Structure–Property Relationships in Ionic Liquids: A Study of the

Article pubs.acs.org/IECR

Structure−Property Relationships in Ionic Liquids: A Study of the Influence of N(1) Ether and C(2) Methyl Substituents on the Vaporization Enthalpies of Imidazolium-Based Ionic Liquids Dzmitry H. Zaitsau, Andrei V. Yermalayeu, and Sergey P. Verevkin* Department of Physical Chemistry and Faculty of Interdisciplinary Research, Department “Life, Light and Matter”, University of Rostock, Dr-Lorenz-Weg 1, 18059 Rostock, Germany

Jason E. Bara* and Alexander D. Stanton Department of Chemical & Biological Engineering, University of Alabama, Tuscaloosa, Alabama 35487-0203, United States S Supporting Information *

ABSTRACT: In this work, the QCM and TGA methods were used concurrently to study the two alkoxy-substituted ionic liquid (IL) series: 1-[oligo(ethylene glycol)]-3-methylimidazolium bis(triflamide) ([Pxmim][NTf2]) and 1-[oligo(ethylene glycol)]2,3-dimethylimidazolium bis(triflamide) ([Pxmmim][NTf2]). For comparison, enthalpies of vaporization measured at elevated temperatures were adjusted to the reference temperature 298 K and tested for consistency. It was found that the vaporization enthalpies of the alkoxy-substituted ILs are significantly lower than those of the analogous ILs with the alkyl-substituted cation. This is in contrast to molecular solvents, for which alkoxy groups are typically observed to increase vaporization enthalpy relative to those of the hydrocarbon analogues. Two useful group contributions for the quick estimation of vaporization enthalpies of various alkoxy-substituted IL cations (e.g., imidazolium, ammonium, pyridinium) are recommended based on the findings of this work.

1. INTRODUCTION Although ether-functionalized ionic liquids (ILs) have been reported in the literature for some time,1−4 they have been much less studied than their alkyl-functionalized counterparts.4 As such, far fewer thermophysical property data and application results have been published. Most commonly, oligo(ethylene glycol) or poly(ethylene glycol) [“PEG”, i.e., −(CH2CH2O)x−] groups have been employed,4−6 although other more complicated ether groups have also been utilized.4,7 Oligo(ethylene glycol) groups attached to imidazolium cations have been shown to alter the physical properties4,8−10 of ILs compared to those of the alkyl-functionalized analogues. Specifically, it has been noted that ether functionalities tend to lower IL viscosity and melting point, although perhaps at the expense of thermal stability.4 Furthermore, differences in molecular-level interactions caused by the presence of ethers have been attributed to H-bonding between the Lewis basic ether groups and the acidic proton at the C(2) position of the imidazolium ring.11−16 Figure 1 depicts the general structures of 1-R-3-methylimidazolium bis(triflamide) ILs, denoted [Pxmim][NTf2] and [Cnmim][NTf2]. Bara and co-workers first noted that, because polar solvents such as MeOH and dimethyl ethers of poly(ethylene glycol) are more effective than hydrocarbons at removing CO2 from industrial gas streams,17,18 [Pxmim][NTf2] ILs would also provide improved separation selectivity for CO2/N2 and CO2/ CH4 mixtures compared to their [Cnmim][NTf2] analogues.8 This tuning of the IL structure also improved the selectivity of © 2013 American Chemical Society

Figure 1. General structures of 1-R-3-methylimidazolium bis(triflamide) ILs with incremental (a) oligo(ethylene glycol) units (Px) and (b) n-alkyl chains (Cn).

polymerized IL [poly(IL)] gas separation membranes for these same gas pairs.19−22 1,2,3-Trifunctionalized imidazolium ILs (Figure 2) have typically received very little attention in the literature when compared to their 1,3-difunctionalized counterparts.23 Studies

Figure 2. General structure of 1,2,3-trifunctionalized imidazoliumbased ILs. Received: Revised: Accepted: Published: 16615

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Scheme 1. Synthesis of [Pxmim][NTf2] ILs by (a) Methylation of Ether-Functionalized Imidazoles50 and Subsequent Ion Exchange8 and (b) Reaction of 1,2-Dimethylimidazole with an Oligo(ethylene glycol) Monomethyl Ether Iodide and Subsequent Ion Exchange

