Interactions of Gases with Ionic Liquids: Molecular Simulation - ACS

Chapter 11

Interactions of Gases with Ionic Liquids: Molecular Simulation

Downloaded by PENNSYLVANIA STATE UNIV on May 31, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch011

Johnny Deschamps and Agilio A. H . Pádua Laboratoire de Thermodynamique des Solutions et des Polymères, Université Blaise Pascal, Clermont-Ferrand, 63177 Aubière, France

The interactions of argon, methane, nitrogen, oxygen and carbon dioxide with room temperature ionic liquids of the dilakylimidazolium family, 1-butyl-3-methylimidazloium hexafluorophosphate, [bmim][PF], and 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF], were studied by molecular simulation. The ionic liquids were modeled using a recently proposed forcefieldand the solute gases represented by multi-site models from the literature. The calculated solubilities are in qualitative agreement with experiment concerning the relative magnitude of this property for the different gases. The temperature dependence of the solubilities is correctly predicted for carbon dioxide but has the wrong sign for the remaining gases. Particular attention is devoted to the structure of the solutions of carbon dioxide in the ionic liquids. It was observed that the solubility of carbon dioxide can be correctly predictedfromphysical interaction only. Our results show that the molecule of carbon dioxide does not interact in a relevant manner with the hydrogen connected to the C carbon of the imidazolium ring (the carbon atom bonded to the two nitrogens), as could be expected. 6

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© 2005 American Chemical Society

In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

151 Hie solvation properties of room temperature ionic liquids are being elucidated by a number of experimental and theoretical studies. The present work concerns the interactions of gases with ionic liquids and was closely articulated with an experimental study (/). Here those interactions are investigated using atomistic computer simulation, with two objectives: (i) to verify the performance of a force field newly proposed for imidazolium salts in predicting properties that depend on unlike interactions, and (ii) to contribute to understand the nature of solute-solvent interactions.


Computational Methods The ionic liquids 1 -butyl-3-methyHmidazloium hexafluorophosphate, [bmim][PF], and l-buiyl-3-methylimidazoliumtetrafluoroborate,[bmim][BF ], were modeled by an all-atom force field developed recently for the family of dialkylimidazolium salts (2). The force field is derived from OPLS-AA (3) and AMBER (O: it adopts the same functional form and, whenever available, parameters were taken from these two sources. The solute gases were represented by intermolecular potentials of the type Lennard-Jones plus charges, obtainedfromthe literature. Argon was represented by a Lennard-Jones potential (J), nitrogen and oxygen were modeled by two centers (6,7), methane was considered as defined in OPLS-AA (J), and for carbon dioxide therigidthreesite model with partial charges proposed by Harris and Yung was adopted (8). No adjustments of unlike interaction parameters were performed within this work, therefore all results presented below are predictive. Simulations were performed using the molecular dynamics method, implemented in the DLJPOLY package (P). Solubilities were obtainedfromthe residual chemical potentials of the solute gases at infinite dilution, calculated using afreeenergy route such as dietest-particleinsertion method. The chemical potential was calculated at infinite dilution, in systems constituted by 250 ion pairs. Simulations were run for 350 ps, in the NpT ensemble at 1 bar, and samples of 1000 configurations were stored for post-treatment. In each configuration, 90 000 insertions at random positions were attempted. Radial distribution functions were obtainedfroma different set of runs lasting 200 ps, of systems containing one solute molecule and 250 ion pairs. Previous to the production runs, the systems were allowed to equilibrate for at least 200 ps. A time step of 1 fs was adopted and a spherical cutoff of 13 A for atomic interactions was considered. Tail corrections to short-range interactions were included and the long-range coulombic interactions were calculated by the Ewald summation method. Except for C-H bond lengths, which were constrained, all other internal modes in the imidazolium cations were left flexible. 6

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Results and Discussion

Solubility of gases The solubility of thefivegases calculated in the ionic liquids [bmirn][PF] and [bmim][BF ], at 323 K, is presented in Figure 1 and compared to experimental data obtained by Brenneckc and co-workers in [bmim][PF] (JO). Although agreement is not quantitative, the relative solubility orderforthe different gases is reproduced by simulation, with C0 showing a much larger solubility, and N a lower solubility, than Ar, CH4 and 0 . Experimental values for N were not published because its solubility is below the detection limit of the measuring device (10). 6

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T = 323K 10.0

S

Ο

0[bmim][PFJexp • (bmimj[BFj8im •tbmim][PFj8Jm

i

1.0

0.1 h

Ar

CH

N

4

CO

2

a

solute gaz

Figure 1. Comparison ofcalculated gas solubilities in the ionic liquids experimental values. Solubility is given as the molefractionofgas dissolv partial pressure of 1 bar.

