a narrow temperature range. The upper limit of the temperature range was reduced due to the fact that the ionic nature of this kind of fluids confer them a high electrical conductivity, which could provide alternative paths for the electrical current and thereby affecting the performance of the device [26,30]. For [C2C1Im][C2SO4] the uppest temperature of our measurements is 323.02 K at which the electrical conductivity is 1.01 S·m−1 according to Vila et al. [32] and a little lower according to Yu et al. [33] and Pereiro et al. [34]. For [C4C1Py][NTf2] the upper temperature is 310.19 K, at which the electrical conductivity according to Zhang et al. [56] is 0.45 S·m-1. For the third IL [N4,1,1,1][NTf2] we have measured up to T = 328.27 K being at 16 the higher the pressure, the higher the effect. However, this trend diminishes with temperature, in fact, at T = 353.15 K for [C4C1Py][NTf2] the temperature-viscosity coefficient,  (T), presents a lower pressure dependence than for [N4,1,1,1][NTf2]. 3.4. Suitability of ILs as Lubricants in the Elastohydrodynamic Regime of Lubrication According to the American Gear Manufacturers Association, in this regime of lubrication the central film thickness of a lubricant in a rolling element is given by the following expression: ( ) ( ) 560 1690 0 . . cent GPa)smPa(mh −   (11) where  0 is the viscosity of the oil at atmospheric pressure and  is the universal pressure-viscosity coefficient [5]. Thus, only the  0 and  are oil dependent, whereas the other parameters depend on the rolling element characteristics. To apply Eq. (11), a suitable definition of the viscosity pressure coefficient  independent of the pressure is required. For that purpose the universal pressure-viscosity coefficient or the reciprocal asymptotic isoviscous pressure coefficient are used. In Table 9 we present the universal pressure-viscosity coefficient  film and the reciprocal asymptotic isoviscous pressure coefficient,   for [N4,1,1,1][NTf2] and [C4C1Py][NTf2] at several temperatures obtained using the eqs (1-3, 8) with the parameters of Table 6. The  film and  are higher (9 %-15 %in the studied range) for [N4,1,1,1][NTf2] than for [C4C1Py][NTf2].  values are slightly lower than the  film values. This fact agrees with the results found for seven ILs by Pensado et al. [20]. In similar way as for [N4,1,1,1][NTf2] and [C4C1Py][NTf2] we have calculated the  and  film values for other ILs measured with the viscometer VisLPT1 [39,40] ([C4C1C1Im][NTf2], [C1OC2C1Pyrr][NTf2], [C4C1Pyrr][NTf2], [C1OC2C1Pyrr][FAP], [C4C1C1Im][FAP] and [C2C1Im][C2SO4]. The values obtained at (313.15, 343.15 and 363.15) K are reported in the Table S3 of the supplementary information. Gaciño et al. [24] have already reported the  and  film for [C4C1Pyrr][FAP] and [C4C1Pyrr][CF3SO3]. Taking into account all these results we have found the following trend for  film at T = 313.15 K with the IL cation: [N4,1,1,1]+>[C4C1C1Im]+>[C4C1Pyrr]+>[C4C1Py]+>[C1OC2C1Pyrr]+. This trend remains 17 up to temperatures closer to T = 343.15 K, although at higher temperature [C4C1Py]+ ILs present the lower  film values of these fluids. [N4,1,1,1]+ present the highest values in the analyzed range. Similar trends were found for  *. In addition, we have calculated the  and  film values of [C₄2C₁Py][BF₄], [C₄3C₁Py][BF₄], [C₄4C₁Py][BF₄], [C₄Py][CF₃SO₃], [C₃Py][BF₄], [C83C1Py][BF₄], [C₄Py][BF₄] and [C43C1Py][N(CN)2] from viscosity correlations proposed by Bandrés et al. [46] (Table S3). At T = 313.15 K the  film values range from 5.5 GPa-1 (for [C43C1Py][N(CN)2]) to 11.2 GPa-1 for [C44C1Py][BF4]. The  film values for 30 ILs at T = 313.15 K are showed in Fig. 9 where it can be seen that the lowest value for  film is for the [C43C1Py][N(CN)2] IL, whereas the highest values (up to 19 GPa-1) are for the four [FAP]- ILs. According with the expression (11) the product 56.069.0 0film  is directly proportional to the hcent under the same conditions of operation (surface geometry and relative speed U). Fig. 8 shows this product as a function of temperature using the  