To our knowledge, no studies on the vaporization of etherfunctionalized ILs or 1,2,3-trifunctionalized ILs have been reported, and it was only recently that Verevkin and co-workers demonstrated that the presence of a methyl group at the C(2) position of the imidazole ring reduces the vapor pressure of alkylimidazoles (i.e., neutral analogues of imidazolium cations).45−47 Previously, we also showed that, when PEG units were appended to the N(1) atom of imidazole and benzimidazole rings, the vapor pressure of the compound was reduced by almost an order of magnitude compared to that of an analogous compound with an n-alkyl chain with the same number of atoms in the chain.48 Correspondingly, the standard enthalpy of vaporization (Δgl Hm ° ) was larger in the etherfunctionalized imidazoles/benzimidazoles than in the corresponding alkylimidazoles/alkylbenzimidazoles.45−49 Herein, we present experimental results on vaporization enthalpies that detail the influence of two structural variables on the enthalpy of vaporization of ILs: the inclusion of ether groups and the presence of a methyl group at the C(2) position. The results indicate that the presence of ether groups actually lowers the enthalpies of vaporization of ILs relative to those of their alkyl-functionalized analogues. This behavior is in contrast to that of molecular solvents but can be rationalized as a disruption of cation−anion interactions caused by the Lewis basic ether linkages. The experimental data were used to develop group contribution values for the influence of ether groups on vaporization enthalpies of imidazolium-based ILs.

have shown that functionalization of the imidazole ring at the C(2), C(4), and/or C(5) positions can result in increased viscosities, higher melting points, and reductions in conductivity,1,24−26 whereas reports on the effect of functionalization at C(2) on CO2 solubility have been contradictory.27−29 From a molecular design standpoint, there might be advantages of employing ILs with more than two substituent groups as building blocks for poly(IL)s, poly(IL)−IL composites, or other soft materials based on ILs, as they can add new dimensions of control/functionality.23,30 Jin et al. recently reported a number of properties, including density, viscosity, melting point, conductivity, and decomposition temperature, for 30 different 1,2,3-trifunctionalized imidazolium [NTf2]-based ILs with at least one ether group as R1 and/or R3 and a methyl or ethyl substituent as R2 (cf. Figure 2).31 With no acidic proton present at C(2) in these ILs, their properties did not tend to vary much when compared to those of analogous 1,2,3-trifunctionalized ILs with only alkyl groups (e.g., 1-butyl-2,3-dimethylimidazolium cations). Triazoliumbased ILs also offer similar opportunities to further control properties through the inclusion of three substituent groups on the cation.32,33 Extremely low vapor pressures are considered to be one of the most useful properties of ILs,34 and as such, quantifying the relationships between vaporization enthalpy and IL structure is an important (yet challenging) task; however, available data are scarce.35 Several groups have studied the influence of the length of the alkyl chain on the vaporization enthalpies of [Cnmim][NTf2] ILs, resulting in the discovery of a linear relationship between chain length (i.e., Cn) and vaporization enthalpy.36−44 However, as [Cnmim] cations represent just a small fraction of the possible structures, exploring other systems such as [Pxmim] cations and investigating the influence of substitution at the C(2) position are logical progressions in this research area.

2. EXPERIMENTAL SECTION 2.1. Materials. [Pxmim][NTf2] (1−3) and [Pxmmim][NTf2] (4−6) ILs were synthesized according to published procedures (Scheme 1).4,8,31 1H NMR data for 1−6 were consistent with published data.8,31 (Spectra are provided as Supporting Information.) 16616

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Table 1. Standard Enthalpies of Vaporization for Ether-Functionalized ILs Derived from QCM and TGA T range (K)

Tav (K)

Δg1Hm ° (Tav) (kJ·mol−1)

Δg1Cpm ° (J·mol−1·K−1)

Δg1Hm ° (298 K)a (kJ·mol−1)

method

−48

121.6 ± 1.0

QCM TGA

−79

132.9 ± 1.2

QCM TGA

−98

142.4 ± 1.0

QCM TGA

−124

132.8 ± 1.0

QCM TGA

−126

140.4 ± 1.0

QCM TGA

−130b

147.8 ± 1.0

QCM TGA

[P1mim][NTf2] 330.5−392.6 509.8−575.0

365.5 542.4

118.4 ± 1.0 109.9 ± 1.4

352.9−407.7 512.5−575.3

378.3 543.9

126.6 ± 1.2 113.6 ± 1.6

362.9−417.7 519.3−582.6

388.2 551.0

133.6 ± 1.0 117.8 ± 1.8

357.9−405.5 511.8−575.1

381.1 543.5

122.5 ± 1.0 102.5 ± 1.2

368.0−417.8 512.2−575.5

390.9 543.9

128.7 ± 1.0 109.5 ± 2.0

377.9−427.8 519.3−582.6

400.9 551.0

134.4 ± 1.0 decomposition

[P2mim][NTf2]

