Solubility Parameters for Nine Ionic Liquids - Industrial & Engineering

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Solubility Parameters for Nine Ionic Liquids Brian Yoo,† Waheed Afzal,† and John M. Prausnitz*,†,‡ †

Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720-1462, United States Energy Biosciences Institute, University of California, Berkeley, California 94720-5230, United States

‡

S Supporting Information *

ABSTRACT: Solubility parameters are useful for estimating solubilities of solutes in ionic liquids when no solubility data are available. Solubility parameters are reported for nine ionic liquids; they were obtained from solubility data for a variety of solutes. The ionic liquids are 1-butyl,3-methylimidazolium hydrogen sulfate, 1-ethyl,3-methylimidazolium acetate, 1-methyl,3trimethylsylilimidazolium chloride, 4-methyl,n-butylpyridinium tetrafluoroborate, ethylammonium nitrate, 1-ethyl,3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-methyl,3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl,3methylimidazolium bis(trifluoromethylsulfonyl)imide, and 1-ethyl,3-methylimidazolium ethylsulfate.

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INTRODUCTION Ionic liquids (ILs) have gained much recent attention due to their beneficial properties that include negligible vapor pressure, wide liquid range, and good thermal stability. To characterize ILs, many researchers1−13 have reported a variety of their physical properties including their ability to dissolve common solutes. Solubility data have been reported as activity coefficients at infinite-dilution, Henry’s constants or Flory χ (chi) parameters; from these, we have calculated solubility parameters for nine ionic liquids. When no solubility data are available, solubility parameters for ILs are useful for estimating solubilities of other solutes in ionic liquids. Solubility parameters for solutes and solvents that are close in value to each other indicate greater likelihood of miscibility.14 Therefore identifying and compiling the solubility parameters for ILs can provide good use as a complementary prescreening tool in solvent selection and in understanding the interactions of solutes and solvents. Following the work of Patterson et al.,15 Merk et al.16 demonstrated how to use chromatographic retention data to obtain solubility parameters for polymers. Using Flory− Huggins theory, they obtained the chi parameter at infinite dilution of a solute in a polymer. Using regular-solution theory for the enthalpic part of the Flory−Huggins equation, the chi parameter is related to the solubility parameters of the polymer and the solute. Application of gas−liquid chromatography to determine solubility parameters for ILs (rather than for polymers) has been the subject of several publications.17−22 Mutelet19,20 and Marciniak et al.21,22 have shown that inverse gas chromatography (IGC) provides a reliable method for determining the solubility parameters for ILs, superior to other methods based on intrinsic-viscosity data or on Kamlet−Taft correlations. In this work, using gas−liquid chromatography data from our previous work and from solubility data in the literature, we have obtained the chi parameter for a variety of infinitely dilute binary mixtures where the solvent is an ionic liquid. From the chi parameters we have obtained solubility parameters for nine ionic liquids. Table 1 identifies the ionic liquids, their molecular © 2012 American Chemical Society

structures, and data sources. For these ionic liquids, solubility parameters have not been previously reported.

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DATA REDUCTION AND RESULTS As shown by Cruikshank,24 chromatographic retention data provide the activity coefficient at infinite-dilution γ∞ 12 of solute 1 in solvent 2 when the solute has a negligible finite pressure correction γ12∞ =

n2RT VN1P1s

(1)

where n2 is the number of moles of solvent in the chromatographic column, Ps1 is the saturated vapor pressure of the solute, R is the gas constant, T is the column temperature in Kelvin, and VN1 is the net retention volume for the solute given by VN1 = J(tr − tg )U0

273.15 T

(2)

Here, tr is the retention time of the solute, tg is the retention time of essentially insoluble hydrogen or air, U0 is the volumetric flow rate at the column outlet, and J is a correction factor24 that takes into account the pressure drop in the column J=

2 3 (Pi /P0) − 1 2 (Pi /P0)3 − 1

(3)

where Pi and P0 are, respectively, the pressure of the column at the inlet and outlet. Using the Flory−Huggins theory, Patterson et al. showed that when the gas phase is ideal, the chi parameter at infinite dilution χ12 can be determined from chromatography data Received: Revised: Accepted: Published: 9913

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Table 1. Abbreviations and Structures of Nine Ionic Liquids Studied in This Worka

a

The references shown provide solubility data for some of the solutes listed in Table A2.