film values for [N4,1,1,1][NTf2] and [C4C1Py][NTf2]. The 56.069.0 0film  values are higher for [N4,1,1,1][NTf2] than for [C4C1Py][NTf2] (around 30 %-50 % in the studied range). This is expected because [N4,1,1,1][NTf2] presents higher viscosity and  values at the same conditions than [C4C1Py][NTf2]. In Fig. 9 we have plotted the film thickness factor, 56.069.0 0film  , for 30 ionic liquids whose values are reported in the literature [20,21,24,25] together with those studied in this work. Thus, ILs containing the anions [PF6]- and [FAP]- provide the thickest film thickness, whereas [NTf2]- and [N(CN)2]- based ILs provide the thinnest films. Within the EHL regime, higher values of film thickness are desired when boundary wear has to be avoided, whereas lower values are also needed to save friction energy, reduce subsurface stress and also pressure peaks [24]. In addition, it is very important that the variations with temperature of 56.069.0 0film  are as low as possible. This can be found in ILs with high viscosity index as those based on [NTf2]- [61] and/or ILs with slight () dependence as those based on [NTf2]- and [C2C1Im][C2SO4] [40]. 4. Conclusions 18 The viscous behaviour of [C2C1Im][C2SO4], [C4C1Py][NTf2] and [N4,1,1,1][NTf2] was analysed at temperatures between 278. 15 K and 373.15 K and at p = 0.1 MPa by means of new data obtained with vibrating-wire and rotational viscometers. Dynamic viscosities were compared with literature values obtaining maximum AADs of 7.89 %, 3.58 % and 9.88 %, for [C2C1Im][C2SO4], [C4C1Py][NTf2] and [N4,1,1,1][NTf2], respectively. The viscosities at high pressure of [C4C1Py][NTf2] and [N4,1,1,1][NTf2] measured with a falling body viscometer up to p = 150 MPa were successfully correlated by means of a modified VFT-equation as a function of pressure and temperature. We found that both pressure-viscosity and temperature-viscosity coefficients are higher for [N4,1,1,1][NTf2] than for [C4C1Py][NTf2]. From the analysis of 30 ILs it have been concluded that the highest  film values are obtained for [FAP]- ILs whereas the lowest for [C43C1Py][N(CN)2], [C3Py][BF4] and [C2C1Im][C2SO4]. The highest 56.069.0 0film  values were obtained for [C8C1Im][PF6] and [P6,6,6,14][FAP] whereas the lowest are for[C43C1Py][N(CN)2]. Acknowledgments This work was supported by the Spanish Ministry of Economy and Competitiveness and the Program European Regional Development Fund (CTQ2011-23925 and ENE2014-55489-C2-1-R). Moreover, this work was funded by the Xunta de Galicia (AGRUP2015/11, GRC ED431C 2016/001 and R2014/015). MD thanks AECID of Spain for the allocation of a MAEC-AECID grant and to the EMMAG Programme mobility. The authors gratefully acknowledge partial financial contribution from COST project CM1206 on “Exchange on Ionic Liquids”. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http Notes The authors declare no competing financial interest. References 19 [1] A.E. Somers, P.C. Howlett, J. Sun, D.R. MacFarlane, M. Forsyth, Transition in Wear Performance for Ionic Liquid Lubricants under Increasing Load, Tribol. Lett. 40 (2010) 279-284. [2] R. Stribeck, Kugellager für beliebige Belastungen, Z. Ver. Deutscher Ingenieure 45 (1901) 73-125. [3] R. Stribeck, Die wesentlichen Eigenschaften der Gleit- und Rollenlager, Z. Ver. Deutscher Ingenieure 46 (1903) 1341-1348. [4] B. Jacobson, The Stribeck memorial lecture, Tribol. Int. 36 (2003) 781-789. [5] Selecting Oils with High Pressure-Viscosity Coefficient - Increase Bearing Life by More Than Four Times. http://www.machinerylubrication.com/Read/586/viscositycoefficient-bearing, 2004. [6] D.A. Lauer, Industrial Gear Lubricants. in: L.R. Rudnick, (Ed.), Synthetics, Mineral Oils, and Bio-Based Lubricants: Chemistry and Technology, CRC Press, Boca Raton, 2013, pp. 473-490. [7] T. Bartels, Gear Lubrication Oils. in: T. Mang, W. Dresel, (Eds.), Lubricants and Lubrication, Second Edition, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2007, pp. 230-273. [8] F. Zhou, Y. Liang, W. Liu, Ionic liquid lubricants: designed chemistry for engineering applications, Chem. Soc. Rev. 38 (2009) 2590-2599. [9] S.A.S. Amiril, E.A. Rahim, S. Syahrullail, A review on ionic liquids as sustainable lubricants in manufacturing and engineering: Recent research, performance, and applications, J. Clean. Prod. 168 (2017) 1571-1589. [10] Y. Zhou, J. Qu, Ionic Liquids as Lubricant Additives: A Review, ACS Appl. Mater. Inter. 