[P3mim][NTf2]

[P1mmim][NTf2]

[P2mmim][NTf2]

[P3mmim][NTf2]

a Calculated with eq 4 from QCM and TGA data measured in this work using Δg1Cpm ° values from column 4. bAssessed according to the trend between [P1mmim][NTf2] and [P2mmim][NTf2].

dt, measured by the QCM, the molar enthalpy of vaporization, Δgl H°m(T0), is obtained as

Prior to the vaporization experiments, the ILs were dried by vacuum evaporation at 333 K and 10−3 mbar for at least 24 h. IL samples were subjected to additional purification inside the experimental equipment to remove possible traces of volatile impurities. 2.2. Measurements of Vaporization Enthalpy by Quartz Crystalline Microbalance (QCM). Enthalpies of vaporization of ILs were measured using the QCM technique. The experimental setup and measuring procedure were reported elsewhere.51 Our technique is principally different than the well-established Knudsen technique. In contrast to the Knudsen method, in which the sample cell is sealed with a membrane and only a small hole connects the sample container to the vacuum, in our method, a sample of IL is placed in an open cavity (Langmuir evaporation) inside the thermostatted block and exposed to a vacuum (at 10−5 Pa) with the whole open surface of the loaded IL. The QCM is placed directly over the measuring cavity containing the IL. During vaporization into the vacuum, a certain amount of the IL is deposited on the quartz crystal. The change in the vibrational frequency of the crystal, Δf (which is a measure of the amount of IL deposited on the cold QCM), is recorded as a function of time at different temperatures of the sample. The quartz crystal in this work was part of a commercially available device (BSH-150, Inficon) that measures the change in vibrational frequency, Δf, of the quartz crystal with a Q-pod transducer (Inficon). The distance between the surfaces of the quartz crystal and the IL was kept constant at 25 mm. The temperature of the QCM and its holder was maintained at 303.1 ± 0.1 K with a Julabo F12-MC cooling/heating circulator. The change in vibrational frequency, Δf, is directly related to the mass deposition, Δm, on the crystal according to the equation51,52

Δf = −Cf 2 ΔmSC−1

° T0 ⎛ 1 Δgl Hm° (T0) − Δgl Cpm ⎛ df ⎞ 1⎞ T ⎟ = A′ − ln⎜ ⎜ − ⎟ ⎝ dt ⎠ R T0 ⎠ ⎝T +

° Δgl Cpm R

⎛T ⎞ ln⎜ ⎟ ⎝ T0 ⎠

(2)

where the constant A′ is essentially unknown and includes all empirical parameters specific to the apparatus and the substance under study. T0 in eq 2 is an arbitrarily chosen reference temperature, which we have set to 298 K. The value Δgl C°pm = C°pm(g) − C°pm(l) is the difference between the molar heat capacities of the gas [C°pm(g)] and liquid [C°pm(l)] phases. The temperature-dependent vaporization enthalpy, Δgl Hm ° (T), obtained from the QCM study is given by ° (T − T0) Δgl Hm° (T ) = Δgl Hm° (T0) + Δgl Cpm

(3)

To detect and avoid any possible effect of impurities on the measured rate of change of the frequency (df/dt), a typical experiment was performed in a sequence with a few increasing and decreasing temperature steps. Each step consisted of 7−11 determinations of the rate of mass loss at each temperature. Several runs were performed to test the reproducibility of the results. The study was finished when the enthalpy of vaporization, Δgl Hm ° (298 K), obtained in the sequential runs by adjusting eq 2 to the temperature-dependent rates (df/dt) agreed within the assessed experimental uncertainty of ±1 kJ· mol−1. To confirm that the IL did not decompose under the experimental conditions, the residual IL in the crucible and the IL deposit on the QCM were analyzed by attenuated-totalreflection infrared (ATR-IR) spectroscopy. No changes in the spectra were detected. Experimental results of the QCM studies are provided in Table S1 of the Supporting Information. 2.3. Measurements of Vaporization Enthalpy by Thermogravimetric Analysis (TGA). A Perkin-Elmer Pyris 6 TGA instrument was employed to measure vaporization enthalpies from the temperature dependence of the mass-loss-

(1)

where f is the fundamental frequency of the crystal (6 MHz in this case), with Δf ≪ f; SC is the surface area of the crystal, and C is a constant.52 Using the rate of change of the frequency, df/ 16617

