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Acoustics as a Tool for Better Characterization of Ionic Liquids: A Comparative Study of 1‑Alkyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]imide Room-Temperature Ionic Liquids Edward Zorębski, Monika Geppert-Rybczyńska,* and Michał Zorębski Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, Poland S Supporting Information *

ABSTRACT: Acoustic properties of three (1-ethyl-, 1-butyl-, and 1-octyl-) 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide room-temperature ionic liquids are reported and discussed. The speeds of sound in RTILs were measured as a function of temperature in the range 288−323 K by means of a sing around method. The densities and isobaric heat capacities were determined from 288.15 to 363.15 K and from 293.15 to 323.15 K, respectively. The related properties, like isentropic and isothermal compressibilities, isobaric coefficients of thermal expansion, molar isochoric heat capacities, and internal pressures, were calculated. It was found that for some ionic liquids, temperature dependence of isobaric coefficients of thermal expansion is small and negative. All investigations were completed by the ultrasound absorption coefficient measurements in 1-ethyl- and 1-octyl-3methylimidazolium bis[(trifluoromethyl)sulfonyl] imide as a function of frequency from 10 to 300 MHz at temperatures 293.15−298.15 K. The ultrasound absorption spectra indicate relaxation frequencies in the megahertz range.

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INTRODUCTION Room-temperature ionic liquids (RTILs) are substances that have low melting points in comparison with the conventional molten salts. They are usually composed of a large unsymmetrical organic cation and an organic or an inorganic anion. Thermophysical properties of different families of RTILs are widely tested to find the new potential applications in chemical and industrial fields.1−9 However, very little effort was made in the systematic study of acoustic properties, like speed of sound, c, and acoustic absorption coefficient, α. Although the later technique gives valuable information on liquid structure, molecular dynamic, and the kinetics of molecular processes in liquids, only very few experimental results in RTILs have been reported so far.10−12 For our comparative study, speed of sound in the temperature range 288−323 K and ultrasound absorption in the frequency range 10−300 MHz at temperatures of 293.15 and 298.15 K, we chose three methylimidazolium-based ionic liquids with large anion, that is, 1-ethyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl]imide ([EMIm][NTf2]), 1-butyl3-methylimidazolium bis[(trifluoromethyl) sulfonyl]imide ([BMIm][NTf2]), and 1-octyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl]imide ([OMIm][NTf2]). They belong to the class of hydrophobic liquids of high thermal stability that differ in alkyl chain length in imidazolium cation (the structures of the cations and anion are shown in Figure 1). Because we have not found any reports on systematic ultrasound absorption investigations in these liquids, the necessary experiment was carried out for two of them, [EMIm][NTf2] and [OMIm][NTf2]. Despite that the speed © 2013 American Chemical Society

Figure 1. Structures of cations and anion methylimidazolium-based ionic liquids investigated in this work: (a) 1-ethyl-3-methylimidazolium; (b) 1-butyl-3-methylimidazolium; (c) 1-octyl-3-methylimidazolium; and (d) bis[(trifluoromethyl)sulfonyl] imide.

of sound results for RTILs have been already reported,11,13,14 we decided to determine c to obtain the set of consistent data by application of the same experimental technique. Density, ρ, and isobaric heat capacity measurements, Cp, are measured to complete the acoustic investigations.

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EXPERIMENTAL SECTION The provenance and purity of [EMIm][NTf2], [BMIm][NTf2], and [OMIm][NTf2] are given in Table 1. From volumetric Karl Fischer method, we found that the water content was at the Received: January 20, 2013 Revised: March 19, 2013 Published: March 19, 2013 3867

dx.doi.org/10.1021/jp400662w | J. Phys. Chem. B 2013, 117, 3867−3876

The Journal of Physical Chemistry B

Article

Table 1. Sample Table liquid

CAS

supplier

mass fraction purity

water content (ppm)

halides (ppm)

molar mass

[EMIm][NTf2]a [BMIm][NTf2]b [OMIm][NTf2]c

174899-82-2 174899-83-3 178631-04-4

IoLiTec IoLiTec IoLiTec

>0.99 >0.99 >0.99

50 70 40

[BMIm][NTf2] > [OMIm][NTf2] and with the temperature, so the negative temperature coefficient (∂Pint/∂T)p at atmospheric pressure is observed. The decrease of Pint with increasing temperature at atmospheric pressure is typical for most organic liquids (with the exception of substances with a spatial network of H-bonds49−51). The decrease of Pint from [EMIm][NTf2] to [OMIm][NTf2] implies a decrease of intermolecular cohesion caused by