9 (2017) 3209-3222. [11] A. Somers, P. Howlett, D. MacFarlane, M. Forsyth, A Review of Ionic Liquid Lubricants, Lubricants 1 (2013) 3-21. [12] S. Zhang, N. Sun, X. He, X. Lu, X. Zhang, Physical properties of ionic liquids: database and evaluation, J. Phys. Chem. Ref. Data 35 (2006) 1475-1517. [13] J. Fernández, F.M. Gaciño, Physical Properties of Pure Liquids. in: A.P. De los Ríos, F.J. Hernández-Fernández, (Eds.), Ionic Liquids in Separation Technology, Elsevier, Amsterdam, 2014, pp. 3-12. [14] N.E.A. Cousens, L.J. Taylor Kearney, M.T. Clough, K.R.J. Lovelock, R.G. Palgrave, S. Perkin, Preparation and characterisation of high-density ionic liquids incorporating halobismuthate anions, Dalton Trans. 43 (2014) 10910-10919. [15] P.J. Carvalho, T. Regueira, L.M.N.B.F. Santos, J. Fernandez, J.A.P. Coutinho, Effect of Water on the Viscosities and Densities of 1-Butyl-3-methylimidazolium Dicyanamide and 1-Butyl-3-methylimidazolium Tricyanomethane at Atmospheric Pressure, J. Chem. Eng. Data 55 (2009) 645-652. [16] A.-E. Jiménez, M.-D. Bermúdez, Ionic liquids as lubricants for steel–aluminum contacts at low and elevated temperatures, Tribol. Lett. 26 (2007) 53-60. [17] X. Liu, F. Zhou, Y. Liang, W. Liu, Tribological performance of phosphonium based ionic liquids for an aluminum-on-steel system and opinions on lubrication mechanism, Wear 261 (2006) 1174-1179. [18] F. López-Sánchez, I. Otero, E.R. López, J. Fernández, Tribological Behavior of Two 1-Ethyl-3-methylimidazolium Alkyl Sulfates as Neat Lubricants for a Steel–Steel Contact, Tribol. Trans. 60 (2017) 729-738. [19] A. Libardi, S.R. Schmid, M. Sen, W. Schneider, Evaluation of ionic fluids as lubricants in manufacturing, J. Manuf. Process 15 (2013) 414-418. 20 [20] A.S. Pensado, M.J.P. Comuñas, J. Fernández, The Pressure–Viscosity Coefficient of Several Ionic Liquids, Tribol. Lett. 31 (2008) 107-118. [21] X. Paredes, O. Fandiño, A.S. Pensado, M.J.P. Comuñas, J. Fernández, Pressure– Viscosity Coefficients for Polyalkylene Glycol Oils and Other Ester or Ionic Lubricants, Tribol. Lett. 45 (2012) 89-100. [22] G. Mordukhovich, J. Qu, J.Y. Howe, S. Bair, B. Yu, H. Luo, D.J. Smolenski, P.J. Blau, B.G. Bunting, S. Dai, A low-viscosity ionic liquid demonstrating superior lubricating performance from mixed to boundary lubrication, Wear 301 (2013) 740- 746. [23] J. Fernández, X. Paredes, F.M. Gaciño, M.J.P. Comuñas, A.S. Pensado, Pressure-viscosity behaviour and film thickness in elastohydrodynamic regime of lubrication of ionic liquids and other base oils, Lubr. Sci. 26 (2014) 449-462. [24] F.M. Gaciño, M.J. Comuñas, T. Regueira, J.J. Segovia, J. Fernández, On the viscosity of two 1-butyl-1-methylpyrrolidinium ionic liquids: effect of the temperature and pressure, J. Chem. Thermodyn. 87 (2015) 43-51. [25] T. Regueira, L. Lugo, M.J.P. Comuñas, J. Fernández, High Pressure Rheological Behavior of 1-Ethyl-3-methylimidazolium n-Hexylsulfate and Trihexyl(tetradecyl)phosphonium Tris(pentafluoroethyl)trifluorophosphate, J. Chem. Eng. Data 62 (2017) 2927-2936. [26] J.C.F. Diogo, F.J.P. Caetano, J.M.N.A. Fareleira, W.A. Wakeham, Viscosity Measurements on Ionic Liquids: A Cautionary Tale, Int. J. Thermophys. 35 (2014) 1615-1635. [27] M.E. Kandil, K.N. Marsh, A.R.H. Goodwin, Measurement of the Viscosity, Density, and Electrical Conductivity of 1-Hexyl-3-methylimidazolium Bis(trifluorosulfonyl)imide at Temperatures between (288 and 433) K and Pressures below 50 MPa, J. Chem. Eng. Data 52 (2007) 2382-2387. [28] K.R. Harris, M. Kanakubo, L.A. Woolf, Temperature and Pressure Dependence of the Viscosity of the Ionic Liquid 1-Butyl-3-methylimidazolium Tetrafluoroborate: Viscosity and Density Relationships in Ionic Liquids, J. Chem. Eng. Data 52 (2007) 2425-2430. [29] C.A. Nieto de Castro, F.J.V. Santos, J.M.N.A. Fareleira, W.A. Wakeham, Metrology of Viscosity: Have We Learned Enough?, J. Chem. Eng. Data 54 (2009) 171-178. [30] J.C.F. Diogo, F.J.P. Caetano, J.M.N.A. Fareleira, W.A. Wakeham, Viscosity measurements of three ionic liquids using the vibrating wire technique, Fluid Phase Equilibr. 