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rate measurements. Between 50 and 70 mg of an IL sample was placed in a plain platinum crucible inside the measuring head of the TGA instrument. The sample was heated in a stepwise manner, and the mass loss from the crucible was recorded at each isothermal step. The isothermal rate of mass loss (dm/dt) was monitored in the temperature range of 513−586 K at a N2 flow rate of 140 mL·min−1. According to the results reported in our previous work,53 the optimal conditions for the reliable determination of vaporization enthalpies of ILs are as follows: • mass loss at each temperature = 0.1−0.8 mg • duration of isothermal steps > 10 min • temperature range (ΔT) > 60 K Prior to the measurement of vaporization enthalpy, a careful conditioning of the sample inside the TGA instrument was performed. A heating ramp of 10 K·min−1 was used, followed by a 4-h static holding period at 423 K, allowing for the slow removal of volatile impurities and traces of water prior to the stepwise isothermal runs. The conditioning was repeated until a reproducible mass loss within two consequent runs was recorded. The absence of decomposition of the IL under the experimental conditions was confirmed by ATR-IR spectroscopy. No changes in the spectra taken from the initial and residual IL in the crucible were detected for the ILs under study. Experimental results of the TGA studies are collected in Table S2 of the Supporting Information.

Figure 3. Validation of consistency in comparing linear relationship of vaporization enthalpies for [Pxmmim] ILs (y axis) and [Pxmim] ILs (x axis).

vaporization enthalpy relative to those of analogous ILs with alkyl groups, namely, the [Cnmim][NTf2] series.54 To perform the comparison properly, we applied the homomorph concept.55 In fact, the enthalpy of vaporization of an appropriate alkane obtained by replacing the O group in the ether chain by a CH2 group (R−CH2−R) essentially represents the van der Waals contribution of the ether group to the enthalpy of vaporization. For example, for the molecular compound ethylene glycol dimethyl ether (CH3OCH2CH2OCH3), the suitable homomorph is n-hexane [CH3(CH2)4CH3]. For the IL [P1mim][NTf2], the suitable homomorph for the alkyl chain is [C4mim][NTf2]. Such a simple assumption allows for the comparison of the vaporization enthalpies of the two parent series [Pxmim][NTf2] and [Cxmim][NTf2] in the same plot in Figure 4. It is apparent from this plot that the vaporization enthalpies of the etherfunctionalized ILs are systematically ∼10 kJ·mol−1 lower than those of the corresponding members of the [Cnmim][NTf2] series. This observation contradicts our expectations from

3. RESULTS AND DISCUSSION The standard enthalpies of vaporization at Tav for the six etherfunctionalized ILs investigated in this work are presented in Table 1. It is apparent from Table 1 (column 3) that the enthalpies of vaporization, Δgl Hm ° (Tav), derived from QCM and TGA are not comparable because they refer to significantly different Tav values. For comparison, enthalpies of vaporization, ° (Tav), have to be adjusted to a reasonable common Δgl Hm temperature, such as the reference temperature of 298.15 K. In a recent work,54 we studied the parent [Cnmim][NTf2] series and developed a practical procedure, called the “Tav procedure”, for estimating Δgl Cpm ° values. This procedure is based on the experimental measurements of vaporization enthalpies, Δgl H°m(Tav), with two different methods at two different Tav values (e.g., with QCM and TGA). According to eq 3, the Δgl Cpm ° values were estimated as o Δ1g Cpm = [Δ1g Hmo(Tav)QCM − Δ1g Hmo(Tav )TGA ]

/[(Tav )QCM − (Tav )TGA ]

(4)

Δg1C°pm,

The experimental differences between heat capacities, derived with eq 4 (see Table 1, column 4) were used to calculate Δgl Hm ° (298 K) (see Table 1, column 5). Now, the enthalpy of vaporization values are comparable. A valuable check for internal consistency of the experimental vaporization enthalpies is a comparison of the Δgl H°m(298 K) values of [Pxmim][NTf2] and [Pxmmim][NTf2]. This comparison is presented in Figure 3. As can be seen in Figure 3, the vaporization enthalpies of the parent IL ether series are in excellent agreement with a correlation coefficient of 0.998, and this fact confirms the reliability of the experimental results measured in this work. Having confirmed the consistency of the experimental data for the [Pxmim][NTf2] and [Pxmmim][NTf2] series, it is interesting now to compare how the inclusion of ether groups in the alkyl chain of the imidazolium cation impacts the