f/MHz

T = 293.15 K

T = 298.15 K

10.002 12.505 15.005 18.005 22.119 26.584 60 79 100 150 200 250 300

2166 2144 2131 2079 2039 1976 1658 1540 1370 1146 960.8 863.8 789.4

1762 1737 1720 1706 1685 1635 1424 1336 1244 1057 906.7 804.9 722.9

seen from these tables (and Figure 7a,b as well), the dependence of the quotient α·f−2 on frequency changes with temperature. It appears also that for [EMIm][NTf2] and [OMIm][NTf2] within the investigated frequency range the quotient α·f−2 is dependent on f. However, the [OMIm][NTf2] shows strong dependence on frequency as early as above 10 MHz, whereas for [EMIm][NTf2], α·f−2 reveals frequency dependence above 100 MHz. Thus, in both RTILs the dispersion characteristic d(α·f−2)/df < 0 is observed. Moreover, [EMIm][NTf2] can be characterized as a middle absorbing substance, whereas [OMIm][NTf2] is rather highly absorbing; that is, the frequency normalized attenuation (at T = 298.15 K and f = 100 MHz) is 470 × 10−15 and 1244 × 10−15 s2 m−1, respectively. It is generally known that the attenuation of ultrasound wave in liquids is usually composed of two contributions according to the relation: 3872



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Table 8. Coefficients of the Relaxation Equation for the Liquids Studied Together with Correlation Coefficients R 1015·A/m−1 s2

T/K 293.15 298.15

205 ± 22 137 ± 17

293.15 298.15

1440 ± 35 1099 ± 35

298.15

112.7 ± 8.6

1015·B/m−1 s2

τ/ns

R

± 27 ± 33

0.56 0.52

0.997 0.997

± 4.6 ± 6.5

1.74 1.47

0.998 0.997

± 52

0.45

0.997

f/MHz

[EMIm][NTf2] 361 ± 22 282 346 ± 17 309 [OMIm][NTf2] 702 ± 35 91.4 635 ± 35 108.2 1-Octanol 46.7 ± 8.5 358

Table 9. Classical Ultrasound Absorption αcl·f−2 According to the Stokes Formula, the Ratios α/αcl of the Observed to the Classical Absorption in the Nondispersion Region,a and the Temperature Coefficients of Absorption α−1·dα/dT in the Nondispersion Regiona for the Liquids Studied T/K 293.15 298.15 293.15 298.15 293.15 298.15 a

Figure 7. (a) The values of the ultrasound absorption coefficient per squared frequency α·f−2 plotted against f for [EMIm][NTf2] at temperatures: ○, 293.15 K and ■, 298.15 K; solid lines were determined according to eq 8; dotted line, αcl for 293.15 K; dashed line, αcl for 298.15 K (from eq 9). (b) The values of the ultrasound absorption coefficient per squared frequency α·f−2 plotted against f for [OMIm][NTf2]; ○, 293.15 K and ■, 298.15 K; solid lines were determined according to eq 8; dotted line, αcl for 293.15 K; dashed line, αcl for 298.15 K (from eq 9).

α ·f −2 = (α ·f −2 )cl + (α ·f −2 )ex

(7)

(8)

where A and f rel are the relaxation amplitudes and relaxation frequency, respectively. B represents the sum of the classical part of absorption and contributions from processes with relaxation frequencies considerably higher than f rel. The values of the parameters A, B, and f rel estimated by the least-squares method are collected in Table 8 where relaxation times τrel (τrel = (2·π·f rel)−1) are also given. According to the Navier−Stokes formula, a classical absorption coefficient αcl can be calculated from the formula: αcl = 8·π 2·η ·(3·ρ ·c 3)−1 ·f 2

1015·αcl·f−2/m−1 s2 344.1 294.7 α/αcl 1.655 1.639 102·α−1·Δα/ΔT −3.03 −3.57

[OMIm][NTf2] 1180.3 980.9 1.835 1.796 −3.73 −4.59

At frequencies below the relaxation region.

respectively), the determined values of α·f−2 are smaller than those predicted by the Stokes relation (eq 9). In other words, the absorption curves shown in Figure 7b indicate that α > αcl at lower frequencies and α < αcl at higher frequencies, and that the frequency for which α = αcl decreases with the increasing temperature. This kind of behavior would result from a relaxation mechanism of the viscous type. As is known, in the Stokes relation the static shear viscosity values are used; that is, the shear viscosities with shear rate ≈ 0. Thus, a shear viscosity relaxation in the megahertz range (above 150 MHz) can be supposed, and the dispersion of α·f−2 can also be attributed to a relaxation behavior of shear viscosity. From the results shown in Figure 7b, it is obvious that ηS must decrease with frequency. Similar behavior has been reported previously for 1dodecanol53,54 and castor oil,55 for instance. The small ratios (Table 9) of the observed absorption to that calculated from the Stokes rule (i.e., classical absorption) and the negative temperature coefficients of absorption in the nondispersion region are characteristic for structural relaxation. Generally, it seems that the differences in ultrasound absorption spectra of [EMIm][NTf2] and [OMIm][NTf2] are caused mainly by the increase of conformational contribution due to an elongation of carbon chain in the cation. At the same time, the negative charge of anion is delocalized on nitrogen and oxygen atoms, which suppose several types of cation···anion coordination. Also, the conformational flexibility of the anion cannot be completely ignored, because it was shown in ref 56 that [NTf2]− anion forms different conformations in crystalline phases of [EMIm][NTf2]. [NTf2]− anion conformers (cis and trans) could be identified by IR or Raman spectroscopy in the liquid phase as well.57,58 It appears that the trans conformer is somewhat more favorable energetically than