The present results indicate that the solubility of gases is higher in [bmim][PF] than in [bmim][BF]. Spectroscopic evidence for C0 (//), however, shows that its interaction is stronger with BF ~ than with PF ", due to the smaller size of theformeranion. The same authors postulate that this effect 6

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153 might be compensated byfree-volumecontributions, which would be larger in [bmim][PF] (//), explaining thus the higher solubility of C0 in imidazolium ionic liquids with die PF " anion when compared to those with BF ~ anion (12). We have attempted in this work to address the issue offreevolume in the ionic liquids, through die calculation offreeenergy of cavity formation. The solvation process is commonly decomposed in two steps: creation of a cavity in the solvent capable of hosting a solute molecule, and activation of solute-solvent interactions. The free energy of cavity formation can be obtained from simulations of die pure solvent, using the technique of test particle insertion to find the probability of inserting a hard sphere at random locations (13). The free energy of cavityformationin the two ionic liquids calculated in this work is shown in Figure 2, where it is compared to that of an organic solvent, /i-hexane, and water. These two liquids were represented by OPLS-AA (3) and TIP5P (14) models, respectively. 6

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T-303K

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Τ-303 Κ

Figure 2. Distribution ofspherical cavity sizes (left) andfreeenergy of cavity formation(right)as afunction ofcavity size in the ionic liquids compared to th in other solvents.

It is observed that spontaneously present cavities are of a smaller size in the ionic liquids than in n-hexane or water at the same temperature. Accordingly, reversible creation of a cavity requires more work in the ionic liquids than in the two otherfluids.As a result, if solute-solvent interactions were similar, any gas would be less soluble in the ionic liquids than in the two other solvents. Since C0 has a larger molecular size than any of the other gases considered in this work, it is clear that it is the strength of its interactions with the ionic liquids that is die cause of its higher solubility. 2


154 Both ionic liquids considered in this study have very similar distributions of cavity sizes, meaning that thefreevolume available in the two should be comparable. Nevertheless,fromdie plot offreeenergy of cavityformationit appears indeed to be slightly easier to create cavities in [bmim][PF] than in [bmim][BF]. This agrees with the view expressed in the literature (11,12). In Figure 3 are plotted the calculated solubilities of Ar and C0 in the two ionic liquids, as a function of temperature, compared to experimental results (/). It is observed that the temperature dependence of the solubility of Ar obtained from simulation is opposite to the experimental trend. For C0 , simulation agrees with experiment, both giving solubilities that decrease with temperature. The temperature dependence of the solubility is related to the enthalpy of solvation, therefore the interaction models used do not yield the correct sign for the enthalpy of solvation of Ar. We have yet no explanation for thisfoot,since it appears to indicate that, in our simulations, the interactions of the non polar gases with the ionic liquids are too strong. The experimental results in room temperature ionic liquids are consistent with data on inorganic molten salts (15). In molten salts it was observed that non (quadru)polar gases like Ar, N and He have indeed solubilities that increase with increasing temperature—endothermic solvation—whereas the solubility of C0 decreases with increasing temperature—exothermic solvation. 6

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CO,

Ar 30

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OPmbnlPFJwp

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ο 0.8 α o

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ο ο

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0Ό 300

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320 3ί Τ/Κ

340

350

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I

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310

OpxnimlPFJtKp •(bmim]BFJtim •IbmlmlPfjttm

Ο

a

ι 320 330 Τ/Κ

α ο

α ο

Χ π 340

350

Figure 3. Temperature dependence ofthe solubility ofAr and C0 in the liquids. The solubility corresponds to the molefractionofdissolved gas a partial pressure of 1 bar. 2

A significant aspect concerning the interactions of quadrupolar gases like C0 or even N with the ionic liquids is the relevance of the electrostatic term. If this term is very important, then it may be necessary to include it in the 2

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description of N ,fromwhere it is often absent. Also, properties that depend on the unlike gas-liquid interactions will be very sensitive to details of die electrostatic part of the models. In Table I are included die solubilities calculated for C0 and N in [bmim][BF] and [bmim][PF], in two situations: with frill electrostatics, and with the partial charges set to zero. This means that a distribution of three charges was added to the model of N in order to reproduce its experimental quadrupole moment (16). It is concluded that the effect of removing the quadrupole is dramatic for C0 , causing a diminution of the solubility, whereas for N an effect is detectable but not very significant Addition of a quadrupole to the N model does not change much the values of the calculated solubilities. 2

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Table I. Effect of the quadrupole on the solubility of C0 and N in [bmim](BF ] and (bmim](PF*] calculated at 303 Κ 2

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Χ2/ΗΓ

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Full electrostatics

No electrostatics C0

[bmim][BF] [bmimirPFid

8.6 2CM)

