Solubility of CO2 and N2O in an Imidazolium-Based Lipidic Ionic

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Solubility of CO and NO in an Imidazolium-Based Lipidic Ionic Liquid Jacob V. Langham, Richard A. O’Brien, James H. Davis, and Kevin Neal West J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b05474 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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The Journal of Physical Chemistry

Solubility of CO2 and N2O in an Imidazolium-Based Lipidic Ionic Liquid Jacob V. Langham,†,§ Richard A. O’Brien,‡ James H. Davis, Jr.,‡ Kevin N. West†,* †

Department of Chemical &Biomolecular Engineering, University of South Alabama, Mobile, AL 36608 ‡

Department of Chemistry, University of South Alabama, Mobile, AL 36608

ABSTRACT: Imidazolium-based ionic liquids have been extensively studied for their ability to dissolve a wide variety of gases and for their potential to be used as separation agents in industrial processes. For many short chain 1-alkyl-3-methylimidazolium bistriflimde salts, CO2 and N2O solublities are very similar. In this work, the solubility of CO2 and N2O has been measured in the lipidic ionic liquid 1-methyl-3-(Z-octadec-9-enyl)imidazolium bistriflimide ([oleyl-mim][NTf2]) at 298 K, 310 K and 323 K up to ~2 MPa. N2O is found to have higher solubility than CO2 at the same conditions, similar to the behavior observed when olive oil, a natural lipid, is the liquid solvent. However, the solubility of each gas on a mole fraction basis is lower in the ionic liquid than in olive oil. Comparison of the gas solubilities on a mass fraction basis demonstrates that CO2 solubility is nearly identical in both liquids; N2O solubility is higher than CO2 for both liquids, but more so in the olive oil. The difference is attributed to the high mass fraction of the olive oil that is lipid-like in character.

The differential solubility of

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N2O/CO2 in this ionic liquid, in contrast to that of shorter chain 1-alkyl-3-methylimidazolium bistriflimide salts, gives physical insight to the solvent properties of this class of ionic liquids and provides further support for their lipid-like character.


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Introduction Ionic liquids (ILs), molten salts with melting points below 100°C, have been of interest as media for gas separations for a number of years. Their vanishingly low vapor pressures and structural diversity make them ideal candidates for media in which to absorb volatile gases. Because of this interest, the solubility of a wide range of gases has been studied in a broad variety of ionic liquids.1-10 Much of the interest has focused on environmental applications such as the absorption of CO2, both physical and chemical,11-14 from industrial process streams. The absorption of N2O (nitrous oxide), another volatile gas with negative environmental impact, in ILs has also been the subject of a number studies.15-19 N2O has two significant and deleterious effects on the environment: it is both a significant greenhouse gas, with a long atmospheric lifetime of over 100 years and global warming potential almost 300 times that of CO2,20 and it is estimated to be the dominant ozone depleting gas of the 21st century.21 Nitrous oxide emissions arise from both biological and industrial origins, and industrial emission sources range from adipic acid production to its use as an anesthetic.20 For these reasons, there has been considerable interest in developing technologies to separate N2O from process streams, often involving the separation of N2O from CO2. This separation is somewhat challenging as CO2 and N2O are chemically and thermodynamically similar. The two compounds have nearly identical molar masses (MMN2O = 44.013 g/mol, MMCO2 = 44.01g/mol), similar critical properties (CO2: Tc = 304.1 K, Pc = 7.377 MPa; N2O: Tc = 309.5 K, Pc = 7.245 MPa)22 and they are isoelectronic; nevertheless, N2O has a small dipole (µ = 0.161 D) and a modest electric quadrupole (Θ = -11.0 x10-40 Cm2),23 while CO2 has no dipole but a 30% larger quadrupole (Θ = -14.3 x10-40 Cm2).24 However, it is the significant Lewis acidity of CO2, not present in N2O, that gives rise to the most significant chemical differences between the