353 (2013) 76-86. [31] J.C.F. Diogo, F.J.P. Caetano, J.M.N.A. Fareleira, W.A. Wakeham, C.A.M. Afonso, C.S. Marques, Viscosity Measurements of the Ionic Liquid Trihexyl(tetradecyl)phosphonium Dicyanamide [P6,6,6,14][dca] Using the Vibrating Wire Technique, J. Chem. Eng. Data 57 (2012) 1015-1025. [32] J. Vila, P. Ginés, J.M. Pico, C. Franjo, E. Jiménez, L.M. Varela, O. Cabeza, Temperature dependence of the electrical conductivity in EMIM-based ionic liquids: Evidence of Vogel–Tamman–Fulcher behavior, Fluid Phase Equilibr. 242 (2006) 141- 146. [33] Y.-H. Yu, A.N. Soriano, M.-H. Li, Heat capacities and electrical conductivities of 1-ethyl-3-methylimidazolium-based ionic liquids, J. Chem. Thermodyn. 41 (2009) 103-108. [34] A.B. Pereiro, J.M.M. Araujo, F.S. Oliveira, C.E.S. Bernardes, J.M.S.S. Esperanca, J.N. Canongia Lopes, I.M. Marrucho, L.P.N. Rebelo, Inorganic salts in 21 purely ionic liquid media: the development of high ionicity ionic liquids (HIILs), Chem. Commun. 48 (2012) 3656-3658. [35] K.R. Harris, M. Kanakubo, L.A. Woolf, Temperature and Pressure Dependence of the Viscosity of the Ionic Liquids 1-Methyl-3-octylimidazolium Hexafluorophosphate and 1-Methyl-3-octylimidazolium Tetrafluoroborate, J. Chem. Eng. Data 51 (2006) 1161-1167. [36] K.R. Harris, M. Kanakubo, L.A. Woolf, Temperature and Pressure Dependence of the Viscosity of the Ionic Liquids 1-Hexyl-3-methylimidazolium Hexafluorophosphate and 1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide, J. Chem. Eng. Data 52 (2007) 1080-1085. [37] K.R. Harris, L.A. Woolf, M. Kanakubo, Temperature and Pressure Dependence of the Viscosity of the Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate, J. Chem. Eng. Data 50 (2005) 1777-1782. [38] K.R. Harris, M. Kanakubo, L.A. Woolf, Temperature and Pressure Dependence of the Viscosity of the Ionic Liquid 1-Butyl-3-methylimidazolium Tetrafluoroborate: Viscosity and Density Relationships in Ionic Liquids, J. Chem. Eng. Data 52 (2007) 2425-2430. [39] F.M. Gaciño, X. Paredes, M.J. Comuñas, J. Fernández, Pressure dependence on the viscosities of 1-butyl-2, 3-dimethylimidazolium bis (trifluoromethylsulfonyl) imide and two tris (pentafluoroethyl) trifluorophosphate based ionic liquids: New measurements and modelling, J. Chem. Thermodyn. 62 (2013) 162-169. [40] F.M. Gaciño, X. Paredes, M.J. Comuñas, J. Fernández, Effect of the pressure on the viscosities of ionic liquids: Experimental values for 1-ethyl-3- methylimidazolium ethylsulfate and two bis (trifluoromethyl-sulfonyl) imide salts, J. Chem. Thermodyn. 54 (2012) 302-309. [41] D. Tomida, A. Kumagai, S. Kenmochi, K. Qiao, C. Yokoyama, Viscosity of 1- Hexyl-3-methylimidazolium Hexafluorophosphate and 1-Octyl-3-methylimidazolium Hexafluorophosphate at High Pressure, J. Chem. Eng. Data 52 (2007) 577-579. [42] D. Tomida, A. Kumagai, K. Qiao, C. Yokoyama, Viscosity of [bmim][PF6] and [bmim][BF4] at High Pressure, Int. J. Thermophys. 27 (2006) 39-47. [43] D. Tomida, A. Kumagai, K. Qiao, C. Yokoyama, Viscosity of 1-Butyl-3- methylimidazolium Hexafluorophosphate + CO2 Mixture, J. Chem. Eng. Data 52 (2007) 1638-1640. [44] A. Ahosseini, A. Scurto, Viscosity of Imidazolium-Based Ionic Liquids at Elevated Pressures: Cation and Anion Effects, Int. J. Thermophys. 29 (2008) 1222- 1243. [45] S. Aparicio, R. Alcalde, B. García, J.M. Leal, High-Pressure Study of the Methylsulfate and Tosylate Imidazolium Ionic Liquids, J. Phys. Chem. B 113 (2009) 5593-5606. [46] I. Bandrés, R. Alcalde, C. Lafuente, M. Atilhan, S. Aparicio, On the Viscosity of Pyridinium Based Ionic Liquids: An Experimental and Computational Study, J. Phys. Chem. B 115 (2011) 12499-12513. [47] N. Voeltzel, P. Vergne, N. Fillot, N. Bouscharain, L. Joly, Rheology of an Ionic Liquid with Variable Carreau Exponent: A Full Picture by Molecular Simulation with Experimental Contribution, Tribol. Lett. 64 (2016) 25. [48] B.J. Hamrock, D. Dowson, Isothermal Elastohydrodynamic Lubrication of Point Contacts: Part III—Fully Flooded Results, J. Lubr. Technol. 