Density and viscosity of several pure and water-saturated ionic liquids

TLDR: In this paper, the authors measured the viscosity and density of six ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate, 1-Butyl 3methylammonium bis(trifluoromethylsulfonyl)imide, butyltrimethylammmonium, ethyl sulfate and butyl trifluoric acid) with an accuracy of 10−3 g cm−3 and 1%, respectively.

About: This article is published in Green Chemistry.The article was published on 2006-02-03 and is currently open access. It has received 762 citations till now. The article focuses on the topics: Ionic liquid & Viscosity.

Figures

Table 4 Calculated molar volumes (Vm/cm 3 mol21) and the coefficients of the thermal expansion (ap/cm 3 mol21) of dried and water-saturated ILs at 293.15 K and 343.15 K

Table 5 Experimental viscosities (g) of dried and water saturated ILs as a function of temperature at atmospheric pressure

Fig. 2 Experimental viscosities of the dried ILs as a function of temperature: ($), Bmim+PF62; (#), Bmim+NTf22; (&), Bmim+BF42; (%), N4111+NTf22; (m), Emim+NTf22; (n), Emim+EtSO42. The lines correspond to the fit of the data by eqn (5).

Table 6 Correlation parameters of the Arrhenius equation (g3, Ea), and of the VFT equation (k, A and T0) with the deviations of the fit sr for the viscosity of dried and water saturated ILs as a function of temperature determined from measurements between 293 K and 388 K

Fig. 3 Relative deviations (100(rlit 2 rfit)/rfit) of the literature densities for Bmim+NTf2 2 from our fitted data for the dried sample: ($), Bonhote et al.;16 (#), Huddleston et al.;17 (e), Krummen et al.;28 (.), Dzyuba et al.;15 (r), Fredlake et al.;12 (+), Tokuda et al.;11 (&), this work—water saturated IL; (%), this work—dried IL.

Fig. 4 Relative deviations (100(rlit 2 rfit)/rfit) of the literature densities for Bmim+PF6 2 from our fitted data for the dried sample: (e), Suarez et al.;21 (#), Huddleston et al.;17 (m), Hyun et al.;29 (n), Gu et al.;14 (–), Blanchard et al.;30 (|), Seddon et al.;31 (,), Seddon et al.;10 (.), Dzyuba et al.;15 ($), Harris et al.;32 (+), Tokuda et al.;11 (&), this work—water saturated IL; (%), this work—dried IL.

Frequently asked questions

Q1. What is the way to measure the viscosity of organic salts?

The great interest of these organic salts, composed of bulky ions, is their negligible vapour pressure, low melting point and good thermal stability which make them liquid over a large temperature range (typically 300 K) including ambient temperature. 

Q2. What is the reason for the decrease in viscosity of ionic domains?

The formation of the microstructures is likely to be responsible for the increase in viscosity and for the decrease in ion mobility (meaning a decrease in conductivity and in diffusion). 

Q3. What is the effect of water on the viscosity of the samples?

While the evolution of the volumetric properties of ILs with the presence of water is almost negligible (1–2%), it strongly decreases the viscosity of the samples. 

Q4. What is the common equation to correlate the variation of viscosity with temperature?

The most commonly used equation to correlate the variationof viscosity with temperature is the Arrhenius-like law:g = g‘exp(2Ea/RT) (4)This journal is The Royal Society of Chemistry 2006 Green Chem., 2006, 8, 172–180 | 177Viscosity at infinite temperature (g‘) and the activation energy (Ea) are characteristic parameters generally adjusted from experimental data. 

Q5. What is the importance of the identification of impurities in the samples?

The identification of the impurities in the samples is of importance as their presence has a strong impact on the physico–chemical properties of ILs.3 

Q6. What is the reason for the increase in the van der Waals interactions?

The justification put forward by Bonhôte et al.16 is that it is the increase in the van der Waals interactions due to the presence of a long alkyl chain that leads to higher viscosities. 

Q7. What is the ap of the ILs with longer alkyl chains?

They reported a Newtonian behaviour for ILs of alkylimidazolium BF4 2 family (with the alkyl chain length between 4 and 8 carbon atoms) while the ILs with longer alkyl chains (number of carbon atoms typically 12) are thixotropic fluids whose viscosity decreases when increasing the shear rate. 

Q8. What is the main reason for the use of ILs as stationary phases?

The dual nature of the interactions (nonpolar-dispersive and ionic domains) has also been reported connected to the use of ILs as stationary phases for gas chromatography. 

Q9. What is the main difference between ILs and density?

The influence of temperature on viscosity is much more important than on density: a strong decrease is observed with increasing temperature10,15,17,18 making ILs easier to apply at superambient conditions. 

Q10. What is the effect of water on the viscosity of the dried Bmim+?

Thus it is obvious that as the viscosity decreases with temperature the effect of water is much less important, as documented in Table 5. 

Q11. What is the importance of preparing ILs of well-defined purity?

In this context, the preparation of ILs of well-defined purity is absolutely necessary and the development of analytical techniques to quantify major impurities in samples appears to be crucial.