⎛ 273.15R ⎞ ⎛ ⎛ρ ⎞ V*⎞ ⎟⎟ − ⎜⎜1 − 1 ⎟⎟ + ln⎜⎜ 1 ⎟⎟ χ12 = ln⎜⎜ s V 2* ⎠ ⎝ ρ2 ⎠ ⎝ P1 VgM1 ⎠ ⎝

γ1∞ = (4)

VN1 w2

(6)

Activity coefficient γ∞ 1 is related to χ12 by

where Vg is the specific retention volume, M1 is the molecular weight of solute, V*1 and V*2 and ρ1 and ρ2 are, respectively, the molar volume and (mass) density of solute and solvent. The specific retention volume is obtained from Vg =

H1 P1s

⎛ρ ⎞ ⎛ 273.15γ ∞M 2 ⎞ ⎛ V* ⎞ 1 χ12 = ln⎜ ⎟ − ⎜1 − 1 ⎟ + ln⎜⎜ 1 ⎟⎟ TM1 V 2* ⎠ ⎝ ⎠ ⎝ ⎝ ρ2 ⎠

(7)

Equation 7 gives the chi parameter from activity-coefficient data at infinite dilution. Activity coefficient at infinite dilution γ∞ 1 (based on mole fraction) is converted into a weight-fraction activity coefficient by multiplying M2/M1. For numerous binary systems, we obtained activity-coefficients at infinite dilution (or Henry’s constants) from our experimental work and from the literature to calculate chi parameters.26−31 In the Appendix, Table A1 shows chi parameters for numerous binary systems at several temperatures near 298 K.

(5)

where w2 is the mass of the solvent. Equation 4 is useful for polymer solutions when the molecular weight or polydispersity of the solvent are not precisely known.15 The activity coefficient at infinite dilution γ∞ 1 is related to Henry’s constant by 9914

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Table 2. Solubility Parameter δ2 for Nine Ionic Liquids Obtained from Solubility Data for (Nearly) Nonpolar Solutes at Several Temperaturesa ionic liquid

T/K

δ2/MPa0.5

[BMIM][HSO4]

313 333 353 373 393 313 333 353 373 393 313 333 353 373 393 313 323 333 343 303 308 313 318 323 303 313 323 333 293 303 313 323 293 303 313 323 303 313 323 333

23.1 22.7 23.5 23.1 23.1 23.1 22.6 22.7 22.4 22.0 21.1 21.0 20.6 20.8 20.5 23.5 23.3 22.7 22.4 25.1 25.0 25.0 24.9 24.8 21.5 21.2 21.1 20.4 21.4 21.1 20.7 20.4 19.7 19.4 19.9 18.8 24.0 23.5 23.1 21.9

solubility parameters δ2,P/ MPa0.5 [EMIM][TFSI] 293 303 313 323

29.7 29.4 29.1 28.7

[EMIM][Ac]

Figure 1. An example of the graphical determination of the solubility parameter for [BMIM][HSO4] at 313 K. The data-point labels correspond to the solute numbers in A2. For the ordinate, the units are δ21 = MPa; T = K; V1* = cm3 mol−1; R = cm3 MPa K−1 mol−1.

[SiMIM][Cl]

[M4B-py][BF4]

[EAN]

[MMIM][TFSI]

[EMIM][TFSI]

Figure 2. Solubility parameters δ2 as a function of temperature for [EMIM][Ac], [SiMIM][Cl], [EAN], [EMIM][TFSI], and [BMIM][TFSI].