99 (1977) 264-275. [49] S. Bair, Y. Liu, Q.J. Wang, The Pressure-Viscosity Coefficient for Newtonian EHL Film Thickness With General Piezoviscous Response, J. Tribol. 128 (2006) 624- 631. 22 [50] H.A. Spikes, P. Cann, G. Caporiccio, Elastohydrodynamic film thickness measurements of perfluoropolyether fluids, J. Syn. Lub. 1 (1984) 73-86. [51] H. Arora, P.M. Cann, Lubricant film formation properties of alkyl imidazolium tetrafluoroborate and hexafluorophosphate ionic liquids, Tribol. Int. 43 (2010) 1908- 1916. [52] G. Xie, J. Luo, D. Guo, S. Liu, Nanoconfined ionic liquids under electric fields, Appl. Phys. Lett. 96 (2010) 043112. [53] L. Zhang, J. Pu, L. Wang, Q. Xue, Synergistic Effect of Hybrid Carbon Nanotube–Graphene Oxide as Nanoadditive Enhancing the Frictional Properties of Ionic Liquids in High Vacuum, ACS Appl. Mater. Inter. 7 (2015) 8592-8600. [54] M.J. Assael, S.K. Mylona, A Novel Vibrating-Wire Viscometer for High- Viscosity Liquids at Moderate Pressures, J. Chem. Eng. Data 58 (2013) 993-1000. [55] X. Paredes, A.S. Pensado, M.J.P. Comuñas, J. Fernández, Experimental dynamic viscosities of dipentaerythritol ester lubricants at high pressure, J. Chem. Eng. Data 55 (2010) 3216-3223. [56] Q.-G. Zhang, Y. Wei, S.-S. Sun, C. Wang, M. Yang, Q.-S. Liu, Y.-A. Gao, Study on Thermodynamic Properties of Ionic Liquid N-Butyl-3-methylpyridinium Bis(trifluoromethylsulfonyl)imide, J. Chem. Eng. Data 57 (2012) 2185-2190. [57] H. Tokuda, S. Tsuzuki, M.A.B.H. Susan, K. Hayamizu, M. Watanabe, How Ionic Are Room-Temperature Ionic Liquids? An Indicator of the Physicochemical Properties, J. Phys. Chem. B 110 (2006) 19593-19600. [58] T. Retsina, S.M. Richardson, W.A. Wakeham, The theory of a vibrating-rod densimeter, Appl. Sci. Res. 43 (1986) 127-158. [59] T. Retsina, S.M. Richardson, W.A. Wakeham, The theory of a vibrating-rod viscometer, Appl. Sci. Res. 43 (1987) 325-346. [60] M. Papadaki, An Absolute Technique for the Measurement of the Viscosity of Liquids PhD Thesis, Aristotle University, Thessaloniki, Greece, 1991. [61] F.M. Gaciño, T. Regueira, L. Lugo, M.J.P. Comuñas, J. Fernández, Influence of Molecular Structure on Densities and Viscosities of Several Ionic Liquids, J. Chem. Eng. Data 56 (2011) 4984-4999. [62] P. Daugé, A. Baylaucq, L. Marlin, C. Boned, Development of an Isobaric Transfer Viscometer Operating up to 140 MPa. Application to a Methane + Decane System, J. Chem. Eng. Data 46 (2001) 823-830. [63] M.J.P. Comuñas, A. Baylaucq, C. Boned, J. Fernández, Dynamic Viscosity for HFC-134a + Polyether Mixtures up to 373.15 K and 140 MPa at Low Polyether Concentration. Measurements and Modeling, Ind. Eng. Chem. Res. 43 (2004) 804-814. [64] M.A. Monsalvo, A. Baylaucq, S.E. Quiñones-Cisneros, C. Boned, Highpressure viscosity behavior of x 1,1,1,2-tetrafluoroethane (HFC-134a) + (1 − x) triethylene glycol dimethylether (TriEGDME) mixtures: Measurements and modeling, Fluid Phase Equilibr. 247 (2006) 70-79. [65] X. Canet, A. Baylaucq, C. Boned, High-Pressure (up to 140 MPa) Dynamic Viscosity of the Methane+Decane System, Int. J. Thermophys. 23 (2002) 1469-1486. [66] S.K. Mylona, M.J. Assael, M.J.P. Comuñas, X. Paredes, F.M. Gaciño, J. Fernández, J.P. Bazile, C. Boned, J.L. Daridon, G. Galliero, J. Pauly, K.R. Harris, Reference Correlations for the Density and Viscosity of Squalane from 273 to 473 K at Pressures to 200 MPa, J. Phys. Chem. Ref. Data 43 (2014) 013104. [67] M.J.P. Comuñas, X. Paredes, F.M. Gaciño, J. Fernández, J.-P. Bazile, C. Boned, J.-L. Daridon, G. Galliero, J. Pauly, K.R. Harris, Viscosity measurements for squalane at high pressures to 350MPa from T=(293.15 to 363.15) K, J. Chem. Thermodyn. 69 (2014) 201-208. 