Figure 4. Comparison of vaporization enthalpies for alkoxy-substituted ILs [Pxmim][NTf2] and [Pxmmim][NTf2] with the homomorph series [Cnmim][NTf2], where N is the total number of all alkyl and O units present in the imidazolium cation in the 1, 2, and 3 positions. 16618

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molecular liquids, for which the enthalpies of vaporization of poly(ether)s are always ∼5 kJ·mol−1 higher than those of their alkane homomorphs, as evidenced by the pairs ethylene glycol dimethyl ether (36.8 kJ·mol−1)56 and n-hexane (31.7 kJ·mol−1), diethylene glycol dimethyl ether (51.2 kJ·mol−1)57 and nnonane (46.4 kJ·mol−1),57 and triethylene glycol dimethyl ether (64.7 kJ·mol−1)56 and n-dodecane (61.5 kJ·mol−1).57 These results for ILs are also in contrast to the findings of our prior works that ether-functionalized imidazoles and benzimidazoles exhibit vaporization enthalpies that are larger than those of their alkyl-functionalized homomorphs.45,48,49 In our opinion, such an unusual behavior of ILs and the significant difference of the series of homomorph pairs [Pxmim][NTf2] and [Cnmim][NTf2] can be explained through the disruption of cation−anion associations. Molecular dynamics simulations have shown that the nanoscale structures of [Pxmim][NTf2] ILs are distinctly different from those of [Cnmim][NTf2] ILs, owing to the competition between the Lewis basic ether groups and the anions for interactions with the protons on the imidazolium ring.11 Such interactions are not possible in [Cnmim][NTf2] ILs, as the nonpolar alkyl chains are incapable of this behavior. The second structural pattern studied in this work was the functionalization of the C(2) position with a methyl group. In fact, the data for the homomorph series [Cnmmim][NTf2], needed for a proper comparison of the ether-substituted series [Pxmmim][NTf2], are absent in the literature. However, considering the general trend for [Pxmmim][NTf2], it is apparent that the vaporization enthalpies for this alkoxysubstituted family are also lower than those of the alkylsubstituted ILs. In addition, the simple structure−property relationship for the ILs under study presented in Figure 3 was used to assess the incremental change in vaporization enthalpy resulting from the introduction of a −(C2H4O)− group into the alkyl chain of the imidazolium cation. These contributions were found to be 10 kJ·mol−1 per −(C2H4O)− group for the 1,3-disubstituted ILs and 7.5 kJ·mol−1 per −(C2H4O)− group for the 1,2,3trisubstituted ILs. These values could be used to rapidly estimate enthalpies of vaporization for additional [Pxmim]- or [Pxmmim]-based ILs with longer alkoxy chains that have not yet been synthesized (e.g., [P4mim][NTf2], [P5mmim][NTf2]). Moreover, based on our experience, the same contributions could be useful for the quick appraisal of vaporization enthalpies for pyrrolidinium-, pyridinium-, and ammoniumbased ILs.

mended that will facilitate estimation of properties for new IL species in the future.

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ASSOCIATED CONTENT

* Supporting Information S

1 H NMR data and spectra for 1−6. Table with primary QCM and TGA data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +49-381-4986508. Fax: +49-381-498-6524. *E-mail: [email protected]. Tel.: +1-205-348-6836. Fax: +1205-348-7558. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the German Science Foundation (DFG) within the framework of priority program SPP 1191 “Ionic Liquids”. Partial support for this work was provided by ION Engineering, LLC, and the U.S. Department of Energy, National Energy Technology Laboratory (DE-FE0005799). Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (Doctoral New Investigator Award 52190DNI9) J.E.B. thanks Ken Belmore of the University of Alabama for the acquisition of 1H NMR spectra.

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REFERENCES

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4. CONCLUSIONS New experimental vaporization enthalpies of [Pxmim][NTf2] and [Pxmmim][NTf2] ILs with alkoxy-substituted chains as R1 (cf. Figure 4) were obtained from the concurrent use of the QCM and TGA methods. These results were adjusted to the reference temperature of 298 K and tested for consistency. Enthalpies of vaporization, Δgl H°m(298 K), for the alkoxysubstituted families were found to be less than those for the analogous ILs with the alkyl-substituted cation, which is in distinct contrast to the observations for molecular liquids. This reduction in vaporization enthalpy can be attributed to the disruption of cation−anion interactions caused by the presence of the Lewis basic ether groups. Using the experimental data, two useful group contributions for the rapid estimation of vaporization enthalpies of alkoxy-substituted ILs are recom16619

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