that is, the classical part (denoted by subscript “cl”) and the excessive part (denoted by subscript “ex”). To represent the ultrasonic absorption data, we applied the function: α·f −2 = A ·(1 + (f /frel )2 )−1 + B

[EMIm][NTf2]

(9)

−2

The obtained values of αcl·f are summarized in Table 9. For the calculations, the literature values of shear viscosity were used.22,24,52 For obvious reasons, we prefer the values obtained by means of capillary method. For [OMIm][NTf2], very interesting is that above some frequency (ca. 150 and 180 MHz at 298.15 and 293.15 K, 3873



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CONCLUSIONS Results obtained during our work for ionic liquids tested show that αp, κS, κT, Cp, and CV increase from [EMIm][NTf2] to [OMIm][NTf2], whereas Pint changes in opposite order. The temperature coefficient of thermal expansibilities is negative or positive, but it is decidedly much smaller than those of many classes of organic compounds. From one side, the change of the cation (like lengthening the alkyl chain) does change the speed of sound significantly. On the other side, the ultrasound absorption spectra over the whole experimental conditions differ significantly, and variation of alkyl chain length in imidazolium cation affects the spectra to a large extent (both relaxation amplitude and relaxation time). For a more detailed analysis of this matter, more experiments with different RTILs are required, which has been recently started.

the cis one, whereas the dipole moment of the cis conformer is significantly larger than those of the trans. It seems that the comparison of the ultrasonic attenuation data obtained for both RTIls studied in this work with those of 1-octanol and ethanol can give some new conclusions to the discussion presented above. The ultrasound absorption has been measured for ethanol (POCh, mass fraction purity 0.998) and 1-octanol (Aldrich, mass fraction purity 0.99) at T = 298.15 K additionally. The results are presented in Table S4 in the Supporting Information. It appears that in in the investigated frequency range 10−300 MHz, α·f−2 for ethanol is independent of frequency (the mean value 49.2 × 10−15 s2 m−1), whereas for 1octanol, α·f−2 depends on frequency only above 100 MHz (Figure 8). Relaxation frequency for 1-octanol calculated

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ASSOCIATED CONTENT

S Supporting Information *

Coefficients ai for the temperature dependencies of the densities ρ, speeds of sound c, and molar isobaric heat capacities Cp, isentropic and isothermal compressibility coefficients, κS and κT, molar isochoric heat capacities CV, Cp/ Cv ratio and internal pressures Pint at various temperatures for liquids studied, isobaric coefficients of thermal expansibility αp at various temperatures, and ultrasound absorption coefficients per squared frequency α·f−2 at various frequencies f for ethanol and 1-octanol at 298.15 K (Tables S1−S4). This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

Figure 8. The values of the ultrasound absorption coefficient per squared frequency α·f−2 plotted against f for at 298.15 K for ethanol (○), and 1-octanol (■); solid lines: determined according to eq 8.

*Tel.: (+48 32) 3 591 684. Fax: (+48 32) 2 599 978. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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according to eq 8 is 358 ± 52 MHz. These results are in agreement with the literature reports where a broadband ultrasonic study for 1-octanol shows two relaxation frequencies at T = 298.15 K, 420 MHz ± 10%, and 1.7 GHz ± 20%.54 Next, for ethanol, no relaxation process has been found in similar conditions.59 Thus, as the results obtained show, for alcohols, as well as for ionic liquids studied, the presence of long alkyl chain length (in the 1-alkanol molecule or in the imidazolium cation) can make the relaxation regions broader and cause its shift toward lower frequencies. To sum for alkanols, the lower relaxation region is assigned to fluctuations in the structure of hydrogen-bonded alcohol clusters, whereas only the higher relaxation region is connected with intramolecular conformational isomerism.54 One may say that for ionic liquids the interpretation of absorption spectra is more complicated, but it can arise from the same background as for alcohols. In our opinion, the good start-point can be taking into account some analysis based on molecular simulation supported in some extent to the results of neutron diffraction analysis.60 According to this, pure 1-alkyl-3-methylimidazolium ionic liquids reveal structuring of their liquid phases that is comparable to microphase separation between polar and nonpolar domains. Both “phases” have a kind of structure; thus it is not out of the question that the creation of nonpolar domains arranged as a dispersed phase for longer side-chains RTILs is more obvious than for shorter-chain ones.

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