23 [68] R.D. Chirico, M. Frenkel, J.W. Magee, V. Diky, C.D. Muzny, A.F. Kazakov, K. Kroenlein, I. Abdulagatov, G.R. Hardin, W.E. Acree, J.F. Brenneke, P.L. Brown, P.T. Cummings, T.W. de Loos, D.G. Friend, A.R.H. Goodwin, L.D. Hansen, W.M. Haynes, N. Koga, A. Mandelis, K.N. Marsh, P.M. Mathias, C. McCabe, J.P. O’Connell, A. Pádua, V. Rives, C. Schick, J.P.M. Trusler, S. Vyazovkin, R.D. Weir, J. Wu, Improvement of Quality in Publication of Experimental Thermophysical Property Data: Challenges, Assessment Tools, Global Implementation, and Online Support, J. Chem. Eng. Data 58 (2013) 2699-2716. [69] M. Dakkach, F.M. Gaciño, S.K. Mylona, M.J.P. Comuñas, M.J. Assael, J. Fernández, High pressure densities of two nanostructured liquids based on the bis(trifluoromethylsulfonyl)imide anion from (278 to 398) K and up to 120 MPa, J. Chem. Thermodyn. 118 (2018) 67-76. [70] A.M. Pinto, H. Rodríguez, A. Arce, A. Soto, Combined physical and chemical absorption of carbon dioxide in a mixture of ionic liquids, J. Chem. Thermodyn. 77 (2014) 197-205. [71] E. Quijada-Maldonado, S. van der Boogaart, J.H. Lijbers, G.W. Meindersma, A.B. de Haan, Experimental densities, dynamic viscosities and surface tensions of the ionic liquids series 1-ethyl-3-methylimidazolium acetate and dicyanamide and their binary and ternary mixtures with water and ethanol at T = (298.15 to 343.15 K), J. Chem. Thermodyn. 51 (2012) 51-58. [72] H. Schmidt, M. Stephan, J. Safarov, I. Kul, J. Nocke, I.M. Abdulagatov, E. Hassel, Experimental study of the density and viscosity of 1-ethyl-3- methylimidazolium ethyl sulfate, J. Chem. Thermodyn. 47 (2012) 68-75. [73] A. Arce, E. Rodil, A. Soto, Volumetric and Viscosity Study for the Mixtures of 2-Ethoxy-2-methylpropane, Ethanol, and 1-Ethyl-3-methylimidazolium Ethyl Sulfate Ionic Liquid, J. Chem. Eng. Data 51 (2006) 1453-1457. [74] A. Fernández, J.n. Garca, J.S. Torrecilla, M. Oliet, F. Rodríguez, Volumetric, transport and surface properties of [bmim][MeSO4] and [emim][EtSO4] ionic liquids as a function of temperature, J. Chem. Eng. Data 53 (2008) 1518-1522. [75] A.P. Froba, H. Kremer, A. Leipertz, Density, refractive index, interfacial tension, and viscosity of ionic liquids [EMIM][EtSO4], [EMIM][NTf2], [EMIM][N(CN)2], and [OMA][NTf2] in dependence on temperature at atmospheric pressure., J. Phys. Chem. B 112 (2008) 12420-12430. [76] A.P. Froba, P. Wasserscheid, D. Gerhard, H. Kremer, A. Leipertz, Revealing the Influence of the Strength of Coulomb Interactions on the Viscosity and Interfacial Tension of Ionic Liquid Cosolvent Mixtures, J. Phys. Chem. B 111 (2007) 12817- 12822. [77] E. Gómez, B. González, N. Calvar, E. Tojo, Á. Domínguez, Physical Properties of Pure 1-Ethyl-3-methylimidazolium Ethylsulfate and Its Binary Mixtures with Ethanol and Water at Several Temperatures, J. Chem. Eng. Data 51 (2006) 2096-2102. [78] J. Jacquemin, P. Husson, A.A.H. Padua, V. Majer, Density and viscosity of several pure and water-saturated ionic liquids, Green Chem. 8 (2006) 172-180. [79] G. McHale, C. Hardacre, R. Ge, N. Doy, R.W.K. Allen, J.M. MacInnes, M.R. Bown, M.I. Newton, Density−Viscosity Product of Small-Volume Ionic Liquid Samples Using Quartz Crystal Impedance Analysis, Anal. Chem. 80 (2008) 5806-5811. [80] H. Rodríguez, J.F. Brennecke, Temperature and Composition Dependence of the Density and Viscosity of Binary Mixtures of Water + Ionic Liquid, J. Chem. Eng. Data 51 (2006) 2145-2155. [81] J.S. Torrecilla, J. Palomar, J. García, F. Rodríguez, Effect of Cationic and Anionic Chain Lengths on Volumetric, Transport, and Surface Properties of 1-Alkyl- 24 3-methylimidazolium Alkylsulfate Ionic Liquids at (298.15 and 313.15) K, J. Chem. Eng. Data 54 (2009) 1297-1301. [82] J.S. Torrecilla, T. Rafione, J. Garcia, F. Rodríguez, Effect of Relative Humidity of Air on Density, Apparent Molar Volume, Viscosity, Surface Tension, and Water Content of 1-Ethyl-3-methylimidazolium Ethylsulfate Ionic Liquid, J. Chem. Eng. Data 53 (2008) 923-928. [83] A. Bhattacharjee, P.J. Carvalho, J.A. Coutinho, The effect of the cation aromaticity upon the thermophysical properties of piperidinium-and pyridinium-based ionic liquids, Fluid Phase Equilibr. 