Viscosity of Imidazolium-Based Ionic Liquids at Elevated Pressures: Cation and Anion Effects

Abstract

The viscosity of imidazolium-based ionic liquids with four different cations and three different anions was measured to pressures of 126 MPa and at three temperatures (298.15 K, 323.15 K, and 343.15 K). The high-pressure viscosity of 1-ethyl-3-methylimidazolium ([EMIm]), 1-n-hexyl-3-methylimidazolium ([HMIm]), and 1-n-decyl-3-methylimidazolium ([DMIm]) cations with a common anion, bis(trifluoromethylsulfonyl)imide ([Tf 2 N]), was measured to determine the alkylchain length effect of the cation. An increase in the alkyl-chain length increased the viscosity at elevated pressures. [DMIm] exhibited a larger nonlinear increase with pressure over the shorter alkyl substituents. Anion effects were investigated with [HMIm] as a common cation and anions of [Tf 2 N], hexafluorophosphate ([PF 6 ]), and tetrafluoroborate ([BF 4 ]). [HMIm][PF 6 ], with the highest viscosity, demonstrated a very nonlinear pressure dependence even at relatively moderate pressures (to 30 MPa), similar to the results for [BMIm][PF 6 ]. A combined Litovitz and Tait equation was utilized to describe the viscosity of the ionic liquids with pressure and temperature and demonstrated good correlation with the experimental data.

Thermodynamic and thermophysical properties of the reference ionic liquid: 1-Hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide (including mixtures). Part 2. Critical evaluation and recommended property values (IUPAC Technical Report)

“…The combined expanded uncertainty (k = 2) given by the authors was 2 %. Ahosseini and Scurto [36] also reported viscosities at high pressures (to 124 MPa) for a non-IUPAC sample with an estimated uncertainty (undefined) near 3 %. These values are consistent with the recommended values near 0.1 MPa given in this paper and with the high-pressure results of Kandil et al [22].…”

Viscosity Measurements on Ionic Liquids: A Cautionary Tale

“…The table shows that a wide variety of different techniques have been used and also that the purity of the sample, particularly with respect to water content, receives variable attention. If the results of the complete set of early, non-IUPAC, measurements [13][14][15][16][17][18], indicated in Table 1, on the same liquid are examined, as is done in Fig. 1, discrepancies from a later correlation approved by IUPAC are very large (and in both directions), reaching as much as −16 %, despite claims of uncertainties of 2 % or 3 % for the results of individual authors.…”

“…Tables 1 and 2 give a list of the viscosity measurements conducted on one particular liquid, [C 6 mim][NTf 2 ], chosen for reasons that will be explained later. The list includes several sets of early measurements [13][14][15][16][17][18], and the results of a round-robin IUPAC study [8,9,[19][20][21]. In Ref.…”

Viscosity and Thermal Conductivity of 1-Hexyl-3-methylimidazolium Tetrafluoroborate and 1-Octyl-3-methylimidazolium Tetrafluoroborate at Pressures up to 20 MPa

“…However, to design ILs with appropriate thermophysical properties, knowledge of fundamental thermophysical properties such as the viscosity and thermal conductivity is required. The viscosities of ILs at atmospheric pressure have been measured in many studies [1], and a few groups have measured the viscosities of ILs at high pressure [2][3][4][5][6][7][8][9]. However, reports on the thermal conductivity of ILs are scarce [10][11][12][13][14][15][16][17].…”