[BMIM][TFSI]

Calculation of Solubility Parameters for Ionic Liquids. The solubility parameter of an ionic liquid is related to the chi parameter by32

[EMIM][EtSO4]

2

χ12 =

V1*(δ1 − δ2) RT

(8)

where δ2 is the solubility parameter of the IL, and δ1 is the solubility parameter of the solute. For the solute, δ1 is obtained from

⎛ ΔH − RT ⎞0.5 δ1 = ⎜ ⎟ V1* ⎝ ⎠

Also shown at the end are solubility parameters δ2,P for [EMIM][TFSI] obtained from solubility data for polar solutes. Solutes 1−45 in Table A2 were used to determine δ2; solutes 46−55 were used to determine δ2,P. a

(9)

where ΔH is the enthalpy of vaporization provided in the NIST database.33 As shown, for example, by Merk et al., algebraic rearrangement of eq 8 gives χ ⎛ 2δ ⎞ (δ12) δ2 − 12 = ⎜ 2 ⎟δ1 − 2 RT RT V1* ⎝ RT ⎠

obtained using the slope and intercept agrees with each other within 5%. Here, the intercepts were used to determine δ2. Table A2 identifies the solutes and their solvents used to obtain δ2. Assuming experimental data have about 5% experimental uncertainty, the solubility parameters decrease or increase linearly with temperature. For the most of the ILs studied in this work from about 293 to 393 K, solubility parameters

(10)

As shown in Figure 1, when the left side of eq 10 is plotted against δ1, 2δ2/(RT) is the slope of the line and −δ22/(RT) is the intercept. Using linear regression, the slope or intercept can be used to determine δ2. The values of solubility parameters 9915

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Figure 3. Flory−Huggins interaction parameters χ12 for 8 solutes in [BMIM][HSO4] at 313 K (a); for 10 solutes in [EMIM][Ac] at 353 K; for 7 solutes in [SiMIM][Cl] at 333 K (c); and for 20 solutes in [EMIM][TFSI] at 313 K (d) as a function of solute solubility parameter δ1. From these plots, the estimated solubility parameters for the ionic-liquid solvents δ2 are shown by the vertical lines. The data-point labels correspond to the solute numbers in Table A2.

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CONCLUSION Table 2 gives solubility parameters for nine ionic liquids when the solute is (nearly) nonpolar, that is, when δ1 < 23 MPa0.5 . For [EMIM][TFSI], we report two solubility parameters, one for (nearly) nonpolar solutes and another for polar solutes with δ1 > 23 MPa0.5. Because solubility data for polar solutes are limited, we cannot obtain similar results for the other 8 ionic liquids.

decreased with rising temperature. Solubility parameter of [BMIM][HSO4] shows an increasing trend with temperature. Figure 2 shows solubility parameters as a function of temperature for a few of the ILs. Table 2 shows solubility parameters for nine ionic liquids. To exclude highly polar solutes, solubility parameters δ2 were obtained using only solubility data where the solute solubility parameter δ1 < 23 MPa0.5. When δ1 = δ2, χ12 = 0. Solubility parameters for [BMIM][HSO4], [EMIM][Ac], and [SiMIM][Cl] were obtained from our experimental data; they show good correspondence to a near zero chi parameter. Solubility parameters for [MMIM][TFSI], [EMIM][TFSI], and [BMIM][TFSI] were obtained from solubility data in the literature; they also show good correspondence to a chi parameter near zero. Polar Solutes. As demonstrated by Batista et al.,23 a plot of infinite-dilution activity coefficient γ∞ 1 > 1 against solubility parameter δ1 can be used to obtain δ2 from the point on the plot where the activity coefficient attains a minimum. They have shown that plots for polar and nonpolar data provide two minima, indicating two different IL solubility parameters. Following their example, we have plotted chi parameters as a function of solute solubility parameters to illustrate an alternative analysis shown in Figure 3. A chi parameter value close to zero indicates a Flory−Huggins ideal solution. Batista et al. have suggested that an adequate description of an ionic liquid cannot be described by a single solubility parameter but rather by two solubility parameters: one for nonpolar solutes and one for polar solutes.23 Figure 3 shows polar solubility parameters for [EMIM][TFSI]. Solubility parameter δ2,P for polar solutes was determined based on solubility data where δ1 > 23 MPa0.5.

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ASSOCIATED CONTENT

S Supporting Information *

Flory−Huggins parameters χ12 (Table A1) and solutes and their solvents used to obtain δ2 (Table A2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1 510 642 3592. Fax: +1 510 642 4778. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS For financial support, the authors are grateful to the Environmental Energy Technologies Division of the Lawrence Berkeley National Laboratory. We much appreciate advice from Dr. Sasisanker Padmanabhan.

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REFERENCES

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