375 (2014) 80-88. [84] B. Bittner, R.J. Wrobel, E. Milchert, Physical properties of pyridinium ionic liquids, J. Chem. Thermodyn. 55 (2012) 159-165. [85] R.G. Seoane, S. Corderí, E. Gómez, N. Calvar, E.J. González, E.A. Macedo, Á. Domínguez, Temperature Dependence and Structural Influence on the Thermophysical Properties of Eleven Commercial Ionic Liquids, Ind. Eng. Chem. Res. 51 (2012) 2492- 2504. [86] F.S. Oliveira, M.G. Freire, P.J. Carvalho, J.A. Coutinho, J.N.C. Lopes, L.s.P. Rebelo, I.M. Marrucho, Structural and Positional Isomerism Influence in the Physical Properties of Pyridinium NTf2-Based Ionic Liquids: Pure and Water-Saturated Mixtures†, J. Chem. Eng. Data 55 (2010) 4514-4520. [87] J.M. Crosthwaite, M.J. Muldoon, J.K. Dixon, J.L. Anderson, J.F. Brennecke, Phase transition and decomposition temperatures, heat capacities and viscosities of pyridinium ionic liquids, J. Chem. Thermodyn. 37 (2005) 559-568. [88] A.S.M.C. Rodrigues, H.F.D. Almeida, M.G. Freire, J.A. Lopes-da-Silva, J.A.P. Coutinho, L.M.N.B.F. Santos, The effect of n vs. iso isomerization on the thermophysical properties of aromatic and non-aromatic ionic liquids, Fluid Phase Equilibr. 423 (2016) 190-202. [89] A. Bhattacharjee, A. Luís, J.H. Santos, J.A. Lopes-da-Silva, M.G. Freire, P.J. Carvalho, J.A. Coutinho, Thermophysical properties of sulfonium-and ammoniumbased ionic liquids, Fluid Phase Equilibr. 381 (2014) 36-45. [90] H. Liu, E. Maginn, A.E. Visser, N.J. Bridges, E.B. Fox, Thermal and Transport Properties of Six Ionic Liquids: An Experimental and Molecular Dynamics Study, Ind. Eng. Chem. Res. 51 (2012) 7242-7254. [91] Y. Deng, P. Husson, A.-M. Delort, P. Besse-Hoggan, M. Sancelme, M.F. Costa Gomes, Influence of an oxygen functionalization on the physicochemical properties of ionic liquids: density, viscosity, and carbon dioxide solubility as a function of temperature, J. Chem. Eng. Data 56 (2011) 4194-4202. [92] U. Domańska, P. Papis, J. Szydłowski, M. Królikowska, M. Królikowski, Excess Enthalpies of Mixing, Effect of Temperature and Composition on the Density, and Viscosity and Thermodynamic Properties of Binary Systems of {Ammonium- Based Ionic Liquid + Alkanediol}, J. Phys. Chem. B 118 (2014) 12692-12705. [93] J.M. Andanson, X. Meng, M. Traïkia, P. Husson, Quantification of the impact of water as an impurity on standard physico-chemical properties of ionic liquids, J. Chem. Thermodyn. 94 (2016) 169-176. [94] M.J.P. Comuñas, A. Baylaucq, C. Boned, J. Fernández, High-Pressure Measurements of the Viscosity and Density of Two Polyethers and Two Dialkyl Carbonates, Int. J. Thermophys. 22 (2001) 749-768. [95] K.A.G. Schmidt, D. Pagnutti, M.D. Curran, A. Singh, J.P.M. Trusler, G.C. Maitland, M. McBride-Wright, New Experimental Data and Reference Models for the Viscosity and Density of Squalane, J. Chem. Eng. Data 60 (2015) 137-150. 25 [96] N. Anwar, Riyazuddeen, Effect of composition and temperature variations on thermophysical properties of binary and ternary mixtures of 1-ethyl-3- methylimidazolium ethylsulfate with 1-butanol and/or methanol, Fluid Phase Equilibr. 437 (2017) 127-139. [97] M. Larriba, S. García, J. García, J.S. Torrecilla, F. Rodríguez, Thermophysical properties of 1-ethyl-3-methylimidazolium 1, 1, 2, 2-tetrafluoroethanesulfonate and 1- ethyl-3-methylimidazolium ethylsulfate ionic liquids as a function of temperature, J. Chem. Eng. Data 56 (2011) 3589-3597. 32 Table 6 Viscosity values,  a, in mPa·s, measured with VisLPT1 at different temperatures, Tb, and pressures, pc. p/MPa T/K 303.15 313.15 323.15 333.15 343.15 353.15 [C4C1Py][NTf2] 10 58.6 40.1 27.2 20.0 14.8 11.5 15 62.3 42.3 28.7 20.8 15.7 12.1 25 70.4 47.2 32.0 22.8 17.4 13.2 50 94.7 61.7 41.6 29.2 21.7 16.4 75 126.3 80.5 53.5 37.2 26.7 20.2 100 104.9 68.5 46.3 33.2 24.8 [N4,1,1,1][NTf2] 10 93.4 59.7 40.1 28.2 21.0 15.6 15 99.9 63.7 42.6 29.8 22.0 16.5 25 114.0 72.4 47.9 33.3 24.3 18.3 50 158.2 98.5 63.5 43.4 31.0 23.4 75 218.5 132.2 83.6 56.1 39.5 29.4 100 300.8 175.9 109.1 71.9 50.3 36.6 125 413.1 232.5 142.0 91.8 63.8 45.3 150 566.6 306.0 183.4 