[bmim]PF ] [bmim][PFl

0.12±0.02 0.30±0.04

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1.3 2Λ N

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0.10±0.01 0.21±0.03

The site-site solute-solvent radial distribution functions obtained by simulation allow an analysis of the structure of the solution, and help elucidate the nature of the gas-liquid interactions. The COr{bmim][PF ] radial distribution functions (rdfs) are plotted in Figure 4. In the upper-left plot are shown the rdfs of die solute atoms with respect to the C carbon in the imidazolium ring—this is the carbon atom bonded to die two nitrogens. The first peaks in the two curves, at a distance slightly above 5 Â, coincide. The situation is totally different in die lower-left plot, where diefirstand second peaks of die O-C4 5 atoms surround thefirstpeak of the C-€ rdf. Thefirstmaximum of the 0-C , rdf is at a close distance of 3.5 Â. lids structure means that die C0 molecule is closer to the C and C carbons, directed with its longitudinal axis towards these atoms of die ring. The C0 molecule is therefore not interacting strongly with the C , as might be expectedfromthe Lewis-acid nature of the H hydrogen which is bonded to this atom (17). In fact, an IR spectroscopy study points out that the stronger interactions of C0 are with the anion (//). From the upper-right plot, containing the rdfs between the solute atoms and the Ρ atom of die anion, the superposition of die very sharp first peaks at distances slightly above 4 A shows that the C0 molecule is lyingflatagainst the 6

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156 PF \ — τ * — 1 ι ι • 1 • ι. • I ι • • I • 6

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:

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r/A

Figure 5. Site-site solute-solvent radial distributionfiinctionsofthe C0 interaction model with the electrostatic charges removed in [bmim][PFJ. 2

Acknowledgment The authors acknowledge access to the supercomputing centers in France, IDRIS of the CNRS and CINES of the Ministère de la Recherche.


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References 1. Costa Gomes, M.F.; Husson, P.; Jacquemin, J.; Majer, V. ACS Symp. Series 2003, this volume. 2. Canongia Lopes, J.N.; Deschamps, J.; Pádua, A.A.H. J. Phys. Chem. Β 2003, in press. 3. Jorgensen, W.L.; Maxwell D.S.; Tirado-Rives J. J. Am. Chem. Soc. 1996, 118, 11225 ; Kaminski, G.; Jorgensen, W. L. J. Phys. Chem. 1996, 100, 18010. 4. Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, Κ. M.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P.A. J. Am. Chem. Soc. 1995, 117, 5179. Parameters obtainedfromfile parm99.dat corresponding to AMBER versions 1999 and 2002. 5. Michels, Α.; Wijker, H.; Physica 1949, 15, 627. 6. Rivera, J.L.; Alejandre, J.; Nath, S.K.; de Pablo, J.J. Mol. Phys. 2000, 98, 43. 7. Myiano, Y. Fluid Phase Equilibria 1999, 29-35, 158. 8. Harris, J.G.; Young, K.H. J. Phys. Chem. 1995, 99, 12021. 9. Smith, W; Forrester, T.R. The DL_POLY package of molecular simulation routines, version 2.12. The Council for the Central Laboratory of Research Councils, Daresbury Laboratory, Warrington, UK, 1999. 10. Anthony, J.L.; Maginn, E.J.; Brennecke, J.F. J. Phys. Chem. Β 2002, 106, 7315. 11. Kazarian, S.G.; Briscoe, B.J.; Welton, T. Chem. Commun. 2000, 2047. 12. Blanchard, L.A.; Gu, Ζ.; Brennecke, J.F. J. Phys. Chem. Β 2001, 105, 2437. 13. Costa Gomes, M.F.; Pádua, A.A.H. J. Phys. Chem. Β 2003, in press. 14. Mahoney, M.W.; Jorgensen, W.L. J. Chem. Phys. 2000, 112, 8910. 15. Field, P.E.; Green, W.J. J. Phys. Chem. 1971, 75, 821. 16. Raich J.C.; Gillis N.S. J. Chem. Phys. 1977, 66, 846. 17. Aggarwal, Α.; Lancaster, L.; Sethi A.R.; Welton, T. Green Chemistry 2002, 4, 517. 18. Margulis, C.J.; Stern, H.A.; Berne, B.J. J. Phys. Chem. Β 2002, 106, 12017. 19. Morrow, T.I.; Maginn, E.J. J. Phys. Chem. Β 2002, 106, 12807. 20. Hardacre, C.; McMath, S.E.J.; Nieuwenhuyzen, M.; Bowron, D.T.; Soper A.K. J. Phys. Condens. Matter 2003, 15, 159.


Interactions of Gases with Ionic Liquids: Molecular Simulation - ACS

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