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compounds. The Lewis acidity of CO2 allows it to strongly interact and react with Lewis bases (such as water, alcohols and amines) to form reactive adducts or strongly bound complexes that can be exploited to affect separations. Another key difference between CO2 and N2O is the latter’s ability to effectively function as a gaseous anesthetic, while the former is not significantly active in this respect. Although there are a number of proposed mechanisms of action, one theory of N2O’s anesthetic effect is related to its solubility in cellular lipid bilayers and its disruptive effect in these structured membranes through the exertion of a lateral pressure.25 As such, it has been long known that a compound’s efficacy as an anesthetic could be tied to its solubility in olive oil, 26-27 a thermosphysical model for a lipid bilayer. Observing this phenomenon reported in the literature, Shiflett and Yokozeki made the most accurate measurements to date of the solubility of CO2 and N2O in commercially available olive oil,28 a polyunsaturated triacylglycerol mixture, as a function of pressure for several isotherms around human body temperature. These measurements demonstrated the higher solubility of N2O in olive oil and were consistent with previous measurements. Unlike in olive oil, the solubilities of CO2 and N2O in many imidazolium-based ionic liquids are very similar, with N2O typically having a slightly higher solubility.1, 4, 18-19 The higher N2O solubility in many ionic liquids may be attributed to N2O having stronger interactions with the charged domains of the ionic liquid;1 although, we note that this would not be the cause of higher N2O than CO2 solubility in lipids. The selective solubility of CO2 over N2O can be significantly enhanced by using an acetate anion, which is known to interact with the Lewis acidic CO2.17 However, little effect on selectivity is observed in altering the cation alkyl chain length for short chain 1-alkyl-3-methylimidazolium salts.1, 4


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Over the past few years, our group has been interested in developing ionic liquids with “nonpolar-like” solvent properties. As most ionic liquids, being ionic in character, exhibit polar to moderately-polar solvent properties,29-30 we took a cue from biology and introduced cisunsaturated, long (C16-C20) alkyl chains onto imidazolium cations to imbue them with non-polar character. As they were inspired by, and synthesized from, naturally occurring fatty acids, we refer to these species as lipidic ionic liquids.31-33 Recently, we have reported their non-polar character with respect to their higher ethane solubility than ethylene solubility,34 contrary to that of most ionic liquids, and their ability to solubilize linear and cyclic hydrocarbons (LLE) up to ~80 mole %.35 Given the differential solubility of N2O and CO2 in olive oil, and their nearly identical solubility in conventional imidazolium-based ionic liquids, demonstrating a differential solubility in a lipidic ionic liquid would support characterization of the solvent properties of the ionic liquid as “lipid-like.” In this work, we further the characterization of the properties of these species by studying the solubility of CO2 and N2O in a representative lipidic ionic liquid, 1-methyl-3-(Z-octadec-9enyl)imidazolium bistriflimide ([oleyl-mim][NTf2]), shown in Figure 1, at temperatures equal to and near human body temperature (25°C, 37°C and 50°C) to give additional physical insight into their ability to function as ionic liquids with “non-polar-like” solvent properties.

Experimental Methods Materials The ionic liquid, [oleyl-mim][Tf2N], was synthesized by methods previously described32 and verified to have ≥99% purity via NMR. As synthesized, the compound has a water content less than 0.25%; in the temperature range studied, the density ranged from 1.19 to 1.17 g/cm3 and the


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viscosity ranged from 264 to 86 mPa·s.33 Samples were placed in high vacuum (1.5 MPa). However, there is a clear difference in N2O solubility, again with N2O solubility higher in olive oil than in [oleyl-mim][NTf2]. Similar to the data expressed in terms of mole fraction, the mass fraction data is influenced by the fact that the olive oil has a higher mass fraction that is represented by the fatty acid chains (as the NTf2- anion of the ionic liquid is rather massive). Comparing the data in terms of mass fraction highlights the higher lipidic mass fraction in olive oil, similar to the mole fraction comparison, but normalizes the data


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with respect to the different molar masses of the species. This, in turn, demonstrates the effect of the lipidic nature of the liquid on N2O solubility, while at the same time demonstrating the lack of effect on CO2 solubility. Additional insight into how lipidic substances solubilize N2O vs. CO2 can be obtained by comparing the selectivity, defined as the ratio of the mole fraction solubility of N2O to CO2 (xN2O/xCO2), in both the ionic liquids and in olive oil. Figure 10 shows these selectivities at 298 K, 310 K and 323 K for olive oil and for the lipidic ionic liquid. Each solvent exhibits the same trend across all temperatures. At low pressures (and low solubilities) there is a relatively high selectivity for N2O over CO2; approaching ~2 for olive oil and ~1.5 for the lipidic ionic liquid. As the pressure is increased, the selectivity rapid decreases and asymptotically approaches a relatively temperature independent value; ~1.2 for olive oil and ~1.1 for the ionic liquid. The higher selectivity at lower concentration coupled with lower selectivity at higher concentration are consistent with a model of dissolution that involves two different phenomena. At low concentration, the more selective mechanism is dominant. Given the similarity between the two solutes, the mechanism is likely based on a stronger solvent interaction with N2O than CO2, and could be related to interaction between N2O and the cis-double bond present in the lipids and lipidic ionic liquids. N2O is known to oxidize alkenes at higher temperatures and evidence suggest that strong complexing between the N2O and the C-C double bond is key to this reaction;43-46 This interaction may be responsible for the low-pressure N2O selectivity over CO2, although further study would be needed to support this hypothesis. As the pressure is increased, each solute is dissolved into the bulk solution which has a lower selectivity for N2O. Though the specific phenomena giving rise to these differences is still not well understood, these


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comparisons give physical insight into structure/property relationship that may be exploited to affect CO2/N2O separations and further demonstrate the lipidic nature of the ionic liquid.

Conclusions The solubility of CO2 and N2O has been measured in the lipidic ionic liquid [oleyl-mim][NTf2] at 25°C, 37°C and 50°C as a function of pressure up to ~2 MPa using a gravimetric microbalance. The data is fit to a modified RK cubic equation of state which reproduces the data to a high degree of accuracy (0.01 MPa). The data demonstrate that the lipidic ionic liquid behaves more like olive oil than it does shorter chain 1-alkyl-3-methylimizaolium bistriflimide salts, with respect to CO2/N2O solubility near human body temperature. N2O solubility is higher than CO2 solubility in the ionic liquid on both a mole and mass fraction basis. Comparison on a mass fraction basis demonstrates similar CO2 solubility to olive oil, while N2O solubility is significantly greater in both liquids, but more so in olive oil. This is consistent with the concept that the lipid portions of the species are responsible for the higher N2O solubility, as the olive oil has a greater fatty acid chain content on a per mass basis. This work presents further evidence, in addition to ethane/ethylene solubility and alkane solubility, of the lipidic, or “non-polar-like”, solvent properties of these ionic liquids and the potential for this class of ionic liquid to be useful in practical separations. Furthermore, the solubility trends and selectivity give insight into the molecular origins of enhanced N2O solubility in lipids and suggest further study. In particular, experimental assays coupled with molecular dynamic simulation may be useful in understanding the true nature of the interactions that govern this behavior and could lead to further developments in N2O separations and in the understanding of the mechanism by which N2O has a higher solubility in lipidic liquids.


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Figure 1. Structure of the lipidic ionic 1-methyl-3-(Z-octadec-9-enyl)imidazolium bistriflimide ([oleyl-mim][NTf2]); MM = 613.72, Tm = -20.9°C.


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Figure 2. Solubility of CO2 (mole fraction) in [oleyl-mim][NTf2] at 298 K (○), 310 K (□) and 323 K (◊). The solid lines are the RK fit of the data.

2 1.5 1 0.5 0

0

0.1

0.2

0.3

0.4

0.5

x

N O 2

Figure 3. Solubility of N2O (mole fraction) in [oleyl-mim][NTf2] at 298 K (○), 310 K (□) and 323 K (◊). The solid lines are the RK fit of the data.


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2 1.5 1 0.5 0

0

0.1

0.2

0.3 0.4 x ,x CO

2

0.5

0.6

0.7

NO 2

Figure 4. Comparison of the solubility of CO2 (○,●) and N2O (◊,♦) (mole fraction) at 298 K in [oleyl-mim][NTf2] (open symbols) and olive oil28 (filled symbols).

2 1.5 1 0.5 0

0

0.1

0.2

0.3 0.4 x ,x CO

2

0.5

0.6

0.7

NO 2



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2 1.5 1 0.5 0

0

0.1

0.2

0.3 0.4 x ,x CO

2

0.5

0.6

0.7

NO 2


Figure 7. Comparison of the solubility of CO2 (○,●) and N2O (◊,♦) (mass fraction) at 298 K in [oleyl-mim][NTf2] (open symbols) and olive oil28 (filled symbols).


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Selectivity (xN O/xCO ) 2 2


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Olive Oil

[oleyl-mim][NTf2]

Figure 10. Selectivity of N2O/CO2 (xN2O/xCO2) as a function of pressure at 298 K (solid), 310 K (dash) and 323 K (dotted) in olive oil28 (top curves) and [oleyl-mim][NTf2] (bottom curves).


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Table 1. Experimental solubility of CO2 (1) in [oleyl-mim][NTf2] (2) in mole fraction, x1, and mass fraction, m1. Temperature (K)

Pressure (MPa)

x1·100

m1·100

298.2 298.3 298.2 298.2 298.2 298.2 298.2 298.2 298.3 298.2 298.2 298.2 298.2 298.2

0.0253 0.0503 0.0756 0.1001 0.1251 0.1497 0.2499 0.5002 0.7499 0.9998 1.2497 1.4996 1.7497 2.0005

1.43 2.34 3.31 4.28 5.17 6.12 9.56 17.45 24.17 29.95 35.00 39.49 43.45 47.00

0.104 0.172 0.245 0.320 0.389 0.465 0.752 1.493 2.235 2.975 3.717 4.471 5.223 5.980

310.2 310.1 310.1 310.1 310.1 310.1 310.1 310.1 310.2 310.1 310.2 310.2 310.2 310.2

0.0249 0.0499 0.0749 0.1005 0.1252 0.1499 0.2499 0.4997 0.7499 0.9998 1.2498 1.4998 1.7506 1.9997

1.22 2.03 2.83 3.63 4.44 5.19 8.26 15.00 20.90 26.09 30.65 34.76 38.51 41.84

0.089 0.149 0.208 0.269 0.332 0.391 0.641 1.250 1.860 2.469 3.072 3.680 4.297 4.906

323.1 323.3 323.2 323.2 323.2 323.2 323.1 323.2 323.2 323.1 323.1

0.0251 0.0499 0.0762 0.1014 0.1260 0.1506 0.2501 0.4998 0.7498 0.9999 1.2500

1.05 1.68 2.31 2.95 3.62 4.26 6.77 12.33 17.56 22.26 26.30

0.076 0.122 0.169 0.218 0.269 0.318 0.518 0.999 1.505 2.012 2.495


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323.1 323.2 323.2

1.5002 1.7505 1.9998

30.05 33.27 36.74

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2.989 3.452 3.998


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Table 2. Experimental solubility of N2O (1) in [oleyl-mim][NTf2] (2) in mole fraction, x1, and mass fraction, m1. Temperature (K)

Pressure (MPa)

x1·100

m1·100

298.1 298.1 298.2 298.0 298.1 298.1 298.2 298.1 298.1 298.1 298.1 298.1 298.1 298.1

0.0247 0.0502 0.0750 0.1007 0.1250 0.1499 0.2499 0.4997 0.7499 0.9997 1.2487 1.4998 1.7496 2.0005

2.17 3.28 4.40 5.49 6.51 7.53 11.48 20.23 27.41 33.47 38.65 43.37 47.21 50.80

0.159 0.243 0.329 0.415 0.497 0.580 0.921 1.787 2.637 3.482 4.323 5.207 6.026 6.893

310.1 310.1 310.1 310.2 310.1 310.2 310.1 310.2 310.1 310.2 310.1 310.1 310.1 310.2

0.0246 0.0500 0.0756 0.1006 0.1248 0.1509 0.2505 0.4998 0.7498 0.9994 1.2499 1.4997 1.7505 1.9982

1.95 2.95 3.89 4.79 5.67 6.54 9.80 17.36 23.81 29.27 34.17 38.25 41.97 45.28

0.143 0.217 0.290 0.360 0.429 0.500 0.773 1.484 2.192 2.883 3.588 4.254 4.932 5.602

323.2 323.1 323.2 323.2 323.1 323.1 323.1 323.2 323.1 323.1 323.2

0.0250 0.0511 0.0753 0.0998 0.1262 0.1510 0.2505 0.4999 0.7498 1.0002 1.2498

1.44 2.30 3.03 3.76 4.60 5.37 8.31 14.76 20.17 25.39 29.73

0.105 0.169 0.224 0.280 0.344 0.405 0.646 1.227 1.780 2.382 2.945


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323.2 323.1 323.2

1.5002 1.7505 1.9994

33.64 37.09 40.54

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3.508 4.056 4.662


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Table 3. Fit parameters for the RK equation of state for CO2 (1)/ [oleyl-mim][NTf2] (2) and N2O (1)/oleyl-mim][NTf2] (2), including the pure component parameter β1 for the IL fit from binary data, and the model residual standard deviation as calculated by Equation 9. Parameter

CO2

N2O

l12 l21 β1 (IL)

0.02310 0.02952 0.7586

0.03070 0.03932 0.7369

s (MPa)

0.01

0.01


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AUTHOR INFORMATION Corresponding Author *

Kevin N. West Department of Chemical & Biomolecular Engineering University of South Alabama 150 Jaguar Drive Mobile, AL 36688 [email protected], 251-460-7563 Present Addresses §

Current Address: Department of Chemical Engineering Auburn University, Auburn, AL 36849

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. RAO and JHD synthesized and purified the ionic liquids. JVL and KNW took the VLE data and performed the modeling. KNW wrote the manuscript. ¶

These authors contributed equally: KNW,¶ RAO,¶ JHD¶.

ABBREVIATIONS [oleyl-mim][NTf2]

1-methyl-3-(Z-octadec-9-enyl)imidazolium bistriflimide

Acknowledgements This material is based upon work supported by the National Science Foundation under Grant Numbers 1126597 & 1133101.


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Solubility of CO2 and N2O in an Imidazolium-Based Lipidic Ionic

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