116.8 81.0 55.7 a Relative expanded viscosity uncertainty (k = 2, level of confidence=0.95): 3.5 %. b Expanded temperature uncertainty (k = 2, level of confidence=0.95): 0.1 K. c Expanded pressure uncertainty (k = 2, level of confidence=0.95): 0.2 MPa. 33 Table 7 Parameters of the correlation Eq. (8) for both ILs. [C4C1Py][NTf2] [N4,1,1,1][NTf2] E0 / MPa 5217.8 -4520.9 E1 / MPa·K-1 -33.26 25.401 102·E2 / MPa·K-2 6.167 -1.916 D 9.8892 19.27 AAD/% 0.66 0.73 Bias/% 0.05 0.13 MaxD/% 2.99 3.02 34 Table 8 Pressure-viscosity coefficients,  (p)/GPa-1, and temperature-viscosity coefficients,  (T)/K-1, for the two ILs studied with VisLPT1. p/MPa T/K 303.15 313.15 323.15 333.15 343.15 353.15 [C4C1Py][NTf2]  (p)/GPa-1 10 12.2 11.5 10.8 10.0 9.2 8.4 50 11.6 11.0 10.3 9.6 8.9 8.2 100 11.0 10.4 9.8 9.1 8.5 7.8 150 10.4 9.9 9.3 8.7 8.1 7.5  (T)/K-1 10 44.1 38.4 33.7 29.8 26.6 23.8 50 46.5 41.1 36.6 32.8 29.5 26.7 100 49.1 44.2 40.0 36.3 33.0 30.0 150 51.5 47.0 43.1 39.5 36.1 33.0 [N4,1,1,1][NTf2]  (p)/GPa-1 10 13.5 12.3 11.4 10.6 9.9 9.3 50 13.1 12.0 11.1 10.3 9.7 9.1 100 12.7 11.6 10.8 10.1 9.4 8.9 150 12.3 11.3 10.5 9.8 9.2 8.7  (T)/K-1 10 49.1 42.2 36.6 32.0 28.3 25.2 50 54.2 46.3 40.0 34.9 30.7 27.3 100 60.1 51.1 44.0 38.3 33.6 29.8 150 65.7 55.7 47.9 41.6 36.4 32.2 35 Table 9 Universal pressure-viscosity coefficients, reciprocal asymptotic isoviscous pressure and product 56.069.0 0film  for the two ILs studied with VisLPT1. T/K  film / GPa-1  GPa-1 56.069.0 0film  [N4,1,1,1][NTf2] 303.15 13.0 12.9 88.3 313.15 11.9 11.8 62.0 323.15 11.0 10.8 45.2 333.15 10.2 10.1 34.4 343.15 9.5 9.4 27.1 353.15 9.0 8.9 21.9 [C4C1Py][NTf2] 303.15 11.4 11.1 59.9 313.15 10.7 10.5 44.0 323.15 10.0 9.8 33.0 333.15 9.3 9.1 25.6 343.15 8.6 8.4 20.2 353.15 7.9 7.7 16.4 36 Figure 1.Viscosities  measured with the vibrating wire (VW) and Stabinger viscometers for the three ILs: [C2C1Im][C2SO4], (() VW; () Stabinger); [C4C1Py][NTf2], (() VW, () Stabinger); [N4,1,1,1][NTf2], (() VW, () Stabinger). The solid lines represent the values obtained using Eq. (7). 37 Figure 2. Relative deviation 100∙(  𝑒𝑥𝑝 −  𝑓𝑖𝑡)/  𝑓𝑖𝑡 between viscosity values obtained with the proposed VFT correlation Eq. (7) for [C2C1Im][C2SO4]. () Present work with VW; ( ) Present work with Stabinger; () Diogo et al. (VW) [26]; (—) Diogo et al. (C) [26]; () Pinto et al. [70]; (–) Diogo et al.[30]; () Pereiro et al. [34]; () Quijada et al. [71]; ( ) Schmidt et al. [72]; () Gaciño et al. [61]; () Larriba et al.[97]; (–) Torrecilla et al.[81]; () Fernández et al. [74]; () Froba et al. [75]; () McHale et al.[79];() Froba et al. (SLS) [76]; () Froba et al. (RV) [76];() Arce et al. [73]; () Gomez et al.[77]; () Jacquemin et al. [78]; () Rodríguez and Brennecke [80]. 38 Figure 3. Relative deviation 100∙(  𝑒𝑥𝑝 −  𝑓𝑖𝑡)/  𝑓𝑖𝑡 between viscosity values obtained with the proposed VFT correlation Eq. (7) for [C4C1Py][NTf2].() Present work with VW; () Present work with Stabinger; () Bhattacharjee et al. [83]; () Bittner et al.[84]; () Seoane et al. [85]; () Zhang et al. [56]; () Oliveira et al. [86]; () Crosthwaite et al. [87], () Rodrigues et al. [88]. 39 Figure 4. Relative deviation 100∙(  𝑒𝑥𝑝 −  𝑓𝑖𝑡)/  𝑓𝑖𝑡 between viscosity values obtained with the proposed VFT correlation Eq. (7) for [N4,1,1,1][NTf2]. () Present work with VW; () Present work with Stabinger; () Bhattacharjee et al. [89]; () Liu et al. [90]; () Deng et al. [91]; () Jacquemin et al. [78]; () Tokuda et al. [57], () Domanska et al. [92], () Andanson et al. [93]. 40 Figure 5. Viscosities  at high pressure for [C4C1Py][(NTf2], a), and [N4,1,1,1][NTf2], b), at 303.15 K (), 313.15 K (); 323.15 K (); 333.15 K (); 343.15 K () and 353.15 K (). The solid lines represent the values obtained using Eq. (8). 0 20 40 60 80 100 120 140 020 40 60 80 100 120  / mPa.s p/ MPa a) 0 100 200 300 400 500 600 0 20 40 60 80 100 120 140 160  / mPa.s p/ MPa b) 41 Figure 6. Local pressure–viscosity coefficient  as a function of pressure for [C4C1Py][(NTf2] (empty symbols) and [N4,1,1,1][NTf2] (filled symbols). () T = 303.15 K, () T = 323.15 